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The Architecture of Living Buildings

The beautiful photos on this post are by Tim Nowack. Check out his site.

Fire ants are self-architects: they construct buildings using their own bodies. Put a chasm in their path, and they’ll unite into a living bridge. Tip them out of a beaker, and they’ll pour forth like a living, many-legged waterfall, with ants at the top supporting the weight of those below. And if you put them in water, as often happens in the Argentinian floodplains that they come from, they’ll merge into rafts.

The rafts are incredible. They can assemble in minutes, and stay afloat for months. By trapping air pockets and relying on their own waxy waterproof coats, the ants can float en masse, even though each of them is individually denser than water. They’re so good at floating that it’s hard to physically push them down with a stick—try it, and the raft will dent the water but won’t go under.

Can't keep a good ant raft down. Credit: Tim Nowack
Can’t keep a good ant raft down. Credit: Tim Nowack

David Hu’s team at the Georgia Institute of Technology have been studying the ants for years by filming them, dumping them in water, and measuring their physical properties. But one thing was always missing: they had never managed to actually peer inside the ants’ self-made structures, to see how they are built. “Imagine you have a thousand ants,” says Hu. “You’ll only see the ones on the surface. You can peer into the second layer but everything on the inside is inaccessible.”

The team, fronted by student Paul Foster, has now finally unveiled this hidden world. First, they swirled groups of 110 ants around in a tube to make them gather into their living balls. Then, they poured liquid nitrogen over the insects, killing them instantly and freezing them in place. They also infused the balls with superglue vapours so they wouldn’t lose shape as they thawed.

Finally, they put the ants in a CT scanner, and visualised the position of every leg and antenna, every bite and touch. “It’s like looking at a building, and seeing the scaffolding and the individual nails,” says Hu.

The scans showed that the ants are extraordinarily good at finding each other. Even though the team forced them to form into balls very quickly, while being swirled in a beaker, almost all of them attached all of their legs to a neighbour. The team studied four groups of 110 ants, giving a total of 2,640 legs. Of these, 2624 (99 percent) were stuck to another ant.

“It’s like you’re in a mosh pit, someone says, ‘Go!’ and  you have to put all your hands and legs on your neighbours,” says Hu. “The ants do that with six legs, and there are no freeloaders. There are no ants that aren’t sticking to anyone.”

On average, each ant makes 14 connections with its neighbours. They do it not with their claws, but with sticky pads at the ends of their feet; dry the pads out with talcum powder, and the ants can’t form balls, rafts, or bridges.

An ant connection. Note the adhesive pad between the legs. Credit: Foster et al, 2014. J Exp Biol.
An ant connection. Note the adhesive pad between the legs. Credit: Foster et al, 2014. J Exp Biol.

They don’t just stick their pads to the nearest thing they can find; they typically attach to their neighbours legs and feet, rather than their bodies. These connections allow the ants to change the shape of their structures by bending or stretching their legs. That explains why the structures are so elastic, and why they can absorb incoming forces more effectively.

The foot-to-foot connections also suggest that the ants actively control the nature of their balls. The team found other such clues. For example, a ball of living ants is less densely packed than a ball of dead ones, implying that they are actively pushing their neighbours away. This presumably helps to create the air pockets that keep the rafts afloat.

The living ants are also arranged differently than the dead ones. In a dead ball, the ants take on a more parallel alignment, in much the same way that rice grains or staples would if you put them in a jar and shook them around. This minimises the air gaps between them and allows them to stack more efficiently. But in a living ball, the ants are more perpendicular than parallel. This might help to space them apart and keep them afloat. It could also make the balls stronger. “If the ants were all parallel, you’d get fracture planes, and could easily break the ball in half,” says Hu. “In this pattern, you don’t have weak spots.”

A CT-scan of an ant ball. Legs have been digitally removed to make the individuals easier to see. Credit: Foster et al, 2014. J Exp Biol.
A CT-scan of an ant ball. Legs have been digitally removed to make the individuals easier to see. Credit: Foster et al, 2014. J Exp Biol.

None of this requires intelligence; there are many examples of complex animal behaviours arising from incredibly simple rules. Still, the team’s discoveries suggest that the ants are behaving in a more complicated way than anyone suspected. They’re not just randomly grabbing their neighbours. Instead, they’re arranging themselves in very specific ways. “They’re literally building a new type of material with special properties, because of the way they connect up,” says Hu. “It requires some dexterity.”

