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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

Our Skulls Didn’t Evolve to be Punched

Hands evolved to punch faces. Faces evolved to take punches. That’s the hypothesis being bandied about by University of Utah researchers Michael Morgan and David Carrier, the pair proposing that the apparent “protective buttressing” of our skulls and hands is a sign of violent prehistoric fights where fists of fury dictated who would mate and who would exit the gene pool. It’s a great example of a just-so story.

Morgan and Carrier’s new paper, published in Biological Reviews, is a sequel to an initial paper that suggested our hands evolved as cudgels. This was more than a bit of a stretch. “The goal of this study was to test the hypothesis that the proportions of the human hand make it an effective weapon,” Morgan and Carrier wrote in the first study, but they couldn’t provide any evidence that punching was a preferred or even common mode of fighting in the past. The hypothesis rested on a post hoc fallacy of the same sort used by “aquatic ape” devotees – because our hands can be effective weapons, then they must have evolved for that purpose. No surprise that the concept of a spandrel – a trait that wasn’t molded specifically by natural selection, but is an evolutionary byproduct later co-opted for a different use – never appears in Morgan and Carrier’s considerations of pummeling fists.

But the skull paper is even stranger. Although Morgan and Carrier focused on the bludgeoning qualities of modern human hands in their previous paper, their new review suggests that our ancient relatives and forebears – the australopithecines – had faces that were molded into punching bags by natural selection. No sooner did humans come out of the trees, Morgan and Carrier suggest, than they started whaling away on each other. The trouble is that they undercut their own hypothesis, leaving only a crumpled heap of speculation.

Citing crime statistics from western countries, Morgan and Carrier write that fistfights often result in broken noses, jaws, and other facial bones. Therefore, they reason circularly, prehistoric humans that punched each other in the face should have more robust facial bones to cope with such blows. Given that early humans Australopithecus and Paranthropus – the latter often called “robust australopithecines” – had broad faces with wide cheeks and thick brow ridges, they’re obviously perfect candidates for Morgan and Carrier’s favored interpretation.

Morgan and Carrier didn’t study whether or not the hands of the early australopithecines could form a fist. Their previous work was on our species, Homo sapiens. Nor did they look for signs of broken facial bones or blunt-force trauma on prehistoric skulls, or even try to model how early human skulls would have reacted to the stresses of an incoming fist. The entire argument is simply that australopithecine skulls look like they could take a punch.

In Morgan and Carrier’s view, the heavy brows, large jaws, and flaring cheeks of the australopithecines are not signals of the way primates grow or the different plant foods they dined on, as paleoanthropologists have discerned, but were adaptations for reducing damage doled out by males as they competed for mates. There’s no evidence that australopithecines fought like this. The entire conjecture is based on sports like mixed martial arts and modern crime stats. And females don’t even figure into Morgan and Carrier’s hypothesis. Female mate choice, and why sexual dimorphism between the sexes has drastically decreased through time, is either ignored or overshadowed by the belief that we owe our most distinctive features to males walloping each other.  This is bro science – dudes pummeling each other driving human evolution.

The skull of Paranthropus boisei. From Ungar, P., Grine, F., Teaford, MF. 2008. Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PLoS ONE 3(4): e2044
The skull of Paranthropus boisei. From Ungar, P., Grine, F., Teaford, MF. 2008. Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PLoS ONE 3(4): e2044

Those early humans couldn’t make the tight fists we do, though. Australopithecines – Lucy and her kin – were bipedal walkers that retained some signs of their arboreal ancestry, such as more ape-like arms and fingers. The hands and limbs of archaic hominins don’t match up with the supposedly “buttressed” skulls. More than that, our species doesn’t have the reinforced cheek bones, deep jaws, or prominent brow ridges that Morgan and Carrier cast as defensive structures. If our fists are so well-suited for punching, why have our faces lost their osteological protection? Morgan and Carrier suppose that we’re weaker than our ancestors, and therefore don’t need thick facial bones, but this runs counter to the heart of their hypothesis. If our hands evolved as weapons, then we should see a coevolution between striking hands and stout faces. Our prehistory shows no such pattern.

Saying that our hands are adapted to strike or that our skulls evolved to withstand those anatomical truncheons is fine as a hypothesis. But a hypothesis is just the initial fuel for the scientific engine. Morgan and Carrier haven’t let that experimental machinery run, instead looking to isolated tidbits of modern culture and projecting those behaviors onto our past. That’s not science. That’s storytelling.


Morgan, M., Carrier, D. 2013. Protective buttressing of the human fist and the evolution of hominin hands. The Journal of Experimental Biology. 216: 236-244

Carrier, D., Morgan, M. 2014. Protective buttressing of the hominin face. Biological Reviews. DOI: 10/1111/brv.12112

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Testing Vaccines On Captive Chimps To Protect Wild Chimps—Is It Worth It?

