Enjoy Wine? Thank a Wasp

Where would we be without yeast? Sober, for one thing. And stuck assembling our sandwiches between two crackers. Humans have relied on the hardworking microorganism for millennia to keep us fed and festive. Without realizing it, we may also have been relying on yeast's insect helpers: wasps that escort it around, store it during winter, and regurgitate it up for the next generation.

Yeast—avert your eyes now if you're squeamish about fungus—is a fungus. Just by going about its regular business, it makes our wines and beers alcoholic and our bread puffy. We use strains of the species Saccharomyces cerevisiae for most of these tasks, though there are many hundreds of other species in the world. And we've been doing it since ancient times: The DNA of S. cerevisiae used for winemaking has turned up in Chinese pots more than 7,000 years old.

The yeast strains we bake and brew with live all over the world. These days we produce them commercially, but they also appear in the wild. Yeast lives on the surface of ripe grapes, for example, where it can be used in the winemaking process. One of many unanswered questions about the history of our relationship with yeast is how it gets there, or anywhere else.


S. cerevisiae can't float through the air; it needs to be carried from place to place. So how have our favorite strains spread all over the world, even to places where humans don't take them? How does yeast appear among the grape vines just in time for the harvest each year?

Researchers led by Irene Stefanini and Leonardo Dapporto at the University of Florence suspected that wasps might hold an answer. Social wasps (the kinds that live together in colonies) are known for feeding on grapes. Additionally, the scientists had a hunch about how wasps might usher yeast from one year into the next, despite the brief lifespans they share with other insects.

From sites around Italy, the researchers gathered 61 wasps that included species of paper wasps (Polistes) and European hornets (Vespa crabro). Then they dissected their stomachs in search of yeast. Hundreds of varieties turned up, including 17 strains of S. cerevisiae. When they checked some honeybee stomachs for comparison, they found no S. cerevisiae strains. Wasps were the right track.

Unlike the other yeast species inside wasps, which tended to vary depending on the season the wasps were collected (and therefore what they were eating), S. cerevisiae remained at a steady level throughout the spring, summer, and fall. This supported the researchers' idea that wasps have a special relationship with S. cerevisiae, and showed that wasps could be responsible for spreading the yeast onto ripe grapes.

But they still had to demonstrate that wasps could account for the whereabouts of the yeast year-round, including in the winter. For this, they looked to certain celebrity females in the wasp world.

Each fall, as temperatures drop, most of the wasps in a colony die off. They leave behind young females that were fertilized during the mating season. These survivors hunker down to hibernate during winter. When it warms up again, the females emerge to start new nests. After these "foundresses" lay enough of their own fertilized eggs to create an entire new wasp colony, they die off too.

The researchers thought that foundress wasps might be the key: By feeding their larvae regurgitated meals, they could pass on the S. cerevisiae they've harbored in their guts all winter, completing the annual cycle and allowing new wasps to one day spread that yeast back to ripe grapes. They tested this idea by feeding young female wasps yeast that had been tagged with a fluorescent protein.

When those females were cut open after their winter hibernation, the fluorescent yeast was still in their guts. And when females were allowed to found new colonies, the tagged yeast appeared in the guts of their new larvae too. Although the larvae hadn't gone into the world to feed yet, regurgitated food from their mothers' gut had seeded new yeast collections in their bellies.

Just because wasps are a carrier for humans' preferred yeast strains doesn't mean they're the only carrier, the authors point out. Yeast might hide out in other homes during the winter. But social wasps certainly seem to be a close friend to S. cerevisiae. Looking at the genes of yeast found in wasp guts, the researchers found a menagerie that included wine, bread, and beer strains. It seems wasps have helped yeast maintain a diverse population around the world, and ensured that humans are never too far from our favorite microscopic fungus.