The 110-ant balls that Hu studies are also very simple. Natural rafts can contain thousands of fire ants. A bivouac (stationary shelter) of ants can control its own their own temperature and humidity by opening pores in its living walls. These structures are so big that they won’t fit inside the scanner that Hu used, so he now wants to freeze them, take them apart, and study them piece by piece. If a simple ball is already more complicated than anyone imagined, what will a tower, bridge or raft look like?

And while you’re waiting for the answers, why not enjoy a comforting cup of teOH HELL NO.

Fire ants behaving like a fluid. Credit: Tim Nowack
Fire ants behaving like a fluid. Credit: Tim Nowack

Reference: Foster, Mlot, Lin & Hu. 2014. Fire ants actively control spacing and orientation within self-assemblages. J Exp Biol. http://dx.doi.org/10.1242/jeb.093021

PS: Fire ants are named for their painful burning sting, and they are obviously very good at surmounting obstacles. “It takes some practice to work with them,” says Hu. Gloves are essential, as are Teflon-coated containers that stop the ants from climbing out. Despite these precautions, bites are inevitable. Since the team often whirls the ants into balls, individuals sometimes go flying out. “You sometimes get an ant bite many hours after the experiment because there’s an ant crawling on your clothes,” says Hu.

More on ant behaviour:

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Zombies Snipers At The Doorstep

Somewhere in the Brazilian rainforest, a lone carpenter ant marches out of its nest towards its doom.

The ant is infected with a fungus called Ophiocordyceps unilateralis, which has both infiltrated and commandeered its body. While it devours the ant alive, it also sends its zombified host scurrying up a plant stem. The ant walks along the underside of a leaf and vigorously locks its jaws around a vein. This is a death-grip; those jaws will never open again. In a week or so, a long stalk erupts from the ant’s head growing outwards and downwards into a bulbous capsule, which releases a rain of spores. If other ants walk underneath, they become infected.

The fungus controls its zombie host with incredible precision. The ant always climbs to around 25 centimetres above the soil—a zone with just the right temperature and humidity for the fungus to grow.

But why does the ant leave its nest in the first place?

Outside, the fungus needs to drive its host to a spot that offers the best chance of raining spores down upon passing workers. But inside the nest, there are thousands of such workers nearby. With this dense smorgasbord of new hosts to infect, why does the fungus send its current host into the great outdoors?

Perhaps these forays are the ants’ doing. Colony-living insects like ants have a kind of social immune system—they behave in ways that prevent infections from spreading through their nests. They clean each other and remove the corpses of their nestmates. Sick ants, which have been infected by killer fungi, are often shunned by their fellow workers, and sometimes leave the nest to die alone. Biologists often view these as acts of self-sacrifice, as if the ant was a six-legged Captain Oates.

But Raquel Loreto and David Hughes from Pennsylvania State University see things differently. The duo first realised what was going on when they collected carpenter ant nests, and seeded them with ants that had been freshly killed by Ophiocordyceps. The ants detected and removed around half of these remaining cadavers—a clear example of their vaunted “social immunity” at work.  But they needn’t have bothered. It turns out that the fungus simply cannot grow inside the nests of its hosts. Even if the cadavers were placed in nests that had no ants, the fungus still wouldn’t sprout.

“The best place for the sick individuals is at home!” says Hughes, who had been studying the zombifying fungi for many years. “In a naive framework, leaving the nest appears to be altruistic but it’s actually part of the fungus’ manipulation.”

So, what happens outside the nest? To find out, Loreto trekked into the same stretch of Brazilian rainforest for 20 months to study 17 specific carpenter ant nests. She ventured into the jungle at night with infrared torches to map the foraging trails that the workers walk along.

She also marked out a 200-cubic-metre zone around four of the nests nest —an area that she calls the “doorstep” of the colony. It’s the zone that all workers must pass through on their way in and out of the nest. Once a month, Loreto walked through these doorsteps and checked the underside of every single leaf within them for the zombified corpses of infected ants.

“This is important because it’s one of the few field studies of ant-parasite interactions,” says Sylvia Cremer from the Institute of Science and Technology Austria. Most people who study social immunity have exposed ants to parasites within the artificial confines of a laboratory. “It’s always good to bring some realism to these discussions,” says Hughes.

At the end of her Herculean effort, Loreto had found zombified corpses around every one of the 17 nests she studied. And she kept on finding new corpses every month around the four nests she studied in detail. The results were clear: despite the ant’s vaunted social immunity, the fungus infects close to 100 percent of the nests in the area, and does so throughout the year. It’s a permanent and omnipresent threat.

