Lemurs in large social groups steal food smarter, study finds

Lemurs may not be the most academically gifted of the primate family, but don’t turn your back on these sly critters. Those in larger social groups are smarter thieves, new research has found. Just as city kids have a reputation for developing street smarts to survive, lemurs in larger groups develop some serious social savvy.

The findings, published in PLoS ONE, provide a window into the development of social intelligence -- and general intelligence -- in primates, which include humans.

Some researchers have long thought that as humans developed larger groups, it drove an increase in cognition to help navigate this increasingly complex environment. Others say that larger social networks would be linked only to an increase in social intelligence, not intelligence overall.

Either way, previous studies have indicated there’s a link between the size of a primate’s social group and the relative size of its brain, which seemed to imply that social groups were somehow contributing to the brain’s development.

To test these ideas, researchers at Duke University led a study involving six lemur species, each with a different average group size. The ring-tailed lemurs, Lemur catta, had the largest with 15.6 per group; the mongoose lemur Eulemur mongoz had the smallest group size, with just three members on average.

The researchers had about 60 lemurs go through a food test of sorts. They placed two humans in front of trays of food (grape leaves, for the most part) with one key difference between them: One human faced the food, while the other one faced away from the tempting meal.

Those lemurs who naturally lived in larger social groups were far more likely to grab the goods from the person looking away than from the person facing the food, the researchers found. The lemurs were using social cues that would come in very handy in the wild when competing with their peers for scarce resources, the researchers surmised. (In short, it’s easier to steal food when you’re not being watched.) The lemurs in smaller groups, however, would not have had to develop this social intelligence to compete within their own group.

But when the researchers covered the humans’ eyes or mouths with a black band, it didn’t seem to make a difference -- perhaps because blindfolds don’t naturally occur in lemurian society.

“Our data suggest that skills in this domain are related to a species’ typical group size, implicating a possible evolutionary relationship between sociality and cognitive skills for outcompeting others for access to contestable resources,” the authors wrote.

To see if the animals from larger societies were smarter overall, they showed lemurs a clear cylinder with some food in it and watched to see if they were able to figure out how to retrieve it from the side of the container without falling for their impulse to reach straight for the visible food. It’s an ability known as inhibitory control, and it’s been linked to decision-making -- and academic and economic success -- in humans.

They found that the lemurs’ social situation had no bearing on how well or poorly they did at this particular measure of smarts. They also found that brain size had little connection to the lemurs’ success on either task.

“These data suggest the possibility of cognitive evolution in the absence of corresponding changes in brain size,” the authors wrote.