In February 2011, a team of scientists led by Peter Walsh at the University of Cambridge injected six captive chimpanzees with an experimental vaccine against the deadly Ebola virus. At first glance, the study looked like a lot of other medical research, in which drugs that are meant for humans are first tested on other animals. But this was different. These scientists were working with chimps to help chimps.

The twin threats of poaching and habitat loss are driving the African apes—chimps, bonobos, and gorillas—towards extinction. Diseases are also a problem. Our ape relatives are vulnerable to infections like anthrax, malaria, and respiratory viruses that spill over from human tourists and researchers.

They can also get Ebola. Under a microscope, the Ebola virus looks like a malevolent knot. In the body of a human or ape, it causes severe and fast-burning disease. In 2006, Walsh’s team estimated that recent Ebola outbreaks have killed around a third of the world’s gorilla population—some 5,000 animals in the Republic of the Congo—and a slightly smaller proportion of the world’s wild chimps.

For years, Walsh has argued that we should protect the survivors with vaccines. He certainly has plenty to choose from: scientists have developed several potential Ebola vaccines that have protected mice and monkeys against the virus. They’re still years away from passing human trials, and since Ebola is a rare disease that mostly affects poor people in the tropics, a human vaccine may never become a commercial reality.  These orphaned medicines could be used to save wild animals.

Not everyone agrees. Historically, conservationists have been happy to protect a species’ habitat or to fight poaching, but they’ve railed against interfering with the animals directly. In 1966, Jane Goodall stopped a polio epidemic among Tanzanian chimps by hiding an oral polio vaccine in bananas. “There was a backlash against her,” says Walsh. “It’s very difficult to get anybody to agree to do vaccinations.” But this attitude is waning, as the threat of diseases is becoming clearer. Walsh thinks there’s now more appetite for vaccinating wild animals.

Ebola virus. Credit: Dr. Frederick Murphy, CDC.
Ebola virus. Credit: Dr. Frederick Murphy, CDC.

In 2011, his team, led by Kelly Warfield, finally tested an Ebola vaccine on three chimps. They used one that doesn’t contain any live viruses; instead, it comprises a piece of the virus’s coat. It trains the immune system to create antibodies that recognise Ebola, without risking an actual infection.

None of the vaccinated chimps showed any signs of weight loss or disease, and they all had similar numbers of blood cells to three chimps that weren’t vaccinated. As planned, they developed antibodies against Ebola. And when the team injected these antibodies into mice, the rodents were twice as likely to survive an encounter with the virus.

That’s not surprising, since the same vaccine has protected monkeys in earlier experiments, says Nancy Sullivan from the National Institute for Allergy and Infectious Diseases, who has worked on Ebola. “It’s a good first step, but whether the vaccine will protect chimps or not, we don’t know,” she says. After all, the team didn’t actually challenge the chimps with Ebola itself. Walsh thinks this is unnecessary. Based on the monkey data and the mouse experiment, he says the vaccine is ready for use.

This study, in which scientists tested a vaccine on captive chimps to protect wild chimps, rather than humans, is the first of its kind. It may also be the last study of its kind.

The era of biomedical research on chimpanzees is drawing to a close. The United States and Gabon are the only countries that still allow this kind of research, and the US may soon leave this short list. In 2011, the Institute of Medicine issued a report saying that “most current use of chimpanzees in biomedical research is unnecessary”—a conclusion that the National Institutes of Health took seriously. In 2013, it announced that all but 50 of its chimps would be retired to sanctuaries. Meanwhile, the US Fish and Wildlife Service has tabled a proposal to list captive chimps under the Endangered Species Act—a move that would ban medical procedures beyond those that “enhance the propagation or survival of the affected species”.

Walsh’s study might still fit the bill but it wouldn’t matter, since labs with the right facilities to house and work with chimps would shut down. That’s a problem, since national park managers in Africa insist that scientists prove the safety of vaccines in captive apes before using them on wild ones (and monkey data won’t suffice). To Walsh, you need captive chimps to test vaccines that would save wild ones from diseases.

He’s not just talking about Ebola, either. Vaccines could also protect chimps from HRSV—a human virus that they catch from humans, often with fatal results. One HRSV vaccine, developed for our own use, didn’t work well in humans but was great at protecting chimps. Beatrice Hahn at the University of Pennsylvania is also developing a vaccine against simian immunodeficiency virus (SIV), which causes an AIDS-like disease in chimps. (Hahn was unavailable for comment due to travel.)

To continue research on such vaccines , Walsh thinks the US Government should keep a humanely housed population of a few dozen chimps specifically for conservation research. “Potentially preventing the extinction of wild chimps should weigh more heavily on the ethical scale than the discomfiture of chimps in captivity,” he says. “They’re not being vivisected or challenged with Ebola. The nasty bit is that, for a couple of months, they’re in a small isolation cage. I don’t like that. It’s not a good thing, but it’s not horrific. I think it’s worth it for the survival of the species.”