The team thinks that the fungus is so successful precisely because it sends its hosts outside the nest. Inside, the ants’ social immune system can protect them. Outside, it doesn’t work. “Over 20 months, Raquel never saw the colony disinfecting its borders,” says Hughes.

The ants also rely on the same trails, so the fungus is virtually guaranteed to find new hosts if it positions its current one in the right spot. Hughes compares it to a sniper’s alley. “The foragers come out at night and pass under the cadavers of their siblings that are now shooting spores at them,” he says.

But why wouldn’t the carpenter ants evolve a countermeasure? Over time, surely you’d expect them to start removing infected corpses from the leaves around their nests, as other species of ants do elsewhere in the world. Hughes suggests that they might not need to. Young carpenter ants stay exclusively within their nests. They only venture out to forage when they get older, so the fungus can only ever infect ants that are near the end of their lives anyway.

This strategy seems to suit both parties. The ants ensure that only their weakest members face death by zombifying fungus, and the fungus get a continually renewed supply of susceptible hosts to infect. It’s as close to a win-win as you get in evolution.

Reference: Loreto, Elliot, Freitas, Pereira & Hughes. 2014. 3D mapping of disease in ant societies reveals a strategy of a specialized parasite. BiorXiv. http://dx.doi.org/10.1101/003574

For more on surprising and counter-intuitive world of mind-controlling parasites, have a look at my TED talk:

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Mercenary Ants Protect Farmers With Chemical Weapons

Humans have a long history of bolstering their armies by paying mercenaries to fight on their behalf. Now, Rachelle Adams from the University of Copenhagen has found that some ants do the same.

Some ants defend their colonies with special soldiers, which are bigger and wield more formidable weapons. But others lack a proper army. Sericomyrmex ants, for example, are fungus-farmers. They bring bits of vegetation back to their nests and use these to nourish a fungus, which they then eat. They are poorly defended, and a sitting target for raiders and pirates. That’s why they rely on other ants to fight for them.

Another ant called Megalomyrmex symmetochus forms its own colonies, complete with queen and workers, inside those of Sericomyrmex colonies. This guest is present in over 80 percent of the farmers’ nests and, at first glance, it looks like a parasite. It eats the fungus that the farmers so assiduously grow, without contributing any labour of its own. Worse still, it eats some of the farmers’ larvae and clips the wings of their young queens, so they contribute to the gardening rather than flying off to start their own colonies.

But M.symmetochus doesn’t just freeload off its hosts. In some cases, it can be their salvation.

Megalomyrmex mercenary (top right) defends a fungus garden from a Gnamptogenys raider (bottom). Credit: Rachelle Adams
Megalomyrmex mercenary (top right) defends a fungus garden from a Gnamptogenys raider (bottom). Credit: Anders Illum

Sericomyrmex nests are often attacked by a third ant called Gnamptogenys hartmani—a sort of six-legged pirate. It raids the colonies of farming species, drives them out, usurps their nests and gardens, and eats any remaining larvae.

The farmers can do very little against these raiders, since they lack specialised soldiers and have mostly lost their stings. Their powerful jaws can deliver a strong bite, but at such close quarters, they risk getting stung and bitten themselves. When attacked, they’re much more likely to feign death or flee. That is, unless there’s a M.symmetochus colony living in their nests.

Back in 2011, Adams let some raiders loose upon a colony of Sericomyrmex farmers, which was already being parasitized by M.symmetochus. “To our surprise, the hosts hid and the parasites rose to the top of the garden to confront and kill the invaders,” she says.

Unlike their hosts, the parasitic ants are far from defenceless. They raise their stings and release a powerful venom directly into the air—an airborne chemical weapon that kills the raiders and befuddles any survivors. “Rather than uniting as an efficient infiltration squad they turn on each other and attack, sometimes killing their own kin,” says Adams.

M.symmetochus behaves like a colony of mercenaries. They can cause problems for the farmers during peace-time, but they provide an invaluable defence when an invading force arrives.

By pitting the three types of ants against each other, first in one-to-one battles and then in more realistic groups, Adams’ team showed that the mercenaries are much better at subduing the raiders than the farmers. It takes just two of them to overpower a single raider, while the same task requires at least eight farmers. On average, a pair of raiders can kill 70 percent of farmers in an unprotected nest, but just 10 percent of them if there are six mercenaries around.