Irene Stefanini, Leonardo Dapporto, Jean-Luc Legras, Antonio Calabretta, Monica Di Paola, Carlotta De Filippo, Roberto Viola, Paolo Capretti, Mario Polsinelli, Stefano Turillazzi, & Duccio Cavalieri (2012). Role of social wasps in Saccharomyces cerevisiae
ecology and evolution. PNAS


Image: Thomas Quine/Flickr

It Takes an 8-Year-Old to Outsmart a Crow

When Aesop penned his fable about a thirsty hero who drops pebbles into a pitcher to raise the water to a sippable height, he was imagining a crow—not an elementary-schooler. And scientists have given real versions of this test to whole flocks of crows and related birds to test their smarts. Now they've turned the tables and given Aesop's test to children. The results are nothing for humans to brag about. By solving one kind of puzzle that stumped crows, though, the kids may have shown how a human mind treats problems differently than a birdbrain.

The fable-reenacting corvids I wrote about last year were New Caledonian crows. Faced with a clear tube that held a floating bit of food just below their reach, the crows were able to learn that dropping stones into the water made the food rise closer to their beaks. They learned to choose bigger objects over smaller ones, and to reject lightweight styrofoam chunks in favor of sinking rubber pieces. They also learned that none of these tricks worked in a tube filled with sand instead of water.

In a study published this week in PLoS ONE, researchers tested 80 children ranging from age 4 to 10 on tasks that Eurasian jays had faced in earlier studies. They guessed that as children grew older, they would be better able to solve the puzzles. Kids with more developed minds would deduce the relationship between throwing objects into a tube and getting the prize that floated inside. (Rather than a piece of meat, the prize for kids was a token that could be exchanged for a sticker of their choice).

The first two tasks were straightforward. In one, kids saw a tube of water and another of sawdust, both with sticker tokens sitting on top. If they learned (over the course of five trials) to drop stones into the water tube and ignore the sawdust one, they passed.

In the second test, kids saw one tube of water and a bowl of yellow "marbles." Half the marbles were actually balls of cork. The subjects had to learn to use the marbles to get their sticker token, ignoring the useless cork balls.

Between ages 4 and 7, kids were largely able to learn the solutions to these two tasks. This matches the performance of Eurasian jays and New Caledonian crows that took the same tests. It wasn't until children were 8 or older that they solved the puzzles on their first try, outdoing the clever birds. 

"We were surprised" that it took a kid as old as 8 to consistently outsmart a crow, says senior author Nicola Clayton. Of course, the circumstances weren't exactly equal. The birds were working for highly motivating bits of meat, for example, while the kids could only win stickers. And birds' eyes are right next to their beaks. But kids have hands.

The final task was one that the jays in an earlier study had never mastered. Subjects saw three clear tubes. The prize was in the center tube, but only the outer two tubes were wide enough to drop stones into. Invisible to the kids or birds, one of those outer tubes was connected in a U shape to the center one. So dropping stones into this tube would make the water in the middle rise too. It's simple enough to get the prize out, as long as you don't get hung up thinking that what you're seeing is impossible.

The youngest children were, like the birds, at a loss. But 7-year-olds could learn to solve the puzzle, and kids 8 or older mastered it quickly. When the researchers asked them afterward how the puzzle worked, some of the kids had it all figured out: "The purple one has a connecting pipe," said one 8-year-old. Others had learned the trick without guessing the mechanism: "One tube makes it go higher, the other doesn’t, dunno why," offered a 7-year-old.

The researchers who gave Eurasian jays this task—and watched them fail—guessed that the birds couldn't get past the counterintuitive relationship they were seeing. Human kids may have succeeded because their brains approached the problem differently. Seeing a clear cause and effect (a stone going into one tube and water rising in another), they could learn to use it to their advantage, even without understanding it.

Another possibility is that the kids simply had better hints. When the children first faced the baffling U-tube puzzle, researchers had to prompt them to try something and see what happened. The birds didn't have that advantage. Additionally, only two jays were tested in the earlier U-tube study. They were smart enough to solve the other tasks easily, but it's possible they have cleverer peers somewhere who could have solved the third task as well. "We just don't know," Clayton says.

Kids today see impossible-seeming things all the time. A wall switch illuminates an entire room; a Wii controller rolls a bowling ball; Grandma appears in the living room via Skype. It may be, the authors say, that accepting impossibilities isn't a normal stage of human brain development at all. Instead, it might be something humans learn when they grow up in technological societies. "This is a question we would love to pursue," Clayton says. 