John VandeBerg from the Texas Biomedical Research Institute voices similar views in a New York Times op-ed, published last year. “The NIH has not permitted a single chimpanzee that it owns or supports to be enrolled in a new research study since December 2011,” he wrote. “Humans — and chimpanzees and gorillas — may continue to die from diseases that could have been prevented or treated by medical products developed from research with chimpanzees.”

“That is totally silly,” says Brian Hare from Duke University, who does non-invasive research with chimps in African sanctuaries. “They could work with the Pan-African Sanctuary Alliance, because there are more than 1,000 captive chimps in Africa that could be protected.” Kathleen Conlee from the Humane Society of the United States, an advocacy group, agrees. “We don’t need to keep chimpanzees in laboratories for such efforts and could instead work with sanctuaries in Africa, where the chimpanzees actually have a chance of exposure to the virus,” she says, echoing points she made in response to VandeBerg’s op-ed.

But Walsh counters that zoos and sanctuaries don’t offer the controlled conditions necessary for a proper vaccine trial—hence, the need for private facilities.

“Protecting endangered chimpanzees and gorillas against Ebola is certainly a very important topic,” says Tom Geisbert from the University of Texas Medical Branch. But he says that the vaccine Walsh used needs three doses to trigger a protective immune response, which is neither practical nor feasible for wild apes. Instead, the team should check the safety of other vaccines based on weakened viruses, which work after a single injection.

Hare adds that vaccinating wild animals is very tricky. “No tested or proven method that doesn’t cost insane amounts and have extreme risk for wild animals,” he says.

But Walsh argues that it’s easier to vaccinate wild animals than critics think. His field team in Africa have already vaccinated wild gorillas against measles using blow darts, in a trial whose results are not yet published. “It’s not trivial, but it’s not that hard,” he says. In the future, he hopes to develop bait methods that can deliver oral doses of a vaccine without the need for darts. “Again, we need a captive population to test the bait system.”

Reference: Warfield, Goetzmann, Biggins, Kasda, Unfer, Vu, Javad Aman, Olinger & Walsh. 2014. Vaccinating captive chimpanzees to save wild chimpanzees. PNAS http://dx.doi.org/10.1073/pnas.1316902111

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Apes Remember Specific Events From Years Ago

In the climax of the Pixar film Ratatouille, a mouthful of the eponymous dish whisks food critic Anton Ego back to his happier childhood days. The scene is reminiscent of one described by Marcel Proust in Remembrance of Things Past, when the taste of a petite madeleine dipped in lime tea brings back the memory of his aunt Leonie. Many of us can recount similar moments, when a peculiar combination of sensory cues—a smell or taste or sound—transported us back to a point in our past.

Do animals share similar experiences? Do they store deep autobiographies that can be triggered by the right set of cues? According to a new experiment by Gema Martin-Ordas, Dorthe Berntsen and Josep Call, the answer is yes, a least for two species—chimpanzees and orang-utans.

In 2009, the team ushered 12 of the apes into the middle of several connected cages. In full view, they hid two tools in different boxes within the adjacent rooms. Their job was to remember where these tools were—they would need them to reach an inaccessible piece of food in a later test. They had four shots at doing this.

Over the next three years, the apes went about their lives. They ate, slept, socialised, and took part in many more studies. Then, in 2012, eleven of them were led into the same set of rooms with tool-containing boxes in the same locations. And all of them, except for one, went straight to boxes and retrieved the tools. They remembered.

Call was surprised at “how quickly they retrieved the tools as soon as we opened the doors”. They all did this on their first attempt, without prompts or trial-and-error. They didn’t know this test was coming—in 2009, even the researchers hadn’t planned to repeat their experiment three years later. And by contrast, seven individuals that weren’t part of the original experiment didn’t head for the boxes; they just explored the rooms randomly.

Here’s the amazing thing: in the intervening years, the 11 apes had been into these rooms again. They’d worked with other scientists in other studies on tool use or cooperation. Martin-Ordas’s team had even tested them in the same rooms but on different tasks. But they had apparently bound the features of the original experiment—the researchers, the location and the task—into a cohesive memory. The combination of those features, reprised in 2012, took them back to 2009, just like ratatouille or madeleines made Anton Ego and Marcel Proust feel like kids again.

In a second experiment, the team tested 10 chimps and orang-utans (including many of the same animals) on another task. This time, tools had been hidden in trays at different placed within the same rooms. They only saw this set-up once, and two weeks later, they saw it again. As before, all but one of them went straight for the tools in the first half-minute. By contrast, eleven individuals, who had no experience with the experiment, took more than 5 minutes to find the tools.