And the raiders seem to know it. When the team gave them a choice between two nests, only one of which was defended by mercenaries, they were more likely to attack the unprotected nest. In the experiment, a wire mesh prevented the raiders from actually touching the colonies, so they were probably put off by the smell of the mercenaries’ chemical weapons. Just by living in the same colonies, the mercenaries protect the farmers by cloaking them in a defensive miasma.

The thing that clinches this incredible relationship is probably Megalomyrmex’s life cycle. They form a life-long bond with a single colony of farmers, and won’t leave to seek out a different host. They exploit their hosts but they don’t overexploit them, and if raiders attack, they mount a defence rather than abandoning the farmers to their fates.

Next, the team wants to study how this alliance varies over time and space. For example, it should be easier to find the mercenaries in the farmers’ nests in places where the raiders are more common. Similarly, a different farmer called Trachymyrmex zeteki should be relatively unbothered by raiders, since it also hosts a parasitic mercenary but in fewer than 6 percent of its nests.

For now, they very astutely compare the mercenary ants to a human disease called sickle cell anaemia. It’s an inherited genetic disease that warps the shape of red blood cells and causes a gradual building illness. The mutated gene behind the disease is common in Africa because two copies might cause sickle-cell anaemia, but one copy protects a person against malaria. Similarly, M.symmetochus is like a mild chronic disease that exerts a toll upon its host, while protecting it from an acute and more destructive infection—the Gnamptogenys raiders.

Reference: Adams, Liberti, Illum, Jones, Nasha & Boomsma. 2013. Chemically armed mercenary ants protect fungus-farming societies. PNAS http://dx.doi.org/10.1073/pnas.1311654110

Experimental manipulation of the tendon of the large jumping muscle of one hindleg

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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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Ant Bodyguards Get Exclusive Contract from Trees

If animals and plants can’t defend themselves, they often form partnerships with bodyguards. Wasps use zombified caterpillars. Corals recruit goby fish. And acacia trees hire ants. The ants defend the trees against hungry mouths by biting and stinging any invading plant-eaters. Some are so ferocious that they can deter elephants. In return, the trees pay their bodyguards by providing shelter in the form of swollen thorns, and food in the form of nectar or nutritious parcels called “food bodies”.

This alliance between ants and acacias is a staple of textbooks, but it’s even more intimate than anyone suspected. Some acacias don’t just supply their ants with any old food. They offer the biological equivalent of a cheque—a reward that only the ants can cash.

Every partnership is vulnerable to thieves. The acacia’s bright, nutritious food bodies could easily be pilfered by any insect quick enough to avoid the patrolling ants. But insects that steal them are in for a poor and possibly dangerous meal.

Domancar Orona-Tamayo from CINVESTAV-Irapuato in Mexico and Natalie Wielsch from the Max Planck Institute for Chemical Ecology in Germany found that the food bodies of two acacia species are loaded with molecules called protease inhibitors. As their name suggests, these block enzymes called proteases, which animals use to digest the protein in their food.

These acacia enzymes were extremely good at neutralising the proteases of two species of seed-eating beetles, slashing their protein-busting abilities by more than 98 percent.

Pseudomyrmex ferruginea—one of the ants that guards the acacia—has no such problems. Its guts are dominated by a special protease called chymotrypsin-1, which the acacia’s protease inhibitors do not inhibit. When these bodyguards eat the food bodies, they get a nutritious reward. When beetles try to do the same, they get indigestion.

The protease inhibitors aren’t found throughout the acacia, just in the food bodies. They are security measures that protect the tree’s rewards by harming would-be thieves. Only the ants can bypass these defences, and only the right ants at that.

Pseudomyrmex ferrugineus, by April Nobile. Via AntWeb
Pseudomyrmex ferrugineus, by April Nobile. Via AntWeb

Orona-Tamayo and Wielsch found that Pseudomyrmex gracilis—a species that exploits the acacia’s rewards without ever lifting a mandible to defend it—isn’t quite as well-equipped as the P.ferrugineus. It has some chymotrypsin-1, but also plenty of other proteases that are inactivated by the acacia’s neutralising enzymes.  It gets something out of the food bodies, but not as much as the tree’s true partner.

There are other examples in the natural world of alliances where partners lock each other into exclusive contracts. Some do it physically. Many flowers hide their nectar at the bottom of long tubes that only the right pollinators can reach them, whether they’re long-billed hummingbirds or long-tongued flies.