Our familiarity with electronic gadgets might be the only thing giving humans a lead over crows in Aesop's challenges. Until that question is answered, we can hold on to the feeling of slight cognitive superiority for a little while longer.

Lucy G. Cheke, Elsa Loissel, & Nicola S. Clayton (2012). How Do Children Solve Aesop's Fable? PLoS ONE : 10.1371/journal.pone.0040574

Image: Illustration by Milo Winter for "The Crow and the Pitcher" from Aesop for Children, 1919; uploaded to Flickr by AnnaleeBlysse. Figures from paper by Cheke et al.

Geometry Proves Sheep Are Selfish Jerks

Sometimes what looks like friendly behavior is really an attempt to get one's neighbor eaten by a wolf before oneself. Sheep, for instance, seem cozy enough in their flocks. What's a better way to travel than surrounded by 100 percent merino? But the real reason they stick close to their neighbors is to save their own woolly rear ends.

The question of what motivates seemingly community-minded animals is a classic one in biology. Do the birds in a flock, or the fish in a shoal, just enjoy each other's company? Do they rely on each other's eyes to spot a predator before it gets too close? In an often-cited 1971 paper, British evolutionary biologist W. D. Hamilton suggested that the answer was simple geometry.

An animal never wants to be the nearest potential prey to a predator, Hamilton said. To increase its odds of survival against a lion, a wildebeest should put itself closer to other wildebeest. And even better than being near a few other defenseless grazers is being surrounded by a pack of them. The more edible neighbors are between you and a hungry mouth, the safer you are.

Hamilton's theory was called the "selfish herd." To find out whether a non-hypothetical herd really behaved as selfishly as Hamilton predicted, researchers led by Andrew King at the University of London used a flock of sheep wearing GPS-equipped backpacks. And one sheepdog.

Three times, the team gave the Australian kelpie the command to round up the flock of 46 sheep. With their backpacks on, the sheep responded as usual by running around in a panicked clump. Then the researchers used the GPS data from the backpacks to map exactly how each sheep had moved, second by bleating second.

They saw that the sheep started reacting when the running dog was about 70 meters away. A few moments later, the sheep had formed as tight a flock as they could.

A sheepdog (approaching from the top right corner) drives sheep into a clump in seconds.

In keeping with the selfish herd theory, the sheep used their flock to buffer themselves from a threatening attack. But the way they did it was surprising. Hamilton had suggested that animals might condense their herd, when a predator approached, by following an easy rule: Move toward your nearest neighbor. King's team found that the sheep were following a different guideline: Always move toward the center of the group. As if armed with a protractor and a map, sheep zoomed toward the middle of the flock, however dispersed it was to start with.

It's a slightly more complicated maneuver, since sheep must consider the positions of many neighbors at once. The authors write that while "a precise calculation of the flock centroid may be unlikely"—as sheep brains probably aren't equipped for much geometry—"sheep may be able to approximate where that target location ought to be."

King is interested in flocking, selfish or not, as a social behavior. His research group has also studied how individual fish "personalities" affect schools; how baboon troops forage for food and coordinate their travel; and even (using GPS baseball caps) how groups of humans move. But King's coauthor Jenny Morton has a different motivation. She studies brain degeneration in Huntington's disease and uses sheep as an animal model. By learning what rules the sheep in a flock usually follow, she hopes to better understand what happens as their brain cells—and their geometry skills—break down.

Andrew J. King, Alan M. Wilson, Simon D. Wilshin, John Lowe, Hamed Haddadi, Stephen Hailes, & A. Jennifer Morton (2012). Selfish-herd behaviour of sheep under threat. Current Biology, 22 (14) : 10.1016/j.cub.2012.05.008

Images: Sheep with GPS backpacks on. King et al., Current Biology Volume 22 Issue 14.

How We Changed Penguins Just by Watching

If a penguin falls in the forest and no one is there to hear it, I don't know what kind of forest that is—but everyone who's interested in penguins is probably hanging out a lot closer to the South Pole. The charismatic birds let scientists and tourists alike get a close look without too much trouble. And all that familiarity has the potential to change penguins, and other closely watched animals, for good.