Gaps of weeks and years might not seem that long, given that we can remember events that happened decades ago. But bear in mind that as recently as the 1970s, we didn’t even know if animals had these sorts of “episodic memories”—recollections  of what, where and when.

Certainly, Canadian psychologist Endel Tulving didn’t think so, and he coined the term “episodic memories” in the first place. He believed  that such memories relied upon our language skills, and were unique to humans. He was wrong; episodic memory is one of a long line of mental abilities that were once thought to be uniquely human but are actually shared by other animals.

Nicky Clayton from the University of Cambridge has done several seminal experiments with western scrub-jays, showing that they can remember the location of food that they had previously stored, and even which hoards were freshest. Other scientists have found similar examples among great apes, hummingbirds, rats, and perhaps even honeybees. But most of these studies used delays of a week at most. By contrast, Martin-Ordas, Berntsen and Call have shown that chimps and orang-utans can retain details of what, where and when for at least three years.

“We do not know about their upper limits,” says Call. “It’s conceivable that they retain memories for much longer, and some studies are planned to assess this.”

“I am not ready to say that apes possess autobiographical memories just like we do,” he says. For example, these memories form an important part of our social lives because we can share them with each other, as I did in the opening paragraph. We don’t know if other animals do the same. “I would say that we have documented some elements of human autobiographical memories that are also present in apes, and possibly in other animals as well,” says Call.

Reference: Martin-Ordas, Berntsen & Call. 2013. Memory for Distant Past Events in Chimpanzees and Orangutans. Current Biology. http://dx.doi.org/10.1016/j.cub.2013.06.017

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Two New Fossils Reveal Details of Ape/Monkey Split

In 2011, a team of palaeontologists led by Nancy Stevens, unearthed a single molar in Tanzania’s Rukwa Rift Basin. It was a tiny fossil, but its distinctive crests, cusps and clefts told Stevens that it belonged to a new species. What’s more, it belonged to the oldest known Old World monkey—the group that includes modern baboons, macaques and more. They called it Nsungwepithecus.

A year later, and 15 kilometres away, the team struck palaeontological gold again. They found another jawbone fragment, this one containing four teeth. Again, a new species. And again, an old and distinctive one. The teeth represent the oldest fossils of any hominoid or ‘ape’. They called it Rukwapithecus.

Together, these two new species fill in an important gap in primate evolution. Based on the genes of living species, we know that Old World monkeys and apes must have diverged from each other between 25 and 30 million years ago. But until now, there weren’t any fossils from either group during that window. The ones we found were all 20 million years old or younger.

But Nsungwepithecus and Rukwapithecus were both found in sediments that could be precisely dated to 25.2 million years ago. They imply that apes had already split away from Old World monkeys by that time. Finally, fossils had corroborated the story that genes were telling. And they suggested that the split between these two groups took place against a backdrop of geological upheaval.

I wrote about the discoveries for The Scientist so head over there for the full story.

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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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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.


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Unlike humans, chimpanzees only punish when they’ve been personally wronged

When Delta Airlines refused to let Arijit Guha board a plane because his T-shirt made passengers uncomfortable, others made Delta aware of their outrage. When Samsung infringed Apple’s copyright, a jury of independent peers awarded Apple more than $1 billion in damages. When Republican Todd Akin claimed that women could stop themselves from becoming pregnant if raped, people called for his head.

These recent events all illustrate a broad human trait: we seek to punish people who do wrong and violate our social rules, even when their actions don’t harm us directly. We call for retribution, even if we have nothing specific to gain from it and even if it costs us time, effort, status or money to do so. This “third-party punishment” is thought to cement human societies together, and prevents cheats and free-riders from running riot. If you wrong someone, and they’re the only ones who want to sanction you, the price of vice is low. If an entire society condemns you, the cost skyrockets.

Do other animals do the same thing? It’s not clear, but one group of scientists believes that our closest relative – the chimpanzee – does not. Katrin Riedl from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany found that chimpanzees will punish individuals who steal food from them, but not those who steal food from others. Even if the victim was a close relative, the third party never sought to punish the thief. These were the first direct tests of third-party punishment in a non-human animal, and the chimps got an F.


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How orangutan engineers build safe and comfy treetop beds

We normally think of nests as the creations of birds, but our ape cousins build nests too. Orangutans, gorillas, chimpanzees and bonobos all build tree beds, by weaving branches, twigs and leaves together into a bowl-shaped cradle. These nests may provide safety from predators, or help the apes to sleep warm.* But it seems that their main function is to provide a good night’s rest. Sleeping against a tree bough is hard on a large ape, and nests offer a more comfortable option.