In these cases, it’s clear that the flowers and their pollinators evolved alongside one another. As nectar tubes got longer, so did bills and tongues, until both fit together like locks and keys. Is the same true for the acacia and the ant? It’s possible, but the team suspects that both partners came prepared for exclusivity.

The acacia uses the same protease inhibitors as many other related plants, and many ants and spiders* have chymotrypsin-1 in their guts. The tree eventually concentrated its inhibitors into its food bodies, while its ant partners emphasised chymotrypsin-1 and downplayed other proteases. They were already a good match from the start. They just became closer over time.

*This might be why the world’s only vegetarian spider, Bagheera kiplingi­­, can get away with eating acacia food bodies.

Reference: Orona-Tamayo, Wielsch, Blanco-Labra, Svatos, Farias-Rodriguez & Heil. 2013. Exclusive rewards in mutualisms: ant proteases and plant protease inhibitors create a lock–key system to protect Acacia food bodies from exploitation. Molecular Ecology http://dx.doi.org/10.1111/mec.12320

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How The “Anternet” Succeeds by Showing Restraint

Some people show more self-control than others. If a marshmallow is placed in front of me, I will probably eat it. You might exercise more restraint than me, or you might not. Either way, your actions will depend on thousands of neurons in your brain. It would make little sense to describe any one of these neurons as showing restraint. They’re just interacting with each other in simple ways, and restraint emerges from these interactions.

You can see the same emergent behaviours in an ant colony. The red harvester ant mainly eats seeds. Some colonies go out searching for seeds no matter the weather but others hold back on dry days when they risk death by dehydration. Deborah Gordon from Stanford University has found that these restrained colonies are actually more successful as those that forage come rain or shine. And they even pass on their reserved behaviour to the next generation.

But it’s not that each individual ant is showing lots of self-control, any more than a single neuron does. The worker isn’t not weighing up how much food the colony already has. It’s not making active decisions based on the weather.

Here’s all that happens: If workers bump into others who have returned to the nest with food, they’re more likely to go out on their own foraging trips. Every colony has its own bar for the number of interactions it takes to make a worker leave to find food. That’s it.

These rules, which Gordon discovered last year, are similar to protocols that control traffic on the Internet—the Anternet, as she calls it. They allow the harvesters to tune their behaviour to their environment without any conscious knowledge. If there’s lots of food around, more foragers will return with seeds, stimulating more nest-bound workers to venture out. If seeds are scarce, fewer ants leave the nest. The size of the foraging parties automatically change to fit the amount of food around.

Harvesters live in deserts and lose water whenever they leave the nest. But they can only gain water by finding seeds. “Colonies must spend water to get water,” explains Gordon. Colonies balance their water budget differently. In some, workers are slightly and consistently less likely to respond to incoming foragers when the humidity is low. “This adds up, year after year, to less foraging activity by that colony,” says Gordon. On some days, not a single ant will leave these nests.

Each ant is behaving like a neuron in a brain, part of a literal hive-mind. It’s just going about its business and interacting with its colony-mates according to extremely simple rules. From these connections, restraint emerges. It’s a collective behaviour that happens at the level of the colony.

What effect does this have in the long-term? That’s a difficult question since harvester colonies can live for 25 years, and produce daughter colonies for 20 of those. To understand how their foraging differences pay off in the long run, you’d need to study a group of harvesters for decades.

That’s exactly what Gordon has done. Since she was a graduate student in 1985, she has almost single-handedly kept an annual census of around 300 harvester colonies in an area of New Mexico (here’s her TED talk on the work). Her records showed that the colonies that hunker down on dry days are just as likely to survive as those that head out all the time. Even though they may stay indoors up to two-thirds of the time, they can find enough seeds on good days and lose fewer workers in the process. They even seem to produce more daughter colonies. “In much of foraging theory, it’s assumed that ‘bigger is better’ – the more food collected, the more successful the forager,” says Gordon. “These results show the opposite.”

Iain Couzin from Princeton University, who studies collective behaviour, calls the study a “remarkable feat” and is particularly impressed that the collective foraging strategies seem to be heritable. Daughter colonies forage in very similar way to their parents. “Due to the relatively large distance between parents and offspring, it is unlikely that such synchronization is based on cultural transmission of behaviour,” he says.

For decades, scientists have been studying how animal swarms, from locusts to fish to birds, move as one, and even think as one. (You can read more about this in my Wired feature on the science of swarms.) But recently, they’ve begun to look at not just how collective behaviours work, but how they evolve.