King penguins (Aptenodytes patagonicus) appeal to zoo visitors and cold-resistant tourists by doing miniature tuxedoed human impressions, and to researchers by diving 100 meters into the ocean or carrying their chicks on their feet to keep warm. In the Crozet Archipelago, an island chain in the far southern Indian Ocean, scientists study the penguins as well as other polar items of interest from a permanent station called Alfred Faure. For 50 years, the camp has shared a small island with a colony of more than 48,000 penguins.

A group of researchers led by Vincent Viblanc from the University of Strasbourg wondered how decades of living near the human research station has affected this population of king penguins. To find out, they studied some penguins that live close to the camp and see humans at least once a day. They compared them to penguins from farther away on the island, where humans visit once a week or less.

The researchers caught 15 penguins from the frequently bothered group, plus 18 more from the group that was mostly left alone. After taping heart monitors to the birds' backs, they rereleased them. Then they gave the birds a series of three tests designed to stress them out. The tests were done in random order over the course of two days, while the heart monitors recorded how high the birds' heart rates rose and how quickly they returned to normal.

One test involved simply walking up to a penguin. A researcher approached the target penguin from a distance while the bird watched, then stopped 10 meters away and stood still for a minute before retreating. This was meant to mimic a tourist taking pictures, or a scientist recording an observation. In a second test, a researcher snuck up behind the penguin while it wasn't watching and banged two metal bars together loudly. This test, which would exercise most humans' hearts too, was meant to represent the loud noises a bird might hear while cranes and trucks move supplies into the research station.

The final test was a capture: what happens to a penguin if researchers need to physically examine it, say, or attach a band to its flipper. Each bird was immobilized for three minutes with a hood over its head before being let go.

Looking at the results from their heart rate monitors, the researchers saw that birds from the busier side of the island weren't as bothered by the stressors they experienced often. A human walking up close, or a loud mechanical noise, didn't bother these birds nearly as much as it bothered the birds from the quieter side of the island.

This might have been because the penguins were simply used to being harassed by humans. Alternately, over the decades of human activities in the neighborhood, all the most stress-sensitive penguins might have fled to a quieter area. "In the field, we notice that some birds are more sensitive to disturbance than others," Viblanc says.

The results of the capture test suggested the first explanation was right. During their brief kidnappings, the penguins from the frequently disturbed group were just as stressed out as penguins from the quiet population. The researchers think this is because captures don't happen very often, so being captured was probably a new experience for all the penguins in the study. Loud noises and visits from humans, on the other hand, are frequent happenings for the penguins living close to the camp. These everyday stressors have become no big deal.

If human activities had driven all but the most relaxed penguins away from the site, then these remaining unflappable penguins should have been calmer than their peers during a capture. Since this wasn't the case, it seems that "the lower stress responses are not a generalized phenomenon," Viblanc says.

Why does it matter? If humans drive animals with certain traits out of a population, we're performing our own (unnatural) selection. It's the difference between teaching your dog a trick and breeding a whole new kind of dog.

Penguins that have mastered the trick of ignoring harmless humans make scientists' work easier, and keep themselves less stressed out. But if humans cause selection for more relaxed animals—here or at sites with other wild animal populations—we might be creating problems for those animals. For example, could calmer penguins be too calm when facing a predator or other threat? "It is hard to say...what the consequences may be in terms of vulnerability to predators," Viblanc says.

There's another reason researchers like Viblanc are keeping a close eye on how well-meaning scientists and tourists affect animal populations. If we force individuals with certain traits away, the whole population becomes less genetically diverse. This gives them less flexibility for adapting to new challenges in their environment. And when a real cause for worry comes along—say, climate change—we want to be sure we've left the little guys with a fighting chance.