Of all the apes, orangutans reputedly create the sturdiest and most elaborate nests. By studying the physical properties of these treetop bunks, Adam van Casteren from the University of Manchester has found that the apes are skilled engineers. As befits animals of their intelligence, they don’t just mash branches together. Instead, they seem to have an impressive amount of technical knowledge about their construction materials.

Orangutans build their nests between 11 and 20 metres up. Once they choose a good spot on a sturdy branch, they bend or break other branches in towards them, and weave them in place to create a basic foundation. On top of that, they add smaller branches to create a ‘mattress’. That’s the basic model, and some orangutans add deluxe features. They can create blankets, by covering themselves with large leafy branches, or pillows, by clumping such branches together. They can loosely braid branches above their heads to make a roof, or even create a secondary ‘bunk-nest’ over the main one.

Like all apes, orangutans construct new ones every day. This means that intrepid scientists have plenty of old discarded nests to study. Van Casteren, along with Julia Myatt from the Royal Veterinary College, found 14 such nests in the Sumatran rainforest. They hoisted themselves into the canopy, attached ropes to different parts of the nests, and lowered these to the ground where team members were waiting with force gauges. “Climbing up into the high canopy is breathtaking,” says van Casteren. “You enter an area of the forest that isn’t used to having humans hang around in it.”

Van Casteren found that orangutans use thicker branches in the structural foundation of the nest, and thinner branches in the mattress. The structural ones are four times stronger and four times more rigid, and they make the nest sturdy. The mattress branches are thinner and more flexible for comfort.

The orangutans also break the two types of branches in different ways. If you bend a dense branch, it will only break halfway – this is known as a “greenstick fracture (see below). That’s what van Casteren found in the structural part of the nest. Once broken like this, it’s surprisingly hard to fully snap a branch in two, even for a powerful animal like an orangutan. The trick is to twist the branch. The fracture extends outwards until the two halves come apart, producing two pieces with long tapered ‘tails’.  Van Casteren filmed the apes using this technique, and the found plenty of the distinctive tailed branches in their mattresses.

There are plenty of questions about the nests left to answer. For example, orangutans don’t choose their trees randomly, and actually avoid the most common species. What’s special about the ones they pick, and does that factor into the properties of the nests? The apes also learn their craft from adults, so do immature orangutans build nests with less distinctive foundations and mattresses? Van Casteren also wants to look at the nests of other great apes, and of other architects such as beaver or birds, to see if he gets similar results.

But for now, his data already show that orangutans make sophisticated technical choices when they build their nests. He thinks that they account for the different properties of the materials in their environment, and use those properties to make bunks that are both safe and comfortable. While many studies of animal intelligence focus on the use of tools, he argues that nest-building is no less mentally demanding.

Roland Ennos, who was involved in the study, says, “I hope helps to show how the evolution of intelligence can be driven by the need to deal with the mechanical environment, rather than the prevailing orthodoxy that it’s only the social environment that’s important.”

* In writing this story, I stumbled across a wonderful study by Fiona Stewart from the University of Cambridge, who tested the value of chimpanzee nests, by sleeping in them. She spent several nights in Senegal either sleeping in newly made chimp beds or on the bare ground. She was warmer in the nests, and received fewer insect bites. She didn’t get any more sleep, but what she got was less disturbed. “Terrestrial animals, including hyenas, were more concerning during ground sleep, although snakes were always a concern,” she writes, in a wonderfully deadpan way. Van Casteren, however, never tried to sleep in the orangutan nests that he studied. They are higher than a chimp’s and he was “too worried about falling out mid-dream”.

Reference: Van Casteren, Sellers, Thorpe, Coward, Crompton, Myatt & Ennos. 2012. Nest-building orangutans demonstrate engineering know-how to produce safe, comfortable beds. PNAS http://dx.doi.org/10.1073/pnas.1200902109

Images courtesy of Adam van Casteren

More on orangutans


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Why humans stand on giant shoulders, but chimps and monkeys don’t

We are like dwarves standing on the shoulders of giants. This metaphor, famously used by Isaac Newton, describes how humans build on what has come before. Everything in our culture is the result of knowledge and skills that have slowly accumulated over time. Without this “cumulative culture”, we wouldn’t have our deep scientific knowledge, rich artistic traditions, or sophisticated technology. Simply put, you can’t make a car from scratch – first, you need to invent the wheel.

Are we alone in this respect? Certainly, many other animals can learn knowledge and skills from each other, and many of them have cultural traditions. But Newton’s metaphor involves not just the spread of knowledge, but its gradual improvement. We build on the past, rather than just passing it along. As generations tick by, our culture becomes more complex. Do other species show the same ‘cultural ratchet’?

Lewis Dean from the University of St Andrews tried to answer that question by presenting human children, chimpanzees and capuchin monkeys with the same task: a puzzle box with three, increasingly difficult stages, each one building on the last.


