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Lemurs' group size predicts social intelligence

Primates that live with many others know not to steal food when someone is watching

Ring-tailed lemurs like those pictured were less likely to steal food with a human watching than were lemurs that live in smaller groups.

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Never turn your back on a lemur — especially if it’s a member of a big gang.

Lemur species that live in large groups can tell when to steal food from a competitor in a lab experiment, researchers report June 26 in PLOS ONE. The finding supports the idea that brainpower in primates evolved to fit their complex social lives. Because the sneakier lemurs don't have bigger brains than less sneaky ones living in smaller groups, researchers suggest that social smarts don’t always depend on brain size.

Much of the evidence for sociality’s role in the evolution of intelligence comes from indirect measures such as brain size, says study coauthor Evan MacLean of Duke University. But brain size does not always correspond to brainpower, so MacLean uses behavioral tests.

He and his colleagues tested the social intelligence of six species of lemur, primates from Madagascar distantly related to monkeys and apes. Each of the species lives in social groups ranging from families of just three, mongoose lemurs’ preferred posse, to gangs of about 16, a typical size for a group of ring-tailed lemurs.

The researchers trained lemurs to view humans as competitors for food, then presented the animals with a choice between pilfering treats from one of two people: one facing the animals or another with his or her back turned. Species that live in small groups reached for the food under a competitor’s nose as often as they did behind people’s backs. But the ring-tailed lemurs were much more likely to choose the unguarded food.

Most intriguing, says Susanne Shultz, a biological anthropologist at England’s University of Manchester, is that group size was much better than brain size at predicting which species would perform well in the social intelligence test. “We assume that if you have a bigger brain you should be better at solving problems,” she says.

Her own research suggests that brain size in primates does correspond to cognitive ability, but she used a different set of intelligence tests and more kinds of primates. She says she would be interested to see how other species perform on MacLean’s tests.

MacLean agrees that a general relationship probably exists between brain size and cognition. But he speculates that this might not be the case with very closely related species like the lemurs. "Cognition might evolve a little bit ahead of brain size,” he says.

In pre-trial training, a ring-tailed lemur learns that a person watching is a competitor for a morsel of food. The animal then chooses to take food from behind a person’s back, rather than from a watchful person.
Credit: E.L. MacLean et al./PLOS ONE 2013.

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