Couzin, for example, showed that shoals can evolve to fool predators, even if the prey animals don’t know they’re in danger. Another group showed that parasites can make shrimps gather in shoals so they’re more likely to be eaten.

Gordon’s study is slightly different – it’s not about how collective behaviour arose, but how it continues to evolve. “It’s the first study of how natural selection is acting on collective behaviour in a natural population,” she says.

More on collective behaviour:

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Ants disinfect their young by drooling backside poison

Personal hygiene is paramount for insects, as it is for us. They too must contend with a cavalcade of bacteria and fungi that threaten to soil their food and infect their bodies. And cleanliness is one of their most important defences against such infections. This goal is familiar to us, but the methods are… unorthodox.

Take the emerald cockroach wasp. Its larva disinfects its food, which happens to be is a zombie cockroach that the young wasp is devouring from the inside out. Its cleaning technique involves slobbering antibacterial fluids inside its living cradle-cum-larder. I’ve written more about the wasp’s behaviour at Nature News (including the see-through windows that scientists installed in cockroaches so they could watch the larva). Carl Zimmer also has you covered here at Phenomena.

For now, I’m going to deal with another recently discovered case of insect hygiene, involving ants that suck poison from their backsides and drool over their brood.

Ants, bees and other social insects live in dense colonies that could easily be wiped out by a single outbreak of disease. To prevent this from happening, the entire colony often acts as a giant immune system that works together to protect their health. Bees, for example, will sometimes buzz in tandem to raise their body temperature, creating a “social fever” that heats bacteria to death. Ants will groom each other to remove the spores of parasitic fungi.

Sylvia Cremer from the Institute of Science and Technology in Austria has spent years studying these hygienic behaviours, and she has found a new one in the garden ant Lasius neglectus. These ants are known to lick fungal spores off their larvae, which they store in pockets in their mouths. Later, they spit these out in small pellets. Now, Cremer has found that the ants have another trick: They slather a poisonous fluid over their denuded youngsters, turning their mouths into temporary “chemical disinfection chambers.’’

Cremer’s team members Simon Tragust and Barbara Mitteregger noticed that the ants don’t manage to clean every spore from their larvae. But even those they miss are much less likely to germinate. The secret to this suppression lies in the ants’ back ends, and specifically in a hole called the acidopore. If the team plugged this opening, the ants were only half as good at suppressing the growth of fungal spores.

The acidopore connects to an ant’s anus, pheromone gland, and poison gland. It’s that last one that matters. When Tragust ad Mitteregger gently poked the ants from behind, they would release their poison as a droplet. Depleted of this chemical supply, they lost their antifungal powers.

The droplet that comes out of the acidopore contains 37 chemicals, but mostly formic acid—a pungent substance that many ants use to defend themselves from predators. Formic acid even gets its name from ‘formica’, the Latin word for ‘ant’. It makes up 60 percent of the acidopore droplets, and accounts for 70 percent of its fungus-busting powers. The other substances, such as vinegary acetic acid, did very little on their own but gave an extra boost to formic acid’s antifungal properties.

Some of the ants would spray the poisonous cocktail directly onto the larvae, but most of them took an indirect route. They ‘licked’ their acidopore and sucked up their own poison into their mouths. They then cleaned their brood, and slathered the acidic poison over them in the process.

This method has many benefits. Tragust and Mitteregger think that it allows the ants to apply their precious chemicals more accurately and evenly. It also means that they disinfect their own mouths, since the acid kills the spores that they lick off the larvae, and stops any pellets they spit out from germinating.

The garden ant and the cockroach wasp are just two new examples of insects acting as their own pharmacists—using self-made chemicals to defend themselves against bacteria. In the simplest strategies, they groom themselves with defensive chemicals. Ants and termites do this, as do rove beetles groom themselves, which use a substance called stenusine to walk on water as well as repel fungi and bacteria.

Some species use their chemicals to clean their homes, relatives, or food supply. Some bees and wasps incorporate their venom into the building materials for their nests. Fire ants apply antibacterial chemicals onto their eggs, and liberally spray the stuff into the brood chambers, where the eggs are kept. The European beewolf—a  type of parasitic wasp—embalms the honeybees that it provisions for its larvae, by covering them with an oily secretion that stops water from condensing and makes it harder for fungi to grow.

At a time when many human societies lack decent sanitation, and others have only enjoyed it for a few centuries, it’s sobering to remember that insects have been practicing careful hygiene for millions of years.