Viblanc VA, Smith AD, Gineste B, & Groscolas R (2012). Coping with continuous human disturbance in the wild: insights from penguin heart rate response to various stressors. BMC ecology, 12 (1) PMID: 22784366


Image: A king penguin, by Liam Quinn/Flickr

How Placebo's Evil Twin Makes You Sicker

Whenever a pharmaceutical company tests a new migraine prevention drug, nearly 1 in 20 subjects will drop out because they can't stand the drug's side effects. They'd rather deal with the headaches than keep receiving treatment. But those suffering patients might be surprised to learn that the drug they've quit is only a sugar pill: the 5 percent dropout rate is from the placebo side.

Lurking in the shadows around any discussion of the placebo effect is its nefarious and lesser-known twin, the nocebo effect. Placebo is Latin for "I will please"; nocebo means "I will do harm." Just as the expectation of feeling better can make our symptoms ease, the expectation of feeling worse can make it a reality.

In a review paper published last week in the German journal Deutsches Ärzteblatt International, researchers say doctors and drug companies are unwittingly introducing patients to the demon of nocebo. Led by Winfried Häuser of the Technical University of Munich, the authors say that nocebo in the doctor's office can add unnecessary pain and distress to ordinary procedures. In clinical drug trials, it can create side effects that shouldn't be there—and perpetuate them in the patients who will take that drug in the future.

Chemically, nocebo seems to use the same toolkit that placebo does. Say you have a headache and treat it however you normally like to—maybe with an ibuprofen, or a few drops of homeopathic whatever under your tongue. If you expect to start feeling better soon, your body will use internal molecules such as dopamine and opioids to start creating its own pain relief. (Depending on what treatment you've used, you may or may not get some chemical backup once it kicks in.) It's good old-fashioned conditioning, just like Pavlov's hungry dogs salivating before food was anywhere in sight. But in nocebo, when you expect your headache to get worse, your body turns the pain-relief machinery down instead of up.

Nocebo doesn't need a doctor's help to find you. But a doctor can harness it too. The standard assumption in medicine, Häuser and his coauthors write, is that patients should be warned ahead of time about anything painful ("You're going to feel a little pinch!"). But telling a patient to expect discomfort might actually make it worse. In one study, patients getting an injection felt more anxiety and pain when their doctors used words such as "sting," "burn," or "bad," even if the doctor was only trying to express sympathy.

In another study, women receiving epidural injections felt more pain when they were warned that the "big bee sting" would be the worst part of the procedure. When women were instead reminded that the injection would numb them and make them more comfortable, they experienced less pain. The authors point out that patients in emergency situations or facing major surgery are often in a "trance state" that makes them even more suggestible than usual.

Nocebo can really throw a wrench into clinical drug trials. Placebo is well accounted for; these trials always include a large placebo group in which patients are given a sugar pill or other fake treatment. To minimize the effect of suggestion, neither doctors nor patients know which group they're in. After the trial, researchers can subtract the positive effect seen in the placebo group from that in the patients taking a real drug, and see how much good their treatment really did.

In both the real and placebo groups, subjects report any side effects they experienced. When drug companies report the results of their trials, the Food and Drug Administration asks that they only report side effects (or "adverse events") that they have some reason to believe were caused by the drug. But the FDA acknowledges that this is "a matter of judgment."

As we saw with the migraine patients, side effects can be common even with a sugar pill. In one study, 44 percent of lactose-intolerant people reported gastrointestinal problems after taking a fake lactose tablet. (Impressively, a quarter of people without lactose intolerance also reported digestive troubles after taking the tablet.) And in a somewhat cruel prostate drug study, one group of subjects was told that sexual dysfunction was a possible side effect, while the other group wasn't. The better-informed group reported sexual side effects at a rate of 44 percent, compared to only 15 percent in the blissfully ignorant group.

Whatever side effects are attributed to a new drug, doctors may increase patients' odds of feeling those effects just by mentioning them. In Germany, Häuser says, "most of the product inserts contain very many potential non-specific adverse events, raising the risks of nocebo effects." So how can doctors avoid making their patients sicker?

Häuser and his coauthors have a couple of suggestions. Patients could consent to not be informed about mild side effects, knowing that just hearing about these effects makes them more likely. And doctors can phrase their warnings more positively, emphasizing that most patients respond well to a treatment rather than focusing on potential negatives.

