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Weird Worms Swim in Huge Green Circles

FLPA, Alamy
FLPA, Alamy

If you find yourself walking along the beaches of the English Channel, you might come across a mat of green goo, as if someone had tipped a jar of mint sauce onto the beach. But if you get down on your hands and knees, and stare at the goo with a magnifying glass, you’ll see its true nature.


Little green worms, each just a few millimetres long, writhing together in their millions.

Their formal name is Symsagittifera roscoffensis, but Channel Islanders call them mint sauce worms for obvious reasons. Their green colour comes from the algae in their bodies, which provide them with nutrients by harnessing the sun’s energy. The algae also gave the creatures their other name, favoured by early 20th century biologists: “plant-animals.”

One of those biologists, Frederick Keeble, wrote a whole book about these creatures in 1901. Eight decades later, Nigel Franks from the University of Bristol picked up a copy at a second-hand bookstore for the princely sum of 50 pence. Intrigued, he set out to find them, and it took him several trips to the island of Guernsey to do so. “It’s hard to find a patch of them but once you find them, they’re there in huge numbers,” he says. “They’re hugely impressive. When you see them, you’ll think they’re algae.”

Franks scooped up a group of them and started studying their behaviour. He quickly realised that if he added enough of them to the same pool of shallow water, they’d start swimming in mesmerising circular mills, with hundreds of individuals in rotating green rings. “They did them at the drop of a hat,” says Franks. “It was such a telltale symptom of really strong social interactions.”

Having spent most of his career studying the collective behaviour of ants, Franks was the perfect audience for the worms’ display. He knew that many other social animals will produce circular mills, for varying reasons. Army ants do it if you isolate a group of them, slavishly following each other’s chemical trails until they die from exhaustion. Fish do it when confronted by predators. Even virtual animals will form mills if you program them with simple rules. So what about the worms?

By studying videos of the animals, Franks and his colleagues showed that they interact strongly with one another, often swimming in parallel with just a millimetre between them. As their densities increase, they grew disproportionately closer. The mills, it seems, arise from these close-quarter interactions, and from the worms’ tendency to curve in a particular direction. (It’s notable that almost all the mills spin clockwise.)

Using this information, Franks’ colleague Alan Worley built a computer simulation in which virtual worms behaved according to simple rules, and yet spontaneously produced circular mills just like their real counterparts.

Green mint sauce worms.
Green mint sauce worms.
Photograph by Stevie Smith

“But many different models of individual interactions can reproduce the same kind of collective patterns,” says Guy Therauluz, another collective motion researcher at Paul Sabatier University. “Deciphering the real social interactions at work between worms is a task that remains to be done.”

Franks plans to do that. It’s possible, he says, that the worms are dribbling some kind of chemical behind themselves, which the others follow. Or perhaps they are reacting to turbulence in the water. “The rules have yet to be worked out,” he says.

He also wants to know why the worms behave in this way and he has a fascinating suggestion. Perhaps the worms are social sunbathers. By gathering in large mats of biofilms, bound together by mucus that they themselves secrete, they can stabilise their position on sandy beaches or tidepools. In Franks’ words, they “behave collectively as a social seaweed”.

Individual worms are also known to head towards light sources that are almost too bright for them, and would max out the abilities of their algal partners. They could deal with that problem in a mat by ducking down into the darker centre once they’ve had their fill of light. Franks compares them to emperor penguins huddling against bitter Antarctic winds: these flocks continuously rotate as individuals at the edge wheedle their way into the centre.

Dora Biro from the University of Oxford praises Franks’ attempt to explain not just the how of the collective motion, but also the why—something that has been overlooked by scientists in this field.  “The hypothesis is very interesting,” she says. “It would be great to find support for it through future work, including observations in the wild on the formation of the biofilm, and the role of milling in the process.”

Franks is just getting started with the mint sauce worms, but he sees them as great subjects for understanding collective behaviour in animals. Other scientists have analysed their bodies, the way they grow, and their powers of regeneration—but only ever one at a time. “I suspect that if people looked at populations more, as we have been fortunate enough to do with these things, they’d see more and more examples of strange microscopic organisms showing social behaviour like this,” Franks says.

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How a 5-Ounce Bird Stores 10,000 Maps in Its Head

It weighs only four or five ounces, its brain practically nothing, and yet, oh my God, what this little bird can do. It’s astonishing.

Photograph by Diana Tomback
Photograph by Diana Tomback

Around now, as we begin December, the Clark’s nutcracker has, conservatively, 5,000 (and up to 20,000) treasure maps in its head. They’re accurate, detailed, and instantly retrievable.

A Clark's nutcracker with a thought bubble full of maps
Photograph by Glenn Bartley, Alamy; Maps courtesy of jaffne, Flickr; GIF by Becky Harlan

It’s been burying seeds since August. It’s hidden so many (one study says almost 100,000 seeds) in the forest, meadows, and tree nooks that it can now fly up, look down, and see little x’s marking those spots—here, here, not there, but here—and do this for maybe a couple of miles around. It will remember these x’s for the next nine months.

How does it do it?

32 Seeds a Minute

It starts in high summer, when whitebark pine trees produce seeds in their cones—ripe for plucking. Nutcrackers dash from tree to tree, inspect, and, with their sharp beaks, tear into the cones, pulling seeds out one by one. They work fast. One study clocked a nutcracker harvesting “32 seeds per minute.”

These seeds are not for eating. They’re for hiding. Like a squirrel or chipmunk, the nutcracker clumps them into pouches located, in the bird’s case, under the tongue. It’s very expandable …

Drawing of one clark's nutcracker bird without seeds in its cheeks, and another clark's nutcracker with its cheeks full of seeds
Drawing by Robert Krulwich

The pouch “can hold an average of 92.7 plus or minus 8.9 seeds,” wrote Stephen Vander Wall and Russell Balda. Biologist Diana Tomback thinks it’s less, but one time she saw a (bigger than usual) nutcracker haul 150 seeds in its mouth. “He was a champ,” she told me.

Next, they land. Sometimes they peck little holes in the topsoil or under the leaf litter. Sometimes they leave seeds in nooks high up on trees. Most deposits have two or three seeds, so that by the time November comes around, a single bird has created 5,000 to 20,000 hiding places. They don’t stop until it gets too cold. “They are cache-aholics,” says Tomback.

When December comes—like right around now—the trees go bare and it’s time to switch from hide to seek mode. Nobody knows exactly how the birds manage this, but the best guess is that when a nutcracker digs its hole, it will notice two or three permanent objects at the site: an irregular rock, a bush, a tree stump. The objects, or markers, will be at different angles from the hiding place.

a drawing of clark's nutcracker looking at a tree stump and a mountain in the spring, with arrows pointing between the bird, the mountain, and the stump
Drawing by Robert Krulwich

Next, they measure. This seed cache, they note, “is a certain distance from object one, a certain distance from object two, a certain distance from object three,” says Tomback. “What they’re doing is triangulating. They’re kind of taking a photograph with their minds to find these objects” using reference points.

Psychologist Alan Kamil has a different view. He thinks the birds note the landmarks and remember not so much the distances, but the angles—where one object is in relation to the others. (“The tree stump’s 80 degrees south of the rock.”) These nutcrackers are doing geometry more than measuring.

a drawing of clark's nutcracker looking at a tree stump and a mountain in the rain, with arrows pointing between the bird, the mountain, and the stump
Drawing by Robert Krulwich

However they do it, when the snow falls and it’s time to eat, they’ll land at a site. “They will perch on a tree,” says Tomback, “on a low branch, [then light onto the ground, where] they pause, look around a bit, and they start digging, and in a few cases I’ll see them move slightly to the right or to the left and then come up again.”

She’s convinced that they’re remembering markers from summer or fall and using them to point to the X spot—and, “Lo and behold, these birds come up with their cracked seeds,” she says. “And it’s really pretty astounding.”

In the 1970s, Stephen Vander Wall ran a tricky little experiment. He shifted the markers at certain sites, so that instead of pointing to where the seeds actually were, they now pointed to where the seeds were not. Like this …

Picture of a clark's nutcracker standing between two X's and looking confused
Drawing by Robert Krulwich

And the birds, as you’d expect if they were triangulating, went to the wrong place.

But at sites where he left the markers untouched, the birds got it right. That’s a clue that each of these birds has thousands of marker-specific snapshots in their heads that they use for months and months. When the spring comes and the birds have their babies, they continue to visit old sites to gather seeds until their chicks fledge.

The mystery here, the deep mystery, is how do they manage to store so much data in their heads? I couldn’t possibly do what they do (I can’t even remember all ten digits in a phone number, so I’d be one very dead nutcracker in no time). Is their brain organized in some unique way?

Is their brain plastic? Can it grow more neurons or more connections when it needs to? Chickadees are also food hiders, and they do grow bushier brains when they need to, expanding in the “remember this” season and contracting afterward. Do Clark’s nutcrackers do that? We don’t know.

Whatever it is they do, I want what they’ve got.

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Chicks Put Low Numbers on the Left, Just Like Humans

Think about all the numbers between 1 and 10, inclusive.

If you’re like the vast majority of people, you just automatically pictured a row of digits, starting with 1 on the left and ascending to 10 on the right. This is the mental number line. Until recently, it seemed to be unique to humans. That’s not to say that other animals can’t count or deal with numbers—they plainly can. But it looked like we were the only species that thought about numbers in this way.

Not anymore. In a really clever experiment, Rosa Rugani from the University of Padova has shown that baby chickens probably have a mental number line too. Once they fixate on a specific number, they associate smaller numbers with the left side of space, and larger numbers with the right—just like us. This supports the idea “that culture is not crucial for the mental number line,” says Rugani.

The seeds of this experiment were planted in 2007. Rugani put domestic chicks and Clark’s nutcrackers in front of a row of sixteen objects, which extended away from them. She trained them to peck at the fourth one. When she rotated the row so that it ran horizontally instead, the birds mostly pecked the fourth object from the left. They were counting from left to right.