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Bonobos: the self-domesticated ape?

The two apes above might look very similar to the untrained eye, but they belong to two very different species. The one on the right is a bonobo; the one on the left is a chimpanzee. They are very closely related but the bonobo is slimmer, with a smaller skull, shorter canines and tufts of lighter fur. There are psychological differences too. Bonobos spend more time having sex, and playing with one another. They’re less sensitive to stress. They’re more sensitive to social cues. And they are far less aggressive than chimps.

Many years back, a young researcher called Brian Hare was listening to the Harvard anthropologist Richard Wrangham expound on this bizarre constellation of traits. “He was talking about how bonobos are an evolutionary puzzle,” recalls Hare. “They have all these weird traits relative to chimps and we have no idea how to explain them.”

But Hare had an idea. “I said, ‘Oh that’s like the silver foxes!’ Richard turned around and said, ‘What silver foxes?’”


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Ancient Greek athletes did it gibbon-style

In the Olympic Games of Ancient Greece, long-jumpers would leap while carrying weights called halteres in their hands. From either a standing start or a short run, they swung the weights and leapt as their arms came forward. The halteres each weighed up to nine kilograms, and would have added around 17 centimetres to a 3 metre jump. Olympians first used the hand weights in 708 BC, but other apes were jumping with a very similar technique millions of years earlier – gibbons.

Gibbons are undisputed masters of the treetops, best known for swinging around at unfeasible speeds from their long, powerful arms. Their wrists contain ball-and-socket joints, which allow their entire body to easily pivot about their hands. This style of movement, known as brachiation, is a gibbon speciality (see video below). But these apes are also accomplished jumpers. Field scientists have watched them clear gaps as large as 10 metres.


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Charity of the apes – chimps spontaneously help each other

Compared to most other animals, humans are unusual in our tendency to help each other out. We donate to charity. We give blood. We show kindness to strangers, even when we stand to gain nothing in return. This behaviour is so odd that the natural question arises: are we alone in such selflessness? And if any animal could help to answer that question, it’s the chimpanzee, one of our closest relatives.

Dozens of scientists study the behaviour of chimps, looking at how these apes act towards their peers. But the results of these studies have been frustrating for many in the field. People who watch captive and wild chimps have documented hundreds of cases of seemingly altruistic behaviour. They have seen individuals helping each other to climb walls, consoling each other after fights, sharing food, risking death to save companions from drowning, and even adopting the babies of dead and unrelated peers. Anecdotes like these suggest that chimps, like humans, behave selflessly towards each other.

But experiments have often shown otherwise. In some studies, chimps choose to help their peers retrieve out-of-reach objects rather than doing nothing. But when chimps have a choice between two equal actions – say, cashing in a token that leads to personal gain versus another that also benefits a partner – they only looked out for themselves. One paper bore the title “Chimpanzees are indifferent to the welfare of unrelated group members”. Another concluded that “chimpanzees made their choices based solely on personal gain”.

Collectively, these studies championed a view of chimps as reluctant altruists, who only act selflessly in response to pressure, and who generally don’t help unfamiliar chimps, “even when they are able to do so at virtually no cost to themselves”. But Frans de Waal from the Living Links Centre at Emory University thinks that this portrait is wrong. He says, “The authors of these studies moved from not finding evidence for prosocial choice to thinking they had proven its absence.”

De Waal thinks that the previous tests handicapped the chimps by putting them in situations that masked their altruistic tendencies.  They couldn’t communicate, they had to cope with complicated equipment involving levers, and they often sat so far apart that they had little understanding of how their choices affected their fellows. With his colleague Victoria Horner, de Waal designed a new experiment to account for these problems. And, lo and behold, chimps spontaneously helped their partners, even without any prompting.


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Children share when they work together, chimps do not

We are a cooperative ape, and a fair one. We work together to put food on the table and once it’s there, social rules compel us to share it around equitably. These two actions are tied to one another. In a new study, Katharina Hamann from the Max Planck Institute for Evolutionary Anthropology has shown that three-year-old children are more likely to fairly divide their spoils with other kids if they’ve worked together to get them.

The same can’t be said of chimpanzees, one of our closest relatives. Sharing comes less naturally to them, and it doesn’t become any more likely if they’ve worked together to get a meal.


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Our closest relatives – a visual tour of the primates

Few groups of animals hold such special significance for us as the primates – the apes, monkeys, lemurs and more. This is the group that we are a part of. Its members are familiar and charismatic, but our evolutionary history is tangled and occasionally controversial.

Now, Polina Perelman has provided the most comprehensive view of the primate family tree to date. Her team sequenced genes from over 186 species, representing 90% of all the genera that we know of. Her tree confirms some past ideas about primate evolution and clarifies other controversies. It’s a story of island conquests, shrinking bodies, tangled branches and ancient relics.

