Social lemurs make better larcenists, scientists say

If you ever have the occasion to dine with a lemur, choose one from a small social group.

Researchers from Duke University have found that lemurs living in large social groups are smarter thieves, a find that could have implications for how scientists measure primate intelligence, which is usually correlated with brain size.

Researchers first trained 60 lemurs to see humans as their competitors for food and then arranged an experiment that gave the animals a choice between which human to steal food from: A person placidly staring down the lemur, or another person facing away from the lemur, their food left vulnerable. 

The lemurs from small social groups were indiscriminate in which food they went after, reaching as often for the vulnerable food as they did for the well-protected items. But the lemurs that came from large groups were savvier. They could read the social cues, and those cunning animals were more likely to target the food that the humans had left foolishly unguarded.

Scientists said that the lemurs in small and large social groups have brains of roughly the same size. Usually, brain-size is an indicator of how well an animal will score on intelligence tests, say researchers. But this study, says the scientists, suggests that social factors could also influence animal cognition and that more studies are needed that test forms of animal intelligence un-related to brain size.

“These data provide evidence for a relationship between group size and social cognition in primates, and reveal the potential for cognitive evolution without concomitant changes in brain size,” the scientists wrote in the study, published in PLOS ONE.

The results offer tentative support for what is called the “social intelligence hypothesis,” which proposes that a group living selectively favors cognitive skills that help animals compete for food and mates within the group, while also maintaining the stability of large social groups. Animals who have those skills are favored in natural selection, meaning that the group as a whole will evolve to have heftier cognitive abilities than animals that have not experienced that natural selection in their small groups.

The stealthy knowledge that the large-group lemurs used in the experiment would also have been useful in their social groups, in which underling individuals must procure food or seize mating opportunities in secret.