They might have had a mental number line. Then again, they might just prefer to pay attention to objects in the left side of space. (Humans have the same bias; it’s called pseudoneglect.)

To clarify the matter, Rugani designed a new and deceptively simple experiment. She placed three-day-old chicks in front of a panel showing 5 squares, and taught them to walk around it to get some food. She then presented the chicks with two identical panels, one on their left and one on their right. If these showed 2 squares, the chicks walked towards the left one 70 percent of the time. But if the panels showed 8 squares, the chicks’ preference flipped: they approached the right one 70 percent of the time.

Credit: Rosa Rugani.
Credit: Rosa Rugani.

Since both panels were identical, there’s no reason why the chick should prefer one over the other. And if it was showing pseudoneglect, it should always prefer the left panel. That’s not what happened. If the testing panels showed a smaller number than the training one, the chicks walked left; if they showed a bigger number, the chicks headed right. That’s pretty strong evidence for a left-to-right mental number line.

Rugani repeated the experiment with different numbers. This time, the training panel had 20 squares, and the testing ones had either 8 or 32. The chicks behaved in the same way: smaller number, head left; bigger number, head right. Note that the ‘small’ number here—8—is the same as the ‘big’ number from the first experiment. If the chick saw 5 squares first, it headed towards the right 8-square panel. If it saw 20 squares, it approached the left 8-square panel.

This showed that the chicks are comparing the relative size of the numbers. “A number is not either small or large in an absolute sense, but rather it is smaller or larger with respect to another number,” says Rugani. In other words, their number line can slide, just like ours can.

“This finding completely overturns the idea—widely held in the past—that our mental number line is a human invention that depends on culture and on instruction,” says Maria Dolores de Hevia from Paris Descartes University, who studies the same topic in human babies.

There was already some evidence for this. For example, De Hevia showed that seven-month-old babies, who haven’t learned language or maths, still prefer 1-2-3 to 3-2-1. Culture can certainly influence the number line. In reaction time tests, people tend to respond faster to smaller numbers with their left side and to larger numbers with their right side—but people who learn Arabic, where writing goes from right to left, show the opposite preference.

But if a chick—an animal with no reading skills, arithmetic, language—also has a mental number line, that changes things. It strongly suggests that human culture only shapes and tweaks something that is innate. “And it adds to the idea that our most sophisticated numerical abilities derive from a core, and evolutionary ancient, sense of number, which evolved in such a way that it exploits our visuo-spatial resources,” says De Hevia. “The question now is: why?”

There might not be a why. The direction of the line might be totally arbitrary. Then again, in many animals including chicks and humans, the right half of the brain takes the lead in visual, spatial, and numerical tasks. And since the right hemisphere directs our attention to the left side of space, perhaps that’s why the number line begins on the left.

Her team are now carrying out similar studies in other species. They’re trying to identify the specific parts of the brain that deal with numbers, and to identify the genes that influence number sense. “This would help us to understand the evolutionary origin of the mental number line,” she says.

Reference: Rugani, Vallortigara, Priftis & Regolin. 2015. Number-space mapping in the newborn chick resembles humans’ mental number line. Science http://dx.doi.org/10.1126/science.aaa1379

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Scientists Instil New Cultural Traditions in Wild Tits

In the early 20th century, milkmen would deliver milk to British doorsteps, in bottles that were sealed with foil caps. Then, in the 1920s, homeowners started noticing holes in the foil. The culprits were blue tits. They had learned to peck open the bottle caps to drink the layer of cream beneath. The behaviour quickly spread. By the 1950s, it seemed that every blue tit in Britain knew the technique.

This story is now a classic tale among students of animal behaviour. It beautifully showed how a new cultural innovation—the infiltration of milk bottles—could spread among wild animals. But the tradition was well-spread before scientists noticed, which meant that they could only observe what was going on.

Lucy Aplin from the University of Oxford wanted to go one better. She wanted to do an experiment where she deliberately seeded different populations of tits with new behaviours and checked how these baby cultures spread, matured, and clashed over time.

She began in the most obvious place: Wytham Woods near Oxford. The great tits that live there have been carefully monitored since the 1940s, and they are among the best studied birds in the world. Every single individual has now been tagged with a unique microchip tag, and several antennae automatically log their movements as they fly past. These birds are subject to a degree of scrutiny that would make Orwell blush.

Into this surveillance society, Aplin introduced two new behaviours. She captured two pairs of birds from five different populations and trained them to extract food from a puzzle box, by sliding either a blue door or a red one. She then released these birds, along with untrained pairs from three other populations, back into the woods, which by then had been littered with the same puzzle boxes. These boxes could read the birds’ tags and automatically record which individuals drew near, whether they collected food, and which door-sliding technique they used. The data poured in. All Aplin had to do was wait.

After 20 days, she found that in the three populations without any trained birds, between 9 and 53 percent of the tits succeeded in opening the puzzle boxes. They had to work out how to do on their own. But among the five groups with trained demonstrators, 68 to 83 percent of the birds solved the puzzles. They were clearly learning from each other. The team proved this by recording the birds’ arrival at feeders, and working out who flocked with whom. In other words, they mapped the birds’ social network—the original Twitter. And they found that if a tit knew how to solve the puzzle, its associates were 12 times more likely to learn the technique than birds with ignorant friends.

Proportion of birds from different populations that solved the puzzle. Red and blue lines are groups that learned to use the red and blue doors respectively. Green lines are groups without training. Credt: Aplin et al, 2014. Nature.
Proportion of birds from different populations that solved the puzzle. Red and blue lines are groups that learned to use the red and blue doors respectively. Green lines are groups without training. Credt: Aplin et al, 2014. Nature.

Aplin also found that the successful groups split into two different schools, based on what their demonstrators did. If the pioneering pair studied the red-door technique, their neighbours also used the red door. If the pioneers learned the way of the blue-door, so did their neighbours.

Proportion of solutions that used the blue door (option A) or the red one (option B). Credit: Aplin et al, 2014. Nature.
Proportion of solutions that used the blue door (option A) or the red one (option B). Credit: Aplin et al, 2014. Nature.

These traditions are totally arbitrary. The blue and red doors are equally valid solutions and equally easy. But with each passing day, the birds in each population became increasingly likely to use the most popular option. They were conformists. They went with the crowd. Indeed, during the experiment, 14 birds moved to a population with a different colour preference, and 10 of them swapped to match their neighbours’ biases.

“We thought that these traditions would erode over time, but actually we saw the birds being more and more biased towards one side,” says Aplin. “I was surprised to see how persistent [the biases] were.”

That became very clear when the team revisited the birds a year later. In the intervening months, the puzzle boxes had been taken down and around 60 percent of the tits had died. The woods were full of youngsters, most of whom had never seen the puzzles before. Still, the old ways remained. The box-opening techniques spread even faster than they did in the previous year, and the red and blue-door schools stuck to their respective biases.

Several scientists have shown that cultural traditions exist among different groups of wild animals. Blue tits learn to open milk bottles; bottlenose dolphins learn to forage with sponges; humpback whales learn to catch fish with bubbles; chimps in different regions use tools in different ways.

Some groups have even done experiments with captive animals, like chimps and capuchin monkeys, to show that tutors can instil new traditions in their peers. But such experiments—the ultimate proof of cultural transmission—are much harder to do in the wild. The first of these was only published last year: Erica de Waal and Andrew Whiten from the University of St Andrews showed that wild vervet monkeys can learn arbitrary new traditions, like preferring blue corn kernels over pink ones, or vice versa. They too showed conformist tendencies: monkey see, monkey do.

Aplin’s study shows that great tits behave in the same way. “It’s a really important and very timely contribution to our understanding of cultural transmission in animals. It is done with admirable rigour, and uses a reassuring large sample of birds,” says Whiten. These kinds of studies are “steadily building a new picture of the importance of what I’ve called nature’s second inheritance system, in which behaviours are inherited not by the primary system of genetics, but instead hop from brain to brain, via learning from others.”

“We were surprised to see this behaviour, which was traditionally thought of as a complex primate one, in a bird,” says Aplin. “It suggests that animal cultures are more widespread than we might have thought.”

Joe Henrich from the University of British Columbia, who studies cultural evolution, is less surprised. We knew that great tits are good social learners, he says, and mathematical studies predicted that such species should show conformist tendencies. This new study confirms those predictions.

Aplin agrees that conformism makes sense for many animals, great tits included. “If you’re a bird coming into a new area, habitats are variable, and you don’t have a lot of info, it would be adaptive to copy what lots of locals are doing,” she says.

Reference: Aplin, Farline, Morand-Ferron, Cockburn, Thornton & Sheldon. 2014. Experimentally induced innovations lead to persistent culture via conformity in wild birds. Nature http://dx.doi.org/10.1038/nature13998

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The Early Chimp Gets The Fig

At my supermarket, I can buy strawberries in winter and pears in summer. Every fruit is available all year round, and the shelves are always stocked. Thanks to this constant glut, it’s easy to forget what a patchy and fleeting resource fruit can be. Even in a tropical rainforest—a world of supposed abundance—animals might have to walk for miles to find the one tree in every 500 that has ripe fruit on it. That tree might only carry ripe fruit for a few weeks, and any juicy baubles would be rotten or eaten within days.

So fruit-eating animals like chimpanzees need good memories and flexible brains. They need to remember where the best trees are and when they are likely to bear fruit, and they must adjust their behaviour accordingly. Karline Janmaat from the Max Planck Institute for Evolutionary Anthropology compares fruit-eating to a game of chess, “in which the pieces do not only change position but also continuously change their state, with intervals that can last months for some yet only hours for others.”

She has also found that wild chimps are masters of this game. They get up earlier in the morning, often before the sun’s first rays, if they plan on eating short-lived fruits like figs. And they build the previous night’s nests in the direction of these trees, so they can get a headstart on any competitors.