Image by Medeis

Today, the primates’ closest living relatives are the flying lemurs, or colugos, of Southeast Asia. There are two species that both glide between trees, using flaps of skin outstretched between their legs. They can’t actually fly and they’re not really lemurs, making them the second most inaccurately named animal, after Michael Winner.

Flying lemurs aside, our next closest relatives are the treeshrews, which also live in Southeast Asia. Continuing the theme of inaccurate names, they aren’t true shrews, but they do at least (largely) live in trees.

By me

The oldest fossil primates, such as Plesiadapis (pictured), lived around 56 million years ago, but genetic studies suggest that the group may have arisen even earlier than that. Perelman says that her study provides “strong evidence” that the first primates arose from a common ancestor around 90 million years ago. That was around the middle of the Cretaceous period, when dinosaurs like Tyrannosaurus and Triceratops were still around. It’s not clear where the primates first arose but Asia’s the best guess, given that the flying lemurs, treeshrews and many of the earliest primates are all confined there.

No sooner had the primate family tree established itself than it split into two major trunks, around 87 million years ago. The first group – the wet-nosed strepsirrhines – include lemurs, lorises and bushbabies. We belong to the other group, the dry-nosed haplorrhines, which also includes tarsiers, monkeys and other apes.

Images by Kalyan Varma (slender loris, left), OpenCage (Senegal bushbaby, middle), Visionholder (black-and-white ruffed lemur, right)

Once they had split away from the haplorrhines, the strepsirrhines themselves diverged into two major lineages around 69 million years ago. The first gave rise to the Lorisiformes, a group that includes the slow-deliberate lorises and pottos, and the spectacular leaping bushbabies. They diversified around 40 million years ago.

The second lineage gave rise to the Chiromyiformes, solely represented today by the bizarre aye-aye, and the Lemuriformes, which include all other lemurs. There are more than a hundred species and subspecies of lemur and they all live on the island of Madagascar. Every one of them descended from a common ancestor that washed up on the island’s shores around 59 million years ago.

Image by David Haring

The slow lorises have the dubious honour of being the only poisonous primates. A gland on the inside of their elbows secretes a poison that smells a bit like sweaty socks; the loris licks this gland, which gives it a toxic (and agonising) bite. The protein behind the poison is remarkably similar to the one that causes cat allergies.

The lorises as a group have another honour. Perelman found that they are the most divergent of all the primates. If you compared the genes of the seven or so species, you’d see differences that are 4 to 5 times greater than those between humans, chimps, gorillas and orangutans. For now, no one knows why.

Image by Frank Vassen

Carl Zimmer once described the aye-aye to me as a “furry Gollum”. This unusual creature is the most ancient of all the lemurs. Its ancestors were among the first to split away from the main lemur line after it arrived on Madagascar. It hunts for grubs at night by tapping on tree trunks with its grossly distended middle finger and listening out with large ears. If it hears the right sounds, it gnaws away at the bark with rodent-like teeth and hauls the grub out with the same narrow finger, wielded like a hook.

Images by me (ring-tailed lemurs; left), Erik Patel (indri; middle) and Arjan Haverkamp (gray mouse lemur, right)

The aye-aye aside, the remaining lemurs diverged into four main groups, starting 39 million years ago. The Lemuridae or so-called “true lemurs” were the first to emerge – today, they include the sociable and distinctive ring-tailed lemurs (left). The Indriidae were next – they include the group’s largest and most vocal member, the indri (middle), as well as the woolly lemurs and the agile, bounding sifakas. The remaining lineage split into the sportive lemurs (Lepilemuridae), and the dwarf and mouse lemurs (Cheirogaleidae; right). The latter include the smallest members of the group. Madame Berthe’s mouse lemur is the smallest of them all – it can weigh as little as 30 grams.

Image by Nummymuffin (golden lion tamarin)

While the strepsirrhines were diversifying, so too were the haplorrhines. The tarsiers were the first to branch away, around 81 million years ago (more on them in the next slide). The rest of the group (the Simiformes or “simians”) split into two main lineages around 44 million years ago. These were the “flat-nosed” platyrrhines (New World monkeys) and the “narrow-nosed” catarrhines (Old World monkeys and apes).

Image by Jasper Greek Golangco

The tarsiers have been a particularly difficult group to place. Originally, they were grouped together with the strepsirrhines to form the prosimians, from the Greek meaning “before ape”. This group – essentially all primates except monkeys and apes – has less relevance today, because we know that the tarsiers are actually haplorrhines. Perelman’s study confirms that.

These big-eyed, knobbly-fingered animals are found only in the Philippines and three Indonesia islands. But around 50 million years ago, there were tarsiers all over the Northern Hemisphere. Today’s species are but a shadow of a once diverse group, one that branched off early from other primates and has evolved alongside us ever since.