"Doctors can and should be trained to positively use the power of their words," Häuser says. If we know where nocebo is lurking, we may be able to keep it far away.


Winfried Häuser, Ernil Hansen, & Paul Enck (2012). Nocebo phenomena in medicine: Their relevance in everyday clinical practice. Deutsches Ärzteblatt International : 10.3238/arztebl.2012.0459

Image: takgoti/Flickr

Brain Scans Predict When Poker Players Will Bluff

Your gray matter doesn't have much of a poker face. Even if your stony expression reveals nothing, two bits of brain tissue behind your ears leap into action when you decide to make a bluff. But it's not just about lying. Your brain is deciding whether your foe is a worthy opponent whose actions should affect your own.

Poker playing taps into a skill called social cognition. This is the mental power that lets us assess what other people are up to, figure out what they're thinking, and anticipate what they'll do next. It comes in handy not just for guessing when an opponent has pocket aces, but for everyday activities like negotiating a four-way stop or remembering which of your friends can't keep secrets.

To find out what goes on in our brains while we're judging different opponents, researchers at Duke University set up a very simple game of poker. Don't expect to see this new variant on ESPN anytime soon. The game is played between two players, each of whom is dealt either a high card or a low card. (Rather than numbers, each card reads simply "H" or "L.") The player who goes first decides whether to bet a set amount of money or to fold right away, ceding the ante to the other player. If the first player bets, the second player must decide whether to call the bet or fold.

The game had to be simple, since the 18 study subjects were playing inside a brain scanner. At the beginning of the experiment, subjects were introduced to Player Two (the same person each time). Inside the scanner, they played eight seven-minute rounds of the poker game by hitting buttons on a screen. But there was a twist: At the beginning of half of those rounds, subjects learned they'd be playing against a computer instead of a human.

Subjects were also reminded before playing that bluffing some of the time—that is, betting with a low card and hoping their opponent folded, leaving them the ante money—was a good strategy. The computer opponent was programmed to call players' bets half the time. The human Player Two had been promised extra payment from the researchers if he, too, called bets close to half the time. The result was that the computer and human opponents played the game the same way, says senior author Scott Huettel.

While this nail-biter of a competition was going on, researchers gathered fMRI data from their subjects. They searched everywhere in the brain for areas that lit up with activity when subjects were bluffing. Certain brain regions that had previously been tied to social cognition showed up, indicating the researchers were on the right track.

But there was only one brain region that distinguished between human and computer opponents, the authors reported last week in Science. When the subjects lying in the scanner decided to bluff against a human component—but not against the computer—an area called the temporal-parietal junction (TPJ) suddenly blazed with activity.

Breaking down their results subject-by-subject, the researchers discovered a final twist. After the poker games, they'd asked each subject: Who was a better opponent, the computer or human? "Eleven of the participants said the human," Huettel says, "and the effect was present in those people, but not in the others." In other words, the TPJ only jumps into action when people are bluffing against a human opponent whom they respect.

The TPJ effect was so pronounced that the researchers could use it to tell the future. Huettel says they created a model using one part of their fMRI data, then returned to the rest of the data to test that model. Based on activity in the TPJ when subjects were shown a low card, the researchers could successfully predict whether that subject would bluff or fold about six seconds later.

"There has been an ongoing debate about whether this region is involved in social cognition," Huettel says. The poker study suggests that we do use our TPJs for social cognition, but only in cases where we're basing our actions on another person's behavior. People who thought the computer was a more formidable opponent might have ignored their opponents altogether while deciding whether to bet or fold.

In a real poker game—where there are 52 possible cards rather than two, and decisions about betting or folding are complicated—the TPJ might be at work more often. The activity that predicted a bluff in this study might simply show players are thinking hard about their opponents' cards in a true game of Hold 'Em. Either way, until someone develops remote brain imaging technology, there's probably no need to purchase cerebral sunglasses.