“The results are entirely consistent with the idea that the chimps are not simply prisoners of the present, but think about what to do next, and do so a while in advance,” says Carel van Schaik, who was not involved with the study. And by planning their future activities, they can compensate for the fluctuations in their food supply. By applying their considerable brains, they keep their guts full.

Janmaat discovered these abilities by following a group of chimps in the Ivory Coast’s Tai National Park. She would track the animals during the day, mark where they slept at night, and return before the next sunrise to watch them get up. It was a punishing schedule, but it yielded an important observation: sometimes, the chimps descended to the forest floor while it was still dark.

They don’t normally do that, and they were extremely skittish. After all, these are dangerous forests; one of the chimps had recently lost a son to a leopard attack. “These chimps are very used to humans and they never look at you,” says Janmaat. “But during these moments, if I broke a branch, they’d look back immediately. I was really struck by that. They were taking risks.”

Why? When Janmaat followed these early-risers, she saw that they almost always headed for fig trees. Figs aren’t particularly nutritious, but they are only ripe for a short time. And since they are soft and unprotected, they attract a large variety of hungry birds, squirrels, and monkeys. So, only the early chimp gets the figs.

Karline Janmaat observing a chimp.
Karline Janmaat observing a chimp. Credit: Ammie Kalan.

Janmaat and her colleague Simone Dagui Ban followed five of these chimps over 275 days, and collected a wealth of data about their habits and movements. They found that the females left their nests earlier in the morning when they were heading to eat figs, but only if the figs were far away from their nests. For other fruits, which aren’t so heavily contested, the chimps were more relaxed. They would get up later if the trees were further away. If they don’t need to arrive early, they don’t risk walking in the dark.

She also found that the chimps plan the location of their nests with tomorrow’s meals in mind. If they ate breakfast in a fig tree, you could draw a pretty straight line between that spot, the place where they slept, and the place where they dined on the previous night. If they ate other fruits for breakfast, their nesting locations were often off at an angle. If they knew they’d be shooting for figs, they positioned their nests en route. They must also have a really good map of the local fruit-bearing trees in their heads.

It’s very hard to explain these complicated patterns through simple rules. The chimps weren’t just drawn to the sight of figs, because they usually can’t see the trees they are travelling to. They weren’t drawn to smells either, or they would have got up earlier when fig trees were closest. They weren’t just moving according to ingrained patterns, because Janmaat never saw them sleeping and breakfasting in the same pair of trees twice. And other factors like temperature, rainfall, or the presence of males couldn’t explain the patterns in their behaviour.

The best explanation is that when figs are on the menu, chimps get up early to beat the breakfast rush. And when they have far to travel, they get up earlier. “They weigh up different types of information—not just what they were planning to eat, but where that food was,” says Janmaat.

“The work is vintage Karline Janmaat: very detailed, long-term and clever,” says van Schaik. “She is a great observer, in situations where experiments are extremely difficult or even impossible.”

Other scientists have found that captive apes can plan for the future by, for example, choosing tools that might come in handy later, saving tokens that they could later cash in for food, or even stockpiling stones to throw at annoying tourists. All of these examples are about tools. Using a tool is an obvious and dramatic way for an animal to show how clever it is. Getting up early seems mundane by comparison, but Janmaat argues that it is just as impressive. It’s a sophisticated way of getting food and beating the competition, and one that demands intelligence.

A young chimp in the Tai National Forest.
A young chimp in the Tai National Park.

Other apes might share this skill: just last year, van Schaik showed that orang utans communicate their travel plans to rivals and mates the night before, another sign of forward-planning. “Gradually, the burden of evidence is shifting toward the conclusion that at least great apes engage in mental time travel on a routine basis in their normal everyday lives,” he says.

This ability may have been critical in our evolution. Some scientists have suggested that our vaunted smarts evolved to help us cope with scarce and patchy food like fruit. For example, larger-brained primates tend to eat a steadier level of calories all year round even though they eat highly seasonal foods. That may be because they have mental tricks like good memory, the ability to make tools, and flexibility, which allow them to find meals at all times. Janmaat’s study suggests that planning is part of that package.

Unfortunately, she thinks that studies like these may be harder and harder to do. “Sometimes, we wouldn’t find females in the morning because they had to shift their nest due to the gunshots of illegal hunters,” she says. “This may have been one of the last chimpanzee populations that could be studied in West Africa. Our opportunities to gain insight in our own evolutionary history through the observations of the behaviours of our closest relatives are decreasing at rapid speed.” And there’s no way for chimps to plan around that. Only we can.

Reference: Janmaat, Polansky, Ban & Boesch. 2014. Wild chimpanzees plan their breakfast time, type, and location. PNAS http://dx.doi.org/10.1073/pnas.1407524111

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When Your Prey’s in a Hole and You Don’t Have a Pole, Use a Moray

Redouan Bshary is best known for studying cleaner wrasse—tiny underwater hygienists that pick parasites from much larger fish, like the roving coral grouper. In 2006, Bshary decided to follow one of the groupers to see whether it sought the services of several cleaners in a row. Instead, he saw something wholly unexpected. The groupers repeatedly swam up to giant moray eels and made a vigorous head-shaking signal. It was a call to arms—a signal that meant “Hunt with me”.

The eels respond by swimming off with the groupers. They can slink through crevices and flush out hidden prey, while the groupers are lethal in open water. When they hunt together, little fish have nowhere to flee.

Working with Bshary, Alexander Vail from the University of Cambridge found that the groupers also use a different signal—a  headstand—to tell the morays where hidden fish can be found. It’s the equivalent of a human pointing a finger—a gesture that says, “The prey’s in here.” These sorts of referential gestures had only been seen in intelligent animals like humans, apes, ravens, dolphins, and dogs. Their use was often taken as a sign of intelligence. The fact that fish—a group hardly known for their smarts—can use similar signals was surprising.

Vail and Bshary made their discoveries by observing fish in the wild. Now, they’ve brought a closely related species—the coral trout*—into their lab, and tested its partnership with morays through experiments. And they’ve found that the fish’s behaviour is even more sophisticated than we thought. It doesn’t just recruit its partner willy-nilly—it can decide when and with whom to collaborate. It recruits morays when the situation demands it, and it picks the more effective of two possible partners. And it performs just as well as chimpanzees did, when confronted with a similar task in an earlier study.

In 2006, Alicia Melis from the Max Planck Institute for Evolutionary Anthropology presented chimps with an out-of-reach food platform connected to some rope. If chimps could pull the platform closer on their own, they generally did. If they needed a partner, they were more likely to recruit one. And if there was more than one partner available, they chose the most effective one. Melis published the results in a straightforwardly titled paper: “Chimpanzees Recruit the Best Collaborators”.

Vail and his colleagues tried to duplicate the gist of that experiment with coral trout. They placed eight captive trout in tanks with a fake moray eel—a life-size plastic cut-out, nestled in a rocky crevice. They also added a similar cut-out of a small prey fish, which either sat out in the open (where the grouper could snatch it) or hidden under a rock (where a moray was necessary). Right from the first day of testing, the trout tried to recruit the moray far more often when the prey was hidden than when it was exposed.

But not all morays are equal. In the wild, the team saw that some eels are consistently more willing to team up with groupers and trout, while others are reticent collaborators. They simulated this by presenting their captive trout with two morays—one that would launch forwards at the right signal, and one that refused to leave its crevice. On day one of testing, the trout had six chances to recruit a partner, and they went after both eels equally. On day two, they went for the cooperative eel five times out of six.

Vail’s experiment featured the same number of subjects and trials as Melis’s study, and his trout performed as well as her chimps. Of course, there are important differences between the two set-ups. “The trout just had to do something very natural for them, something they’ve practiced for their whole lives,” says Vail. “But the rope-pulling thing was relatively novel for the chimps. It was fairly removed from something they normally do.” However, he adds that the chimps “received quite extensive training in each aspect of their task”, before being exposed to the whole experiment. By contrast, he gave his trout no such training.

“The findings are very exciting. Their results suggest that the fish’s behavior is highly adaptive, and I am not surprised to see similarities in how [chimps and trout] cooperate or choose partners,” says Joshua Plotnik, who has studied cooperation in elephants. “However, as the authors rightly point out, similarities in behavior do not necessarily suggest similarities in intelligence. The authors note that much of the fish’s behavior could be due to learning mechanisms, which do not necessarily require the flexibility of more complex cognition.”

In other words, this doesn’t mean that trout are as intelligent as chimps. They almost certainly don’t show the same wide ranging of sophisticated behaviours. But they can behave in complex ways in the situations that benefit them. They have specific smarts driven by ecological needs, rather than all-round smarts driven by big brains. They remind us, again, that complex behaviour doesn’t necessarily imply complex minds.

Still, this experiment, together with the earlier observations in the wild, suggests that the fish are going beyond simple algorithms like “IF want prey, THEN find moray”. They seem to understand the eel’s role, they recruit it in the right circumstances, and they can direct it to the right place. “It’s always going to be hard to get inside the mind of an animal, but my hunch is that the groupers have an idea of what’s going on,” says Vail. “It shows some of the hallmarks of intentional communication that apes have.”

The study adds to the evidence that fish can be smarter than we thought. We know that several species hunt in teams. Lionfish work together to corral prey with their expansive fins, and have a “Let’s hunt” signal for recruiting their peers. Some electric fish flush out their prey with formation attacks, which they coordinate through electric pulses. And the yellow saddle goatfish hunts in packs where individuals assume specific roles—some chase, others block, not unlike a team of lions, wolves or chimps.

Fish also seems to be particularly good at hunting with other species. The roving coral grouper will also partner with the humphead wrasse, the coral trout will form hunting parties with octopuses, and goatfish sometimes team up with banded sea kraits (a type of sea snake).