Reference: Tragust, Mitteregger, Barone, Konrad, Ugelvig & Cremer. 2012. Ants Disinfect Fungus-Exposed Brood by Oral Uptake and Spread of Their Poison. Current Biology http://dx.doi.org/10.1016/j.cub.2012.11.034

Herzner, Schelcht, Dollhofer, Parzefall, Harran, Kreuzen, Pilsl & Ruther. 2013. Larvae of the parasitoid wasp Ampulex compressa sanitize their host, the American cockroach, with a blend of antimicrobials. http://dx.doi.org/10.1073/pnas.1213384110

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The world’s smallest fly probably decapitates really tiny ants

Illustration by Inna-Marie Strazhnik

Some flies, known as phorids, specialise in decapitating ants in a gruesome way. They lay their eggs inside their victims. When the maggots hatch, they move towards the ant’s head, where they gorge upon the brain and other tissues. The ant stumbles about in a literally mindless stupor until the connection between its head and body is dissolved by a enzyme released from the maggot. The head falls off and the adult flies burst out.

There are hundreds of species of phorid flies, each one targeting its own preferred ants. But some ants are naturally defended against these parasites because they’re incredibly small. Most phorids are a few millimetres long. If an ant is the same size, its head wouldn’t be roomy enough for a developing fly. Thailand, for example, is home to an acrobat ant (Crematogaster rogenhoferi) which can be just 2 millimetres long. Surely these workers are safe from decapitating parasites?

No, they’re not. Brian Brown from the Natural History Museum of Los Angeles County has just discovered a Thai phorid that’s just 0.4 millimetres in length. It’s the world’s tiniest fly, small enough to sit comfortably on the eye of a common housefly. It’s easily small enough to fit inside the head of even the smallest acrobat ant. It just goes to show that there is no way of truly escaping from parasites. If you evolve a miniscule body, they will shrink even further in pursuit.


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Meat-eating plant digests insects using ants

Many insects eat plants, but some plants can turn the tables on their would-be diners. The pitcher plants are among several groups that can capture insects and digest their flesh. And one species – the fanged pitcher plant – goes even further. It digests insects with insects.

There are around 120 species of pitcher plants and all of them have large leaves that fold to produce fluid-filled traps. The rims of the pitchers are usually extremely slippery, and insects that wander by lose their foothold and fall into the pool of fluid within. There, they drown and are digested by the plant.

The fanged pitcher is unusual. Its rim lacks the usual waxy layer and is less slippery than those of its cousins. And it’s the only species that recruits ants. The base of each pitcher contains a swollen tendril that houses ants of the species Camponotus shcmitzi. These insects are permanent residents; they’ve never been seen in any other plant.


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Top tip: do not steal food from ant traps

The Amazonian tree known Hirtella physophora looks rather unassuming, but it is the site of several grisly spectacles. Amid its leaves and branches, an animal, a plant and a fungus conspire to create a nightmarish trap where trespassers become meals, robbers get the death penalty, and assassins are assassinated.

The tree is home to ants called Allomerus decemarticulatus, which defend it from hungry insects. In return, the tree provides the ants with leaf pouches and swollen thorns as shelter, and feeds them with nectar and sugary nodules. These food sources are rich in carbohydrates but low in proteins. To supplement their diets, the ants need flesh, and they get it by shaping the tree into traps.

The ants cut hairs from the plant and weave them together into a hollow gallery, which extends down the side of the tree’s branches. Within the gallery, the ants hide inside small holes, jaws agape. From the outside, nothing can see them. If an insect lands on the trap, hundreds of lurking jaws seize its legs and pull it spread-eagled, as if on a medieval ‘torture rack’. The victim is overpowered and dismembered.


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Return of the supersoldier ants

One massive group of ants has a secret supersoldier programme that’s been locked away for 35 to 60 million years.

The Pheidole ants are an exceptionally diverse group with over 1,100 species. They’re also known as big-headed ants because their soldier caste has unusually large heads. Until now, we knew that a few of the Pheidole – just 8 out of 1,100 – can also produce supersoldiers, which are even larger than normal soldiers and have even more enormous heads. They use their outsized noggins to block their nest entrances against invading army ants.

Now, Ehab Abouheif has found that the supersoldiers are the result of a genetic programme that runs throughout the entire Pheidole dynasty. It’s likely that every single species in the vast group has the hidden ability to make this special caste. In fact, Abouheif managed to induce supersoldiers among species that don’t usually recruit them, with just a dab of hormone.