Carter RM, Bowling DL, Reeck C, & Huettel SA (2012). A distinct role of the temporal-parietal junction in predicting socially guided decisions. Science (New York, N.Y.), 337 (6090), 109-11 PMID: 22767930

Image: Rob Watkins/Paf (Flickr)

Why Successful Leaders Share Their Harems

A male monkey cruising through the grasslands with a harem of females all to himself might seem to have hit the primate jackpot. What simian doesn't want a dozen lady monkeys to bear his long-tailed young and then tote them around? But he's better off sharing the wealth. By letting other males infiltrate his group and mate with his females, the leader lands himself a longer term in office—and more, not fewer, of his own offspring.

The gelada (pronounced not unlike the Italian frozen dessert) is a social animal that lives only in Ethiopia. Males have luxurious hair capes and bright red hourglass shapes on their chests. The lucky ones live in groups called reproductive units that include one "leader" male and up to twelve females. About a third of reproductive units also include one or more "follower" males.

Unattached males, meanwhile, live in bachelor groups that wait for the right moment to attack leader males and try to take over their women.

Attention Marvel: New character concept? Cape is cool but logo needs tweaking.

For females, it doesn't much matter which males they're following around. But for male geladas, society is deeply divided between haves and have-nots. Leader males have mates, and therefore babies. Almost everyone else is stuck waiting their turn to contribute to the gene pool.

Researchers led by Noah Snyder-Mackler at the University of Pennsylvania wanted to investigate gelada society more deeply. Does a leader male sometimes allow follower males to intrude into his group, and mate with his females, only because it's too hard to fight them off? Or does the leader actually gain something from letting his mates have a couple boyfriends on the side?

In a study that will prepare their resumés for future work on daytime talk shows, the team went into the field and did some paternity testing. They collected feces from all the babies born during their three-year study period, as well as the mothers and potential fathers, which gave them enough DNA to find out where all the young monkeys had really come from.

They also closely observed the dynamics of the 21 reproductive units in their study population. When did bachelor groups clash with the males in reproductive units? Were the attacked males leaders or followers, and how effectively did they fight back? For how long did leaders keep control of their groups before being ousted by another monkey?

A gelada reproductive unit.

The researchers found that leader males could be kicked out of their groups at any time; the longer they stayed, the more likely they were to go. The average leader held onto his group for 3 years or so.

While those males were in charge, they kept a tight rein on the females in their groups. Leader males that had a whole reproductive unit to themselves fathered 100 percent of the offspring born. (Impressive, considering gelada groups come together in very large meet-ups—as many as 1100 monkeys at a time—to forage for food. Apparently those conventions are all business.) When follower males were present, all the mating stayed inside the group, but now leaders fathered just 83 percent of the young.

Despite losing out on fatherhood, though, leader males experienced a clear advantage when they let follower males into their groups. Followers helped defend the group against aggressors; units with multiple males were less likely to be taken over by interloping bachelors. Consequently, the leaders of these groups stayed in charge for 30 percent longer than males who had a group all to themselves. Units with more than one male also had greater numbers of females.

Between the extra mates and the longer time in office, leaders who put aside their jealousy and allowed followers into their groups ended up fathering more babies than leaders who flew solo. On average, leaders with followers had 7 babies before getting kicked out of power. Solo leaders had just 4.

Snyder-Mackler writes that systems like the geladas' could help explain how social behavior evolved in mammals in the first place. For the monkeys, playing nice with potential competitors isn't about being friendly—it's a fruitful reproductive strategy. Follower males don't father very many children, but they do better than bachelors. And later, they might become leaders of their own groups.

The system benefits female geladas too: When bachelors take over a group, they may kill the babies that are there already. Living in a group with multiple males gives females a little more security for their infants, since the group isn't as likely to be taken over. Females may even encourage leader males to allow followers. After all, a little mate-sharing is the neighborly thing to do.

Snyder-Mackler N, Alberts SC, & Bergman TJ (2012). Concessions of an alpha male? Cooperative defence and shared reproduction in multi-male primate groups. Proceedings. Biological sciences / The Royal Society PMID: 22764162


Images: Male gelada focusshoot/Flickr; gelada group dave watts/Wikimedia Commons; female gelada mc_bos/Flickr.