And recently, a European team found that three captive cod learned to manipulate a feeding device with a tag attached to their fins, after accidentally getting the tag entangled in the device. It was “one of the very few observed examples of innovation and tool use in fish.”

In fact, Vail suggests that you could view the cooperative hunts of coral trout and morays as a kind of social tool use. “A chimp can get a stick and probe honey out of a hole,” he says. “A grouper has no hands and can’t pick up a stick. But it can use intentional communication to manipulate the behaviour of a different species with the attribute it needs.” In other words: When your prey’s in a hole, and you don’t have a pole, use a moray…

Reference: Vail, Manica & Bshary. 2014. Fish choose appropriately when and with whom to collaborate. Current Biology http://dx.doi.org/10.1016/j.cub.2014.07.033

* Common names are letting us down here. The roving coral grouper and the coral trout are both part of the genus Plectropomus. The coral trout is not closely related to the freshwater trout that you might eat.

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Intelligent Crows Flunk Causality Test (But Babies Pass)

You drop a block onto a box, and a toy pops out. If a baby was watching you, she could deduce that your action caused the happy arrival of the toy, because she understands cause and effect. She’d also realise that she could trigger the same event by placing a block on the box herself, because she can use her knowledge to actively shape her world.

These two abilities—understanding causality, and using that understanding—seem so simple and mundane to us that it feels weird to lay them out, and weirder still to separate them. But they are separate. That much becomes clear when you study an animal that can do one of these things and not the other.

The New Caledonian crow is one such animal. These birds are renowned for their intelligence, problem-solving acumen, and skill with tools. In their attempts to extract grubs from dead wood, they can choose the right tool for different jobs, combine tools together, and improvise from unusual materials. Their skills are impressive enough that many biologists have described them as “feathered primates”.

If these crows are so good at using tools, it stands to reason that they’d understand cause and effect very well. Auguste von Bayern from the University of Oxford showed as much in 2009: she found that crows that got food after pushing a platform with their beaks would then drop stones on the platform if it was placed out of reach. But these birds had experience–they had all previously pushed the platform themselves and been rewarded for their trouble. What happens without that experience?

That’s what Alex Taylor from the Universities of Cambridge and Auckland wanted to find out. He tested some crows with a similar task. The birds saw a Perspex box with several holes in it and a rotating cylinder inside. The first time round, a plastic block sat on a ledge above the cylinder, with a piece of meat attached to it. When the crows pecked at the meat, the block would fall and land on the cylinder, which would rotate and drop a second lump of meat onto the ground next to the crow.

The next time round, the plastic block was sitting on the ground outside the Perspex box. If the crows understood what they had previously seen, they would pick up the block, and drop it into one of the holes overlooking the cylinder. The block would land, the cylinder would roll, and a tasty hunk of meat would drop within reach.

Taylor tested five crows. All of them failed.

Over 100 trials, none of them dropped the box onto the ledge. “We thought they’d be good at this,” he says. “It’s interesting that they really, really struggle.”

It’s also interesting that human babies don’t struggle. Taylor’s team, including child psychologist Alison Gopnik, tested 22 two-year-olds with basically the same task, except with a marble instead of meat. Their initial attempts to reach a marble caused a block to fall off a ledge, rotate a cylinder, and dispense a second marble. The next time round, 16 of them dropped the block directly onto the ledge, within a few trials. They managed it, when the crows uniformly didn’t.

“We’ve got to test more crows and try different types of apparatus and behaviour,” says Taylor. “It’s hard to interpret a failure but given that we have the children passing with flying colours at age 2, for this particular paradigm, it’s pretty clear that the crows really can’t do it.”

The crows weren’t lacking in motivation; they were always quick to approach the plastic block and continued to do so over the course of the experiment. There’s nothing about the task itself that stops them, either. Taylor’s team found that they could train three other crows to drop the plastic block into the right hole, by walking them through the process and rewarding them at every step. They just won’t do it spontaneously; only the babies did that. We see, then do. They need to do before they can do.

This discovery highlights one of the important parts of Taylor’s study: he only worked with wild crows. His team captured the birds in New Caledonia, housed them in an aviary for a few months while they took part in experiments, and then released them. This means that, unlike many similar studies on animal intelligence, these birds had no experience with experiments and no training in the task they were tested on. “You’ve got these minds that evolved to function in the wild, so it’s important to look at the wild cognition if you can do so,” says Taylor.

The crows’ failure means that the ability to “create causal interventions”—that is, to do things that result in a desired effect—can be separated from the ability to understand causality in the first place. We have both; crows (at least as per this study) only have the latter.  “We have the complete package, so it’s really hard for us to know what’s particularly special and what isn’t,” says Taylor. “Studies like this provide a more nuanced view of what’s going on.”

Indeed, Taylor speculates that our ability to learn about causality through observation alone could have been one of the driving forces behind our success as a species. “It seems so obvious to a human but that’s almost the point,” he says. He’s now talking to colleagues who work with primates to see if our closest relatives can pass the same test.

Reference: Taylor, Cheke, Waismeyer, Meltzoff, Miller, Gopnik, Clayton & Gray. 2014. Of babies and birds: complex tool behaviours are not sufficient for the evolution of the ability to create a novel causal intervention. Proc Roy Soc B. http://dx.doi.org/10.1098/rspb.2014.0837

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This is How You Study The Evolution of Animal Intelligence

There are many scientists who study the mental abilities of animals. As intelligent animals ourselves, we’re keen to learn whether other species share our skills, and how our vaunted smarts evolved. We see study after study about whether chimpanzees care about fairness, whether pigeons outsmart humans at a classic maths problem, whether cuttlefish can remember where, what and when, or whether (and how) parrots and crows use tools,

But animals are hard to work with. You need to design tests that objectively assess their mental skills without raising the spectre of anthropomorphism, and you need to carefully train them to perform those tests. These difficulties mean that researchers mostly resort to small experiments with just one species, often with their own bespoke tasks. This makes it very hard to compare species or pool the results of separate studies. If a lemur behaves differently to a monkey in separate experiments, is it because of some genuine biological difference, or some quirk of the respective studies?

These problems mean that the study of animal intelligence is rich but piecemeal. Each study adds a new piece to the jigsaw, but is everyone even solving the same puzzle?

Evan MacLean, Brian Hare, and Charles Nunn from Duke University have had enough. They led a international team of 58 scientists from 25 institutes in studying the evolution of one mental skill—self-control—in 567 animals from 36 species.

Chimpanzees, gorillas, baboons, marmosets, lemurs, squirrels, dogs, elephants, pigeons, parrots and more tried their hands (or trunks or beaks or snouts) at the same two tasks. “It was literally a mouse-to-elephant study,” says MacLean, “or at least a Mongolian-gerbil-to-elephant study.”

“I think it’s really showing the future of the field of cognition,” says Karin Isler from the Universtiy of Zurich. “Instead of just giving glimpses and suggestions, and sometimes contradicting evidence, one can find convincing explanations for the evolution of cognitive abilities.”

The team focused on self-control—the ability to stop doing that, put that down, eat that later. Animals exercise it when they stop themselves from mating in the presence of a dominant peer, when they forgo an existing source of food in favour of foraging somewhere new, or when they share resources with their fellows. In humans, a child’s degree of self-control correlates with their health, wealth, and mental state as adults. It’s important.

It’s also easy to measure. Swiss psychologist Jean Piaget did it in the 1950s when he repeatedly put a toy under a box in front of some infants, and then moved it to a second box. He found that babies under 10 months of age would keep on searching under Box A, despite what they had seen. They couldn’t resist their old habit to do something flexible and different; that ability only kicks in around our first birthday. MacLean, Hare and Nunn’s team gave this “A-not-B” test to their animals, using food rather than a toy.

They also tried a second task, where animals had to reach round the side of an opaque cylinder to get at food within. The team then swapped the opaque cylinder for a transparent one. Now, the animals had to hold back their natural instinct to reach directly for the food (which would have knocked the cylinder over), and go around as before.

The team tested all their animals on one or both tasks, and compared their performance to traits like brain size or group size. They found a few surprises. For example, the animals’ scores correlated with the absolute but not relative sizes of their brains. In other words, it didn’t matter whether the animals’ brains were big for their size, but whether they were big, full-stop.

“That’s funny because brain size and body size scale predictably. Big animals have big brains,” says MacLean. As such, many scientists believed that relative brain size mattered more. There’s even a measure called the encephalization quotient (EQ) that estimates intelligence by comparing an animal’s brain to that of a typical creature of the same size. And yet, for self-control at least, it’s absolute size that’s important. That was true whether they looked at all their 36 species, or just at the primates.

“That makes sense,” says Richard Byrne at the University of St Andrews. “If the brain is, to some extent, an on-board computer, it will be the number of components that determine its power [rather than] the size of the carrying case or body.”

The team also tested two leading explanations for the evolution of primate intelligence. One idea says that our smarts evolved so we could keep track of the relationships within our complex social groups. Indeed, you can make a decent guess about the size of community that a primate lives in by measuring the size of its skull. But the team found no link between group size and performance in their tasks. “That surprised us,” says MacLean. “It’s such a popular hypothesis but we found no evidence for it.”

Instead, the team found more support for a second idea: that primate intelligence was driven by the need to keep track of a wide range of food like fruit, which vary by place and season. They showed that the variety in the animals’ diets (but not the proportion of fruit) was indeed linked to self-control. Together, these two factors—absolute brain size and dietary breadth—explained around 82 percent of the variations in the primates’ scores.

“The nice thing about the tasks is that, because of their simplicity, they are very unlikely to depend a lot on species-specific aptitudes unrelated to cognition or to prior experience,” says Byrne. “I’d trust the results.”

But Robin Dunbar from the University of Oxford felt that the team’s conclusions are “misguided and naive” because their tasks weren’t a good measure of self-control, at least in any sense that matters in an animal’s social life. Instead they were “straight ecological or foraging tasks and nothing more, so it’s not awfully surprising that it correlates with diet,” he says.