Images by Ipaat (bald uakari; top left), Mila Zinkova (emperor tamarin, top right); Luc Viatour (squirrel monkey, bottom left) Hans Hillewaert (mantled howler monkey, bottom right)

Modern platyrrhines live in Central and South America but it’s not entirely clear how their common ancestor got there. At the time, around 25 million years ago, the Panama land bridge that connected North and South American hadn’t formed, and the Atlantic Ocean was narrower. It’s possible that this ancient monkey rafted across from Africa. No matter how it got there, what happened next is clearer thanks to Perelman’s study.

After they reached South America, the platyrrhines diverged into three major families. The first to branch off were the pithecids, including the titis, the bald-faced uakaris (top left), the bearded sakis. Next came the atelids with their long, prehensile tails, including the howler (bottom right), spider and woolly monkeys.

Finally, the cebids. This group includes several species that have previously been classified in separate families; Perelman has decided to united them in one. They diverged in quick succession – first, the capuchins and squirrel monkeys (bottom left), and then, the marmosets, tamarins (top right) and the mysterious owl monkey.

Image by John Morton

The cebids are particularly interesting because as they diverged, they also became smaller. The group’s earliest members, including the night monkey and capuchins are generally larger than the later marmosets and tamarins. The smallest of the them all – the pygmy marmoset, no bigger than a hand – is also the latest to evolve.

Image by Mark Laidre

Meanwhile, in the Old World, the catarrhines had also diverged into three major families. The cercopithecoids, including all the Asian and African monkeys, branched away 32 million years ago and started truly diversifying around 18 million years ago. The remaining catarrhines split into two groups just 20 million years ago – the hylobatids, including all the gibbons; and the hominids, including ourselves and the other great apes.

The history of the cercopithecoids is a convoluted puzzle, not least because the genetic differences within the group are lower than expected. As they evolved, it seems that many subspecies and species mated with one another to produce hybrid lineages. They turned a neat forking tree into a tangled bush.

Images by Lea Maimone (mantled colobus, far left), Thomas Schoch (Hanuman langur, left), Benhamint444 (proboscis monkey, right), Jack Hynes (golden snub-nosed monkey, far right)

Despite the complex history of the cercopithecoids, Perelman thinks that the family splits into two big sub-families. The Colobinae started diversifying around 12 million years ago, but they’ve given rise to a large number of African and Asian species. Most of them live in trees and eat leaves These include the colobus monkeys (far left), the langurs and leaf monkeys (left), and the aptly-named “odd-nosed monkeys” (including the bizarre proboscis monkey (right) and China’s beautiful golden snub-nosed monkey (far right)).

Images by Hans Hillewaert (De Brazza’s monkey, far left) Muhammad Mahdi Karim (crab-eating macaque, left), me (hamadryaz baboon, right) Malene Thyssen (mandrill, far right)

The second cercopithecoid subfamily – confusingly known as the Cercopithecinae – diversified at around the same time as the Colobinae. They split into two tribes. One includes some of Africa’s most beautiful species including the guenons, patas monkey, green monkeys and vervets. The other includes baboons, geladas, mangabeys, mandrills and macaques. This second group in particular has a rich history of hybridisation.

Image by Suneko

They hylobatids, or gibbons, diversified by 9 million years ago and today, there are around a dozen or so species. The largest of them – the siamang – is pictured above. These “lesser apes” have taken the primates’ fondness for trees to a whole new level. Their wrist is made up of a ball-and-socket joint, much like our shoulders or hips. That means a swinging gibbon can rotate its entire body around its wrist, giving them a unique style of movement called brachiation (video). They can zoom through treetops with a top speed of 35 miles per hour.

While the gibbons’ movements are all style and grace, their chromosomes are a chaotic mess. They’ve rearranged around 10-20 times faster than most other mammals and, as with lorises, it’s not clear why. That’s a mystery for a future study to solve.

Images by me (orangutan, far left), Pierre Fidenci (bonobo, left), Mila Zinkova (gorilla, right) Ikiwaner (chimpanzee, far right)

Finally, we come to our small branch of the primate family tree – the hominids. If you follow the forking branches to us, the orangutan subfamily (Ponginae) were the first to split away around 16.5 million years ago. That branch later diverged into the two modern species of orangutan – the Bornean and Sumatran – just over one million years ago. On the other subfamily (Homininae), the gorillas were the next to branch away around 8.3 million years ago. Finally, our ancestors diverged from those of chimpanzees and bonobos around 6.6 million years ago.

The genus Homo has been around for less than 10% of the entire history of the primate order. And it has taken us far less time to put many of the other species at risk of extinction. Nearly half of all species are endangered thanks to a combination of deforestation, bushmeat hunting and illegal wildlife trade.