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Larger monkey groups lose fights because they contain more deserters

In the Battle of Rorke’s Drift, 150 or so British troops defended a mission station against thousands of Zulu warriors. At the Battle of Thermopylae, around 7,000 Greeks successfully held back a Persian army of hundreds of thousands for seven days. Human history has many examples of a small force defeating or holding their own against a much larger one.

Among animals too, the underdogs often become the victors. One such example exists in the rainforests of Panama. There, capuchin monkeys live in large groups, each with its own territory. The monkeys often invade each other’s land. Numbers provide an obvious advantage in such conflicts, but small groups can often successfully defend their territory against big ones. Unlike human underdogs, they don’t win because of superior tactics or weapons. They win because their rivals are full of deserters.

Margaret Crofoot from the Smithsonian Tropical Research Institute, and Ian Gilby from Duke University have spent many years studying wild white-faced capuchins on Barro Colorado Island in Panama. They’ve fitted individuals from six different groups with radio collars, so they can be easily tracked.

In 2008, they showed that on the whole, larger groups are more likely to win fights than smaller ones. Each extra member increases the odds of victory by 10 percent. But these odds vary greatly depending on where the capuchins are. For every 100 metres that they move away from the centre of their territory, their odds of victory fall by 31 percent. If they invade another group’s patch, their distance from home can greatly outweigh the benefits of a large party.

Now, Crofoot and Gilby have worked out why. They simulated incoming invasions with hidden speakers, which played recordings of rival capuchin groups of varying size. Oddly, they found that monkeys are more likely to run away from the speakers when they belonged to larger groups.

Rather than taking strength in the fact that they outnumbered their foes, the monkeys realised that they could afford to cheat. In larger groups, each monkey has a proportionately smaller effect on the outcome of the fight, so each is more likely to desert the battle altogether. If enough do so, the big group can snatch defeat from the jaws of victory. In fact, Crofoot and Gilby found that for every extra group member, the odds that any individual will become a turncoat go up by 25 percent!

As before, location matters: monkeys were almost twice as likely to flee at the edge of their territory as at its centre. When they decide whether to fight or flee, they factor in both where they are and how many friends they have.

The upshot of this is that capuchin territories are remarkably stable. Even though competition is fierce, the monkeys have a natural home-field advantage. When they’re defending the centre of their home ranges, they’re better at converting any numerical advantage into victory. When they’re far from home, and encroaching into another group’s range, even a large force soon loses its advantage as its members flee. This also explains why large groups don’t simply sweep through the jungle, displacing or killing smaller ones on their way.

Now, Crofoot and Gilby want to see if specific groups or other species of primates can overcome the problem of defection to better capitalise on the weight of numbers. The answers could tell us more about our own origins, especially if (as has been suggested) conflict between groups played a central role in our evolution.

Such studies could also inform our behaviour today. The dilemma faced by large groups of monkeys is one that we should all be familiar with. We are facing a multitude of big problems, including a changing climate, a massive loss of other species, and plummeting levels of valuable resources. These problems affect such large groups of people that there are good odds that any one individual will bow out of the fight. If enough do so, defeat will be assured.

Reference: Crofoot & Gilby. 2011. Cheating monkeys undermine group strength in enemy territory. PNAS http://dx.doi.org/10.1073/pnas.1115937109

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11 thoughts on “Larger monkey groups lose fights because they contain more deserters

  1. Quite an intriguing study. It seems to me a prime lesson here is the extra motivation that comes from defending turf as opposed to taking it: the defender has far more to lose than the attacker (and also knows the terrain better). And the threat *feels* proximate and ever so real — another big difference from the case with, say, climate change, where the threat seems distant and diffuse.

  2. A distinction between “defection” and “desertion” seems relevant here. “The problem of defection” hasn’t been demonstrated here, but would be equally interesting. Where possible, I expect it would favor bigger groups, and select against grudge-holders in small groups.

  3. It would be interesting to see if reduced desertion in a monkey group could lead them to take over the world. Of course they would have stiff competition from one that already has.

  4. A cogent argument against invading any foreign country. Defenders are fierce because it is HOME and ferocity can triumph over technology and numbers. Ask anyone who has ever fought a guerilla war. Heating fuel, fuses and cans can disable million dollar military vehicles.

  5. A better example might have been Custer’s last stand. That was only one battle of four or five, all of which U.S. troops lost, in a war that the united states lost. Another company of U.S. troops was wiped out the same day. An entire battalion was kept pinned down for 3 days without food or water. Custer thought he was the advanced force for a powerful column that was supposed to arrive on the other side of the Indian encampment and force its surrender. What he didn’t know was the Indians had forced the withdrawal of that column the previous day.
    That engagement involved the column trying to make its way7 up a valley with its wagons and artillery and finding their way blocked by Indians firing on them from a ridge. The general sent his best troops to clear the ridge, which they did with heavy casualties- and found there entrenchments and rifle pits the Indians knew how to construct because many were civil war veterans. The Indians had accurate repeating rifles and made their living by their marksmanship. They were defending their wives and children. The U.S. soldiers found the ridge raked by accurate rifle fire from entrenchments on neighboring ridges and could go no further. Most of the troops in the Army in those post civil war days were ne’er-do-wells and derelicts and the U.S. general knew night was falling and with his unreliable troops it was going to be pistols, knives, and tomahawks all night long, and come morning he would be lucky if there was any organization left in his command at all. He wisely withdrew. On reaching the little Big horn a few days later and discovering the slaughter there, he decided against pursuing Sitting Bull further- with the casualties he’d already taken, the demoralization of his troops, unreliable out the outset, and the likelihood of further prepared ambushes and counterattacks, he knew he’d been defeated. New York Times articles of the period describe the whole thing.

  6. I’d be curious to see why the center of the home range is of such importance. Are there specific topographic/vegetative features that lend themselves towards victory more than others? If so, can we find evidence of capuchins creating home ranges around more readily defendible areas? Or, is it just a matter of familiarity with the landscape and a sense of security being well away from the territory of your enemies?

    Interesting study!

  7. Good introduction with the analogy to the Greek and Persian armies. Strategy in a war is as important as the number of soldiers… maybe even more important

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