Brain-scanning studies in humans and monkeys have also found links between the size of specific brain regions, size of social groups, and social skills. “It seems bizarre to be running an analysis against measures of total brain size,” says Dunbar.

Of course, this study just looked at one aspect of animal psychology, among many. The team found that the animals’ scores on the self-control tests did correlate with reports of other skills, like innovation, tool use, deception, and social learning. But MacLean suspects that if other teams focused on these skills, they would find different results. Group size may become more important if researchers focused on tasks that looked at social learning—the ability to imitate and learn from others. Alternatively, diet may again win out if scientists looked at memory skills.

This new study doesn’t settle the debates. It just points to a way forward. Each of the scientists in the team could easily have published their own papers using the collected data, but they decided to combine their efforts into one publication. “We thought it would be most powerful if it came out together,” says MacLean. “There’s never been a data set this size. We’re certainly hoping that it’s a game-changer in the way we do comparative psychology.”

And even Dunbar says, “It’s good to see comparative studies of this kind being done at last, and it’s very worthy that they have done the same task on many species.”

Reference: MacLean, Hare, Nunn, Addessi, Amici, Anderson, Aureli, Baker, Bania, Barnard, Boogert, Brannon, Bray, Bray, Brent, Burkart, Call, Cantlon, Cheke, Clayton, Delgado, DiVincenti, Fujita, Herrmann, Hiramatsu, Jacobs, Jordan, Laude, Leimgruber, Messer, Moura, Ostojic, Picard, Platt, Plotnik, Range, Reader, Reddy, Sandel, Santos, Schumann, Seed, Sewall, Shaw, Slocombe, Su, Takimoto, Tan, Tao, van Schaik, Viranyi, Visalberghi, Wade, Watanabe, Widness, Young, Zentall & Zhao. 2014. The evolution of self-control. PNAS http://dx.doi.org/10.1073/pnas.1323533111

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Elephants Hear Age, Gender, Ethnicity in Human Voices

To most people, elephants sound the same. Unless, you’re very experienced, it would be hard to tell the difference between two elephants based solely on their voices. They, however, have no such problems with us.

Karen McComb and Graeme Shannon from the University of Sussex have now shown that wild African elephants can tell the difference between the voices of humans from two ethnic groups, and react accordingly. They can even discriminate between the sounds of men and women, and adults and boys.

This ability matters because, to an elephant, not all humans are equal. They have no quarrel with the agriculturalist Kamba. But they often come into conflict with the cattle-herding Maasai over access to water or land, and they sometimes leave these clashes with a flank full of spears.

Back in 2009, Lucy Bates and Richard Byrne from the University of St Andrews showed that elephants at Kenya’s Amboseli National Park can distinguish between the smell of Maasai and Kamba clothes. If they sniffed eau de Maasai, they were more likely to flee into long grass. They behaved in the same way if they saw the distinctive red colour of Maasai clothes. McComb and Shannon’s study is a sequel of sorts. They showed that elephants can rely on sounds as well as smells to assess the threats they face.

The team recorded 20 Maasai and 15 Kamba saying “Look, look over there, a group of elephants is coming” in their respective languages. They then played these recordings to 48 family groups of Amboseli elephants. The herds obviously couldn’t understand the meaning of the words, but they could tell the difference between the two languages. When they heard the Massai voices, they were much more likely to bunch up into defensive clusters and sniff the air with their trunks. They knew which group was more dangerous.

They also seemed to know which people within the groups pose the greatest threat: they behaved defensively when they heard Maasai men rather than women, and adults rather than boys. “I don’t find this at all surprising, since voice pitch alone enables that distinction,” says Byrne. “But the details that differ between Maasai and Kamba languages are presumably more subtle.”

But when McComb and Shannon altered the Maasai recordings so that the male and female voices had the same pitch, the elephants could still tell them apart. They must have been picking up on some features that are subtler than mere frequency.

Still, that’s not surprising. Elephants are big-brained and extremely intelligent. They communicate with a wide range of sounds. They are long-lived, so they can build up a substantial lifetime of experience, and they live in tightly knit social groups, so youngsters can benefit from the knowledge of their elders. “The surprising thing was just how clued up they were,” says McComb. “They were really able to make these distinctions very well and they rarely got it wrong.”

They can also tailor their responses to different predators. Their main threats are humans and lions. In an earlier study, McComb and Shannon found that elephants can tell the difference between the roars of male and female lions. They react to these roars by forming defensive circles and then noisily mobbing the source of the sound. (Watch them react below.)

But when they heard the Maasai voices, they were much less aggressive. “Coming towards humans with spears would be very detrimental,” McComb deadpans. “They behaved as if they were expecting to see Maasai.” They also went into stealth mode; they only made audible noises 10 percent of the time after hearing Maasai speech, compared to 67 percent after hearing lion roars.

In a related study, Joseph Soltis, who works at Disney’s Animal Kingdom, found that elephants react differently to two distinct threats: talking humans and buzzing bees. Bees can sting elephants in vulnerable places like their eyes or inside their trunks, and elephants are so scared of this that they’ll flee if they hear buzzing.

Soltis’ team showed that Kenyan herds make distinct alarm calls when they hear either humans or bees, and they can modify the tempo and pitch of the calls to show how urgent the threats are. They also react accordingly. When the researchers played the calls back to the elephants, they found that both alarms would prompt the herds to keep watch and run away. But the bee alarm specifically makes them shake their heads, presumably to knock away any nearby stings.

These studies are testament to the keen intelligence, rich social lives, and sophisticated communications of these largest of land animals. As Ferris Jabr beautifully writes, “To look an elephant in the face is to gaze upon genius.”

But the results also speak to the sad history of conflicts between humans and elephants. These conflicts must have played out many times over for the animals to build up enough experience about which humans are the most dangerous.

“The level of spearing has gone down quite considerably in recent years,” says McComb. The Maasai are now partners in Amboseli National Park. They also get compensated if elephants accidentally kill their livestock, which stops them from spearing the animals in retaliation. Still, the sound of Maasai still sets them on edge. This suggests that once at least some members of the family have a bad run-in with humans, the others learn from her and the fear stays in the group.

Still, McComb adds that “elephants are very good at living alongside humans by avoiding dangerous situations. But when we start doing something dramatically different, like a huge increase in poaching or using automatic weapons, they can’t adapt fast enough. That’s when we need to step in and protect them.”

Fritz Vollrath from Oxford University, who was involved in the bee study, adds, “Knowing how elephants perceive their social and physical environments and how the communicate their perceptions between one another will allow us to not only better understand them but also to better protect them in the wild.”

References: McComb, Shannon, Sayialel & Moss. 2014. Elephants can determine ethnicity, gender, and age from acoustic cues in human voices. PNAS http://dx.doi.org/10.1073/pnas.1321543111

Soltis, King, Douglas-Hamilton, Vollrath & Savage. 2014. African Elephant Alarm Calls Distinguish between Threats from Humans and Bees. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0089403

More on elephant behaviour:

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Cuttlefish Remember What, Where and When They Ate

You and I both have the ability to travel back in time… at least in our minds. For example, I can remember that last Monday, I was at my desk, writing a post about stomachless animals. You too have a seemingly endless catalogue of the whats, wheres and whens of your life.

This ability to remember the what, where and when of our past experiences is known as “episodic memory”. The term was first coined in the 1970s by Canadian psychologist Endel Tulving, who thought that such memories depended on language and were unique to humans.

He was wrong. In 1998, Nicky Clayton from the University of Cambridge published the first of many seminal experiments with western scrub-jays, showing that they can remember where they had stored food and which hoards were freshest. In other words, these bird brains also have episodic-like memories. We say “episodic-like” since we can’t really know if the animals store their what-where-when information into single coherent memories in the way that we do. Still, it’s clear that the components are there.

Since then, the episodic-like memory club has grown to include the great apes, rats, hummingbirds, and pigeons. But these are all mammals and birds. Christelle Jozet-Alves from Normandie University wanted to know if the same skills existed in animals that are very different to these usual suspects. She turned to the common cuttlefish (Sepia officinalis).

Like octopuses and squid, cuttlefish are cephalopods—a group of animals known for their amazing colour-changing skin and sophisticated intelligence. Cuttlefish are separated from birds and mammals by almost a billion years of evolution. But Jozet-Alves, together with Clayton and Marion Bertin, has shown that they too can “keep track of what they have eaten, and where and how long ago they ate”.

They are also soft-bodied and nutritious, which puts them on the menu of virtually every major group of ocean predator. Cuttlefish deal with these manifold threats through camouflage, defensive ink, and just plain-old hiding. They spend more than 95 percent of their time hiding in safe places. When they do venture out to search for food, it pays them to be quick about it. “Cuttlefish live fast and die young. They live less than two years, but their size drastically increases between hatching and old age,” says Jozet-Alves. “They definitely need to be very efficient when foraging if they want to grow as fast as possible.”

First, the team trained three cuttlefish to approach a black-and-white symbol to get a morsel of food—either crab, which they were fine with, or shrimp, which they vastly preferred. The cuttlefish also learned that the shrimp supply took a while to refill. If they approached the symbols within 3 hours of their last meal, they got nothing.

Next, Jozet-Alves presented them with two of the same symbols at different positions in their tank. The cuttlefish randomly approached one of the symbols, and Jozet-Alves dropped shrimp in front of one, and crab in front of the other.

She tested them an hour later. At this time, it would have been pointless to approach the shrimp symbol, since it wouldn’t have replenished. And the cuttlefish knew that—they almost always approached the crab symbol the second time around. But if Jozet-Alves tested them three hours later, they almost always approached the shrimp symbol instead. They knew that their favourite morsel would have replenished and that it was worth trying for it.

Schematic of cuttlefish experiment. Credit: Jozet-Alves et al, 2013. Current Biology.
Schematic of cuttlefish experiment. Credit: Jozet-Alves et al, 2013. Current Biology.