I wrote about this study for Nature, so head over there to read all the details. It’s a great evolutionary story.


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Look, no hands: ants kill termites with airborne chemical weapons

Many insects are armed with venom, which they can inject into their enemies via a sting. The African ant Crematogaster striatula is no exception, but its arsenal has a disturbing twist – its venom goes airborne. The ant can raise its sting and release its toxins as an aerosol spray. Its targets are termites, whose nests it raids. Even without making any contact, the ants can induce seizures in the termites, eventually paralysing them.

All Crematogaster ants have a mobile sting. The sting sits on the ant’s rear-end, which connects to its torso by a flexible stalk, so the ant can aim it in virtually any direction. Aline Rifflet from the Jean-Francois Champollion University Center saw this ability in action when she watched C.striatula take on termites.


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Fire ants conquered America by monopolising calorie-rich food

They came to America and found a nation overflowing with calories. Carbohydrate-rich fast food was available on every corner, and with little competition for it, the migrants ate their fill. Soon, they started spreading throughout this new land of opportunity. They are red imported fire ants (Solenopsis invicta) and their invasion is well underway.

The fire ant is an international pest. It devastates native ants, shorts out electrical equipment, damages crops, and inflicts painful stings. It hails from Argentina, but it was carried to the United States aboard cargo ships that docked at a port in Alabama. That was in the 1930s; since then, this invader has spread throughout the southern states, from California to Florida. The country spends over a billion dollars every year in attempts to stem the invasion.

Now, Shawn Wilder from Texas A&M University has found that their remarkable invasion has been driven by partnerships with local insects. The fire ants run a protection racket for aphids and other bugs, defending them from other attackers. In return, they get honeydew, a sweet nutritious liquid that the bugs excrete, after they suck the juices of plants. They are both farmers and bodyguards.


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Fire ants assemble into living waterproof rafts

What happens when you dump 8,000 fire ants into a tray of water? Nathan Mlot from the Georgia Institute of Technology wanted to find out. Mlot scooped the ants into a beaker, swirled it around to roll them into a ball, and decanted them into a half-filled tray.

Over the next three minutes, the ball of ants slowly widened and flattened into a living, waterproof raft. By trapping air bubbles trapped among their interlocking bodies, the ants boosted their natural ability to repel water and kept themselves afloat. Humans build rafts by lashing together planks of wood or reeds; the fire ants do so by holding onto each other.

The experiment might seem odd, but it mirrors conditions that the fire ant (Solenopsis invicta) regularly has to cope with in its natural environment. The ant hails from the Brazilian rainforest floodplains of Argentina, where rising water regularly submerges their nests. They respond by weaving their own bodies into rafts. The ants also come together to construct bridges, ladders and walls, but the rafts are the longest-lasting of these living structures. In this form, they can float and sail for months.


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Pocket Science – wasps airlift ants away from food

It’s not a very fair fight. In one corner is a tiny ant. In the other is a large wasp, two hundred times heavier and capable of flying. If the two of them compete for the same piece of food, there ought to be no contest. But sometimes the wasp doesn’t even give the ant the honour of stepping into the ring. It picks up the smaller insect in its jaws, flies it to a distant site and drops it from a height, dazed but unharmed.

Julien Grangier and Philip Lester observed these ignominious defeats by pitting native New Zealand ants (Prolasius advenus) against the common wasp (Vespula vulgaris). The insects competed over open cans of tuna while the scientists filmed them.

Their videos revealed that ants would sometimes aggressively defend their food by rushing, biting and spraying them with acid. But typically, they were docile and tolerated the competing wasp. Generally, the wasp was similarly passive but on occasion, it picked up the offending ant and dropped it several centimetres away. In human terms, this would be like being catapulted half the length of a football field.

The wasps never tried to eat the ants, and they never left with one in their jaws. They just wanted them out of the picture. Indeed, the more ants on the food, the further away the wasps dropped them. This may seem like an odd strategy but at least half of the dropped ants never returned to the food. Perhaps they were physically disoriented from their impromptu flight, or perhaps they had lost the chemical trail. Either way, the wasps could feed with fewer chances of taking a faceful of acid.

Reference: Grangier and Lester. 2011. A novel interference behaviour: invasive wasps remove ants from resources and drop them from a height. Biology Letters http://dx.doi.org/10.1098/rsbl.2011.0165