Their behaviour shows that they remember what (shrimp or crab), where (which symbol was associated with which food) and when (the time since they last ate). Admittedly, the team only tested three individuals but all of them behaved in the same consistent way. Like scrub-jays, chimps and hummingbirds, they have episodic-like memory.

But cuttlefish, being invertebrates, are very distantly related to these other members of the club. The only other invertebrates with a hint of the same abilities are honeybees, and they were only trained to go to the same place at the same time every day. That’s not quite the same as encoding information about specific events. Still, it’s clear that contrary to Tulving’s claims, the ability to encode what, where and when isn’t a uniquely human trait. It’s probably not even a uniquely vertebrate one.

Jennifer Mather from the University of Lethbridge, who studies cephalopod smarts, isn’t surprised. Years ago, she noticed that octopuses are “win-switch foragers”. That is, if they find food somewhere, they don’t visit it for a few days. “This paper isn’t the first behavioral evidence of episodic-like memory in an invertebrate, but it’s certainly the first experimental evidence of this capacity,” she says. “It underlines the tremendous flexibility and cognitive capacity of these very interesting animals.”

Jozet-Alves agrees. “Everyone who had the chance to work with cuttlefish have seen how amazing and fascinating they are,” she says. “At the same time, working with them is a real challenge to your patience. They are so shy that just making a cuttlefish eat in front of you can take sometimes days or even weeks. But once a cuttlefish gives you its trust, you just enjoy working with them and totally forget the hours waiting that they move out of their shelter!”

Reference: Jozet-Alves, Bertin & Clayton. 2013. Evidence of episodic-like memory in cuttlefish. Current Biology http://dx.doi.org/10.1016/j.cub.2013.10.021

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Madness of Crowds: Single Ants Beat Colonies At Easy Choices

Virtually every article or documentary about ants takes a moment to fawn over their incredible collective achievements. Together, ant colonies can raise gardens and livestock, build living rafts, run vaccination programmes, overpower huge prey, deter elephants, and invade continents. No individual could do any of this; it takes a colony to pull off such feats.

But ants can also screw up. Like all animal collectives, they face situations when the crowd’s wisdom turns into foolishness.

Takao Sasaki and Stephen Pratt from Arizona State University found one such example among house-hunting Temnothorax ants. When they need to find a new nest, workers spread out from their colony to search for good real estate. In earlier work, Sasaki and Pratt have shown that, as a group, the ants are better at picking the best of two closely matched locations, even if most of the workers have only seen one of the options. It’s a classic example of swarm intelligence, where a colony collectively computes the best solution to a task.

But Sasaki showed that this only happens if their choice is difficult. If one nest site is clearly better than the other, individual ants actually outperform colonies.

When a worker finds a new potential home, it judges the site’s quality for itself. Temnothorax ants love dark nests, in particular; with fewer holes, it’s easier to control their temperature or defend them. If the worker decides that it likes the spot, it returns to the colony and leads a single follower to the new location. If the follower agrees, it does the same. Through these “tandem-runs”, sites build up support, and better ones do so more quickly than poorer ones. When enough ants have been convinced of the worth of a site, their migration gathers pace. Workers just start picking up their nestmates and carrying them to the new site.

In past experiments, the team have always found that ant colonies make better decisions than individual workers. Even though each worker might only visit one or two possible sites, the colony collectively explores all the options and weighs them against one another. And since many individuals need to “vote” for a particular site, “this prevents any one ant’s poor choice from misleading the entire colony,” says Sasaki.

This time, the team wanted to see if the colony keeps its superiority for easy tasks as well as difficult ones. They presented Temnothorax ants with two possible nests—one held in constant darkness and another whose brightness could be adjusted. Sometimes, the ants had an easy choice between a dark nest and a blindingly illuminated one. Sometimes, they had to choose between two similar sites, one just marginally dimmer than the other.

As the light difference between the nests got bigger and the task became easier, the ants, whether as individuals or colonies, made more accurate choices. The team expected as much. But to their surprise, the single workers showed the greatest improvements and eventually outperformed their collective peers. In the easiest tasks, they chose the darker nest 90 percent of the time, while the colonies peaked at 80 percent accuracy.

To understand why this happens, consider how the ants choose their nests. If an individual is working by herself, she might visit a few sites in a row and gauge the difference between them. If they’re very similar, there’s a good chance she’ll make the wrong decision. But the colony doesn’t work off the recommendations of any individual; it relies on a quorum, just like the up- and down-voting system of social websites like Reddit. Together, the colony can amplify small differences between closely-matched sites and smooth out bad choices from errant individuals.

Still, this system isn’t perfect. If many ants happen to find a bad site very quickly, they might reach a quorum before other workers have time to rouse support for a better alternative. “A bad choice can happen even if one site is much better than the other, because the ants at the bad site will have no information at all about the existence of the much better alternative,” says Sasaki.

A single ant isn’t as vulnerable to this problem. “She will visit both sites, easily see that one is better than the other, and nearly always make the right choice,” says Sasaki. Colonies, however, put less effort into comparing their options than lone individuals, which sometimes leads them astray.

Does that sound familiar? Perhaps the same vulnerability can explain why the collective intelligence of humans often flips into the so-called “madness of crowds”. Sasaki certainly thinks so. “For example, I just went to an online site to buy a fan,” he says. “Instead of comparing options carefully, I blindly bought the most famous one. This ant-like consuming behaviour may lead to a similar pattern—the crowd fails when quality of options is easy to distinguish.”

Reference: Sasaki, Granovskiy, Mann, Sumpter & Pratt. 2013. Ant colonies outperform individuals when a sensory discrimination task is difficult but not when it is easy. PNAS http://dx.doi.org/10.1073/pnas.1304917110

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On Copycat Whales, Conformist Monkeys and Animal Cultures

This is the story of a whale that tried something new and a monkey that fell in line.

It’s about how wild animals can create cultures and traditions just as we can, through the twin forces of innovation and conformity.


Lunge-feeding humpbak whale, by Jennifer Allen and the Whale Center of New England
Lunge-feeding humpbak whale, by Jennifer Allen and the Whale Center of New England

In 1980, a humpback whale in the Gulf of Maine started doing something different. All its neighbours would catch small fish by swimming in circles below them, blowing curtains of bubbles, and then lunging straight up at the corralled shoal. Then one individual, out of the blue, started smacking the water surface with its tail before diving down and blowing its bubbles.

This behaviour is called lobtail feeding, and no one knows why it works. Maybe it disturbs the water above the bubble curtains and discourages fish from jumping to safety. Whatever the benefit, it went viral. Just eight years after the first innovative whale started doing it, 20 percent of the Maine humpbacks had picked up the technique. Now, it’s more like 40 percent. What began as one whale slapping the water is now a tradition.

The obvious explanation is that the whales were learning from each other. But there could be other reasons. If the technique has a strong genetic basis, it could pass down family lines without any form of social learning. Or maybe environmental changes were responsible. The whales seem to use lobtail feeding specifically to catch small fish called sand lance, and the strategy only started spreading after populations of herring, another important prey species, crashed. Perhaps hunger drove the whales to individually develop a new technique for catching a different sort of prey.

Jenny Allen worked out how to tell these possibilities apart. As a masters’ student, she had worked on whale-watching boats in Maine, and knew that the Whale Center of New England (WCNE) had collected a huge data set of the local animals’ behaviour. Over 27 years, they had recorded almost 74,000 sightings. Allen was looking for ways of using this data when she joined Luke Rendell’s lab at the University of St Andrews as a PhD student. “I realised this was the lobtail-feeding population and asked whether it was still going on,” says Rendell. “She said, ‘Yes, it seemed to still be spreading’. I knew we were in business.”

The team used the Maine data to reconstructed the whales’ social network and simulate the spread of lobtail feeding under different mathematical models—some that included social learning and others that didn’t. The results were so clear that even Rendell was surprised. “It was very, very clear that cultural transmission was important in the spread of the behaviour,” he says.

The models which assumed that the whales were learning lobtail feeding from each other were a far better fit for the actual spread of the behaviour than those which assumed no social connection. “The weight of evidence was up to 23 orders of magnitude greater for these models,” says Rendell. “It’s the difference between the weight of a single person and the weight of planet Earth.”

By contrast, genetics was unimportant. “Having a lobtail-feeding mother makes virtually no difference to whether you will become one,” says Rendell. Ecology mattered more. Whales were more likely to learn the lobtail method in the specific region where the sand lances live, and during years when sand lance numbers were high.

This doesn’t detract from the importance of social learning, which was by far the more important factor in the strategy’s spread. Instead, it shows how useful it can be to pick up skills from your neighbours. “If a species is smart enough to innovate and transfer information socially, it could adapt very quickly to new environmental  pressures. This is why humans are so successful,” says Michael Kruetzen from the University of Zurich. “I find this to be a highly convincing case for a foraging tradition in a cetacean,” adds Susan Perry, an anthropologist from the University of California, Los Angeles.

Critics might point out that Allen’s study relied only on observations rather than experiments, and incomplete observations that were limited by what boat crew could see. But the team took steps to account for this, adjusting their models to account for patchy sightings, or the fact that the most commonly spotted whales would repeatedly pull off the same behaviours. None of that changed the results.

And Rendell scoffs at the notion that you can never know anything for sure from observational data alone. “It would be great to look at this experimentally, but we’re talking about a population of wild humpback whales here,” he says. “Spock and Kirk were able to beam one up in The Voyage Home, but we aren’t going to be doing that any time soon. This is really the best approach we have, and the answer it gives is unequivocal.”


Vervet monkeys choosing pink corn over blue. Credit: Erica van der Waal
Vervet monkeys choosing pink corn over blue. Credit: Erica van der Waal

Meanwhile, thousands of miles away in South Africa’s Mawana Game Reserve, there lived a vervet monkey called Groot, who was a fan of blue corn. One day, two boxes of dyed corn kernels had mysteriously appeared. The pink ones tasted disgusting but the blue ones were tasty, and Groot’s entire group quickly learned to eat the blue ones. Then, as all male vervets do when they grow up, Groot left his family behind and moved to a new group. And when he did, he saw that his new companions liked pink corn instead.  He watched, he processed, and he starting eating the pink corn too.

Groot didn’t know it, but he was part of an ambitious experiment by Erica van de Waal and Andrew Whiten from the University of St Andrews to study the spread of animal traditions. Recently, Whiten’s team has studied whether captive chimps and capuchin monkeys can learn from each other. The answer is yes. Tutors, who are taught new foraging techniques in isolation, can seed their groups with these new innovations when they are reunited.

This approach is impractical in the wild, because it’s very hard to isolate a tutor individual. Instead, scientists have studied differences in behaviour between groups of wild chimps, orang-utans and other species. These studies have been pivotal for our understanding of animal culture, but they’ve run against the same refrain that Rendell dislikes: they’re just observational, not experimental.

So, van de Waal tried something new—she seeded new traditions in entire groups rather than individuals. She gave four groups of wild vervets, which included 109 individuals between them, a choice between blue corn and pink corn. In each case, the group would only ever eat one colour because the other was coated with a repulsive extract from local aloe plants. (They tried vinegar and chilli powder, but the vervets happily ate those. Only aloe worked. “The experimenters tested the corn themselves and had the bitter taste for a whole day in their mouths,” said Whiten.)

Van de Waal took the corn away for 4 to 6 months and during that time, new babies were born into the vervet communities. The corn eventually returned and this time, both colours were tasty and palatable. Even so, it seems you can’t teach an old vervet new tricks, and the monkeys stuck with their existing colour preference.

More importantly, their infants, who had never seen dyed corn before, just ate whatever they saw their mothers eating. Those born into pink cultures ate pink corn. Those born into blue cultures ate blue corn.

It’s not surprising that infants follow their mothers, but the strength of their preferences caught the team off-guard. “Infants chose only what their mother ate despite there being right in front of them a box of perfectly edible corn of a different colour,” says Whiten. “Some even sat on that, to eat the ‘right’ colour of corn!”

Emigrating males also took up the traditions of their new groups. By sheer luck, during the experiment, ten males moved into a group that preferred a different coloured corn than their original group did. Seven of these newcomers seven immediately started eating whatever colour their new comrades preferred, and two more soon followed suit. The only exception was a male called Lekker who immediately took up a dominant rank in his new group, which may explain why he stuck to his old ways.

Perry praises the elegant experiments but notes that the numbers are quite small. “Seven out of ten is only 2 data points greater than chance preference for a particular colour,” she says. “I appreciate the difficulty in obtaining a larger sample—you have to wait for males to migrate—but I hope the authors will persevere in increasing that sample size.”

This degree of conformity is surprising especially for vervets, which “are often thought to be opportunistic”, according to Whiten. This “when-in-Rome” mentality makes sense. In the wild, foraging animals have to make decisions about the nutritional quality of potential foods and the presence of poisons. When moving into a new environment, it pays to copy what local experts are doing, even when it means overriding the knowledge you’ve gained in a different context.

The tendency to conform could also explain other social learning experiments have failed. Scientists have tried to teach new behaviours to wild tutor individuals, including vervets and meerkats, but found that these nascent traditions are difficult to spread. That may be because these traditions face an uphill struggle, says Whiten, “whereas, in our study, the naïve infants and immigrant males were already surrounded by a majority doing the same thing.”

The team now wants to see if the wild vervets will also learn more complicated behaviours from each other, such as techniques for dealing with their food. Based on work with captive monkeys, they think the answer is yes. It’s now time to take these experiments into the field.


Thanks to decades of research, it is now clear that animals can learn from each other in ways that create different cultures in the wild.

As Frans de Waal writes in a commentary accompanying these new studies, “The early debate about animal culture focused on the mechanism of behavioural transmission.” Are apes apeing each other in the way that humans can? When whales and dolphins imitate each others’ songs and actions, do they understand each others’ goals and methods? When blue tits peck open the tops of milk bottles, is it because they’ve picked up the technique from other tits, or because those birds just drew their attention to the bottles?

Now, studies of animal culture are moving beyond just asking whether it happens to probing why it happens and how strongly it does. The humpbacks show that new traditions can easily spread within a group, but the vervets show that the conformity can also suppress new behaviours in favour of old rituals. We see the same tension between innovation and conformity in our own societies, and it’s fascinating to see the same patterns in animal groups.

All of this requires intensive field work and long-term studies. To watch the vervets from the comfort of nearby chairs, de Waal and Whiten had to spend over a year with the monkeys, getting them used to their presence and learning how to recognise over 100 individuals by eye. To understand what the whales were doing, Allen and Rendell had to use a 27-year set of data. “That shows how important it is to have long-term research so you can create these data sets,” says Kruetzen “If people had just gone there for a year or two, it would have been very hard to document these changes.”

Reference: Allen, Weinrich, Hoppitt & Rendell. 2013. Network-Based Diffusion Analysis Reveals Cultural Transmission of Lobtail Feeding in Humpback Whales. Science http://dx.doi.org/10.1126/science.1231976

Van de Waal, Borgeaud & Whiten. 2013. Potent Social Learning and Conformity Shape a Wild Primates Foraging Decisions. Science http://dx.doi.org/10.1126/science.1232769

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Do chimpanzees care about fairness? The jury’s out

A stranger gets a pot of money and offers you a share of it. If you accept the offer, both of you walk away with your proposed shares. If you reject, you both leave with nothing.

This is the ultimatum game—a classic psychological experiment used to study fair play. If both players behave completely selfishly, the proposer might offer as little as possible, while the responder should accept any offer as long as it’s not zero. That way, both of them walk away with something. In practice, responders typically reject any offers less than 20 percent. They care enough about fairness to do themselves out of money in order to punish unfair partners. Meanwhile, proposers from industrialised countries, who are wary of social norms and the potential for punishment, tend to offer between 40 and 50 percent of the pot.

But what about chimpanzees? Do our closest relatives also share our attitudes to equality? Darby Proctor from Georgia State University thinks so. She modified the ultimatum game so that chimps and human children could play it in the same way. In both cases, when proposers needed to cooperate with responders, they became more likely to offer an equal split, rather than trying to hog the rewards for themselves. The conclusion: “humans and chimpanzees show similar preferences regarding reward division, suggesting a long evolutionary history to the human sense of fairness.”

But her study has prompted stern criticism from scientists who have also tested chimps at the ultimatum game and found the opposite. In their studies, our fellow apes did not make fair offers and accepted anything as long as it was greater than zero.

This debate reflects a growing divide between scientists who study chimpanzees, but it is more than an academic spat. It speaks to a fundamental question about our evolution: is our sense of fairness a uniquely human trait, or one that we share with our closest kin?

Old male chimp, by Prabir Kumar Bhattacharyya

The experiments

Keith Jensen, Josep Call and Michael Tomasello were the first to play the ultimatum game with chimps. In their experiments, two chimps sat in adjacent cages, facing a contraption with two sliding trays. Each tray contained one dish on the proposer’s side and another on the responder’s side, and the two dishes carried varying numbers of raisins. The proposer made an offer by using a rope to pull one of the two trays half-way over. The responder could accept this offer by pulling the tray the rest of the way, allowing them both to eat their respective raisins. Alternatively, they could reject the offer by doing nothing, leaving both animals unfed.

The team found that the proposers were likely to choose the tray that gave them the most raisins, and the responders tended to accept any offer, no matter how unbalanced. They wrote that “in this context, one of humans’ closest living relatives… does not share the human sensitivity to fairness.” And in a second study, the team found that our other close relatives—the bonobos—behaved in a similar way.

But Proctor was unimpressed by the team’s set-up. She felt that the “complex mechanical apparatus” was unlike anything that humans use when we play ultimatum games, and may have been too complicated for the chimps to understand. And while humans usually play for money, which is exchanged for other rewards, the chimps were playing for food, which is immediately rewarding.

In her version of the ultimatum game, two chimps play for tokens that are exchanged for six bananas laid out in front of them. A human experimenter offers two tokens to the proposer chimp, one signifying an equal banana split and the other signifying an unequal 5:1 divide. The proposer picks one token and passes it to a responder in an adjoining cage. They can drop the token to reject the offer, or pass it back to the experimenter to accept.

Proctor played the game with three pairs of chimps, one of which swapped roles as proposer and responder. She found that two of the proposers chose the equal-split token more often than expected by chance. And all four of them chose that token more often in the ultimatum game than in a straight preference test, when their choice dictated their reward irrespective of what a responder did. Proctor concluded that when chimps need to cooperate to get a reward—that is, when the proposer depends on the responder—they change their behaviour to favour the fairer option. Young children, aged two to seven, behaved in the same way.

Margot, an orphaned chimpanzee from a sanctuary in Cameroon. By Daniel Bergin

So what does that mean?

The obvious criticism is that Proctor’s study only included six chimps, and only the two animals who played both roles offered the equal token more often than expected by chance. Proctor admits that the numbers were small, but says that these were the only chimps that passed her rigorous pre-tests and clearly understood the nature of the game.

But Jensen disputes Proctor’s claim. He is glad that another team tried to replicate his results, since “one can only conclude so much from one or two studies,” but says that Proctor’s experiment was no ultimatum game. The most important aspect of the ultimatum game is not what the proposer does, but how the responder reacts,” he says. The proposer’s offers are strategic rather than a sign of fairness—they’re a reaction to what the responder might do. It’s the responder’s ability to reject unequal offers that drives fairness in the game.

And among Proctor’s chimps, no responder ever refused an offer, even the many unfair ones. That’s even less rejection than in Jensen’s study. “Not rejecting unfair offers is puzzling if chimps are really playing the ultimatum game,” says Call. “I see that as a fatal flaw,” adds Jensen. At best, it confirms his original experiment by showing that the responders are insensitive to unfairness and only motivated by getting bananas. At worst, it shows that they didn’t understand the task.

Proctor counters that she did extensive tests to be as sure as possible that the chimps understood what the tokens meant. She admits that she did not explicitly train the chimps that they could refuse offers, but says, “This actually makes our results more striking. Without experiencing a refusal, proposers changed their behaviour to be more equitable. They may be responding to the potential for refusals as do adult humans.”

Jensen doesn’t buy it. “There isn’t the tiniest shred of evidence that proposers understood that responder could reject their offers, and no demonstration that responders understood anything of the possible consequences of their choices,” he says.

David Rand, a psychologist from Harvard University who has used the ultimatum game in human studies, agrees with Jensen’s criticisms. While Proctor’s set-up does look like an ultimatum game, “it looks like maybe the chimps didn’t understand the game structure,” he says.

Jensen thinks that this confusion arose because Proctor’s task is not as simple as she claims. Unlike human ultimatum games, where players interact with each other, Proctor’s chimps spent as much time exchanging tokens with humans. “Passing a token is just an intermediate step to getting food from experimenters, something they are highly trained to do,” says Jensen. He doubts that this set-up, which involved tokens exchanging hands three times, is truly simpler than his tray-pulling machine.

The verdict

As Proctor notes, there are many reasons to suspect that chimps care about equality. They help one another, share food, and cooperate extensively to hunt, fight, patrol, defend, and more. But it’s difficult to interpret wild anecdotal behaviour, which is why experiments are valuable.

None of the existing studies is perfect. In all the chimp ultimatum games, the animals could only reject offers passively, by not pulling a tray or not handing over a token; in human games, rejection is an active choice. In the chimp games, the animals could see each other, and played multiple rounds with the same partners; in human games, partners usually play single rounds anonymously to stop social dynamics and reputations from clouding the results.

Given these shared weaknesses, Proctor’s team is right that Jensen’s studies don’t prove that chimps are insensitive to fairness even though they support that hypothesis. After all, absence of evidence is not evidence for absence. But equally, the problems in Proctor’s study prevent it from confirming that chimps are sensitive to fairness. Until more research is done, we’re at an impasse.

Reference:  Proctor, Williamson, de Waal & Brosnan. 2013. Chimpanzees play the ultimatum game. PNAS http://dx.doi.org/10.1073/pnas.1220806110

Note: This study was “contributed” to PNAS by co-author Frans de Waal, a publishing route where members of the National Academy of Sciences can nominate their own peer-reviewers. I try to avoid papers that use this track but did most of the reporting before I noticed, so here’s the piece anyway.

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Scientists breed smarter fish but reveal costs of big brains

In a Swedish lab, Alexander Kotrschal has deliberately moulded the intelligence of small fish called guppies. From a starting population, he picked individuals with either unusually large or small brains for their bodies, and bred them together. It’s what farmers and pet-owners have done for centuries, selectively breeding animals with specific traits, from shorter legs or more muscle.

Or bigger and smaller brains. After just two generations, Kotrschal had one lineage of guppies with brains that were 9 percent bigger than the other. And these individuals proved to be smarter—they outclassed their peers at a simple learning task, where they learned to discriminate between two and four symbols. This may seem like child’s play for us, but it’s a “relatively advanced cognitive task” for a fish, says Kotrschal.

Their boosted smarts came at a price—the big-brained fish developed smaller guts and produced fewer offspring. Brains are expensive energy-guzzling organs. Ours, for example, make up just 2 percent of our body weight but consume 20 percent of our energy. Many scientists think that to pay for our larger brains, we had to scale back other parts of our bodies like our guts or fat stores, and that’s exactly what Kotrschal found in his guppies.

This deceptively simple experiment is an important one. It provides direct evidence for two important ideas about brain evolution:  that bigger brains make for more intelligent animals; and that animals must pay some cost for bigger brains.

Most of the support for these ideas, and there’s rather a lot, comes from comparing different animals or populations. For example, more than 50 studies have shown links between brain size and social complexity, flexible behaviours, innovation, and more. This comparative approach is useful but has many problems. The fact that two traits are correlated does not mean that one caused the other, and there could always be other factors at work. For example, big-brained humans do tend to have higher IQs, but maybe that’s because wealthier people raise healthier children and provide them with better education.

That’s why experimental evidence, of the kind that Kotrschal has found, is so valuable. If we actively manipulate brain size to produce creatures with bigger brains, do they become more intelligent? Yes. Do other body parts get smaller? Yes. It’s “the strongest evidence for a direct effect of brain size on cognitive abilities within a species,” says Karin Isler, who studies brain evolution at the University of Zurich.

Human and chimp brain to scale, by Todd Preuss, Yerkes Primate Research Center

There’s a long history of studies into brain size evolution, and it’s not hard to see why. For our bodies, human brains are relatively huge compared to those of other animals, and it’s reasonable to think that this boost in size was important for our vaunted intellect. On the one hand, comparisons in primates and other animals support this link. On the other, social insects like ants and bees can show sophisticated behaviour, form complex societies and perform advanced mental tasks with brains no bigger than a pinhead.

But Kotrschal writes, “Our results now show that larger brains really can be better.” Brain size may be a crude and unsophisticated indicator of mental abilities, but the link is good enough that selecting for the former can boost the latter.

Kotrschal’s study also reminds us that evolution can’t just pull adaptations out of thin air. There’s always a cost. In the case of the guppies, that took the form of smaller guts and fewer young.

The guts result is important—it’s the first direct support for an 18-year-old hypothesis that during human evolution, we sacrificed guts for smarts. This “expensive-tissue hypothesis”, first proposed in 1995 by Leslie Aiello and Peter Wheeler, noted that our guts and brains are our most energetically expensive organs. As our ancestors started eating a richer diet of meat and tubers, and eventually started cooking food, they lifted some of the digestive load from their bowels. This allowed their guts to shrink, and freed up some surplus energy to fuel our expanding brains.

It’s still a controversial idea. Just last year, Isler led a study that seemed to disprove it, with an intense series of dissections that showed no connection between the size of a mammal’s brain and its other organs (although big-brained species did have smaller fat stores). Again: more comparisons and correlations. “Careful experimental work is what has been lacking,” says Aiello.

That’s what Kotrshcal has provided, even though he was originally sceptical of the idea. “There were multiple comparative studies suggesting such a trade-off but I always found it hard to believe,” he says. “In fact this is one of our strongest results now. [That was] the biggest surprise for me.”

Guppy by Inka Crabs

It’s perhaps even more important that Kotrschal’s big-brained guppies produced 19 percent fewer offspring than the small-brained ones. Big brains are such an obvious part of our lives and anatomy that it’s tempting to see them as naturally desirable adaptations. But studies like these show that there are huge costs to braininess. For example, you might sideline reproduction, which is also an energetically expensive activity

This fits with a pattern seen across other animals. Among mammals, the most intelligent groups—the primates and cetaceans (whales and dolphins)—also have unusually low fertility. And humans, in particular, have the largest brains of any primate and the lowest number of offspring. “Our results suggest that the reduction in offspring number may have been a major cost associated with the evolution of a larger brain among the primates and especially hominids,” says Kotrschal.

This tug-of-war between brains and fertility sets a “grey ceiling” for animals—a point where their brains become so big and they reproduce so slowly, that they flirt with extinction. For bigger brains to take hold, the advantages they offer, such as greater intelligence leading to higher odds of survival, have to outweigh the fact that their owners cannot raise as many young.

There’s one final twist to Kotrschal’s study: Only the female big-brained guppies became smarter. This may just be because females are more active and take more risks while foraging than males, so are naturally suited to the test that Kotrschal used. Females also choose their males based on black spots on their bodies, so their eyes may be better tuned to the black symbols in the test. Or, it’s possible that both sexes use their extra brains in different ways.

Next, the team wants to see what would happen if they repeated their experiment in more realistic settings, where the fish get to compete with one another over limited supplies of food, or face the threat of predators.

It would also be interesting to see more experiments of this kind in other animals. “I am a bit concerned with extending the conclusion beyond guppies,” says Aiello.” The lesson of recent work is a variety of trade-offs that are possible across species to support large brain sizes. I would like to see similar research done in other species and particularly in mammals like mice, to begin to get a handle on what other trade-offs might be involved, or if the brain gut trade-off really is more universal than many would accept now.”

Reference: Kotrschal, Rogell, Bundsen, Svensson, Zajitschek, Brannstrom, Immler, Maklakov & Kolm. 2013. Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and Benefits. Current Biology http://dx.doi.org/10.1016/j.cub.2012.11.058

More on brain evolution

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Q: Why don’t apes have bigger brains? A: They can’t eat enough to afford them

As animals get bigger, so do their brains. But the human brain is seven times bigger than that of other similarly sized animals. Our close relative, the chimpanzee, has a brain that’s just twice as big as expected for its size. And the gorilla, which can grow to be three times bigger than us, has a smaller brain than we do.

Many scientists ask why our brains have become so big. But Karina Fonseca-Azevedo and Suzana Herculano-Houzel from the Federal University of Rio de Janeiro have turned that question on its head—they want to know why other apes haven’t evolved bigger brains. (Yes, humans are apes; for this piece, I am using “apes” to mean “apes other than us”).

Their argument is simple: brains demand exceptional amounts of energy. Each gram of brain uses up more energy than each gram of body. And bigger brains, which have more neurons, consume more fuel. On their typical diets of raw foods, great apes can’t afford to fuel more neurons than they already have. To do so, they would need to spend an implausible amount of time on foraging and feeding. An ape can’t evolve a brain as big as a human’s, while still eating like an ape. Their energy budget simply wouldn’t balance.