Cunningly stealing food while no one is looking instead of pilfering out in the open is a sign of advanced "social intelligence," according to recent research on lemurs, which may indicate that brain size is not the only factor in how intelligent a creature is.

Researchers from Duke University studying personality traits in six species of lemurs suggest that lemurs that live in large social groups tend to have more social intelligence - the capacity to effectively navigate complex social environments.

To reach their conclusion, researchers from Duke tested 60 individual lemurs living at the Duke Lemur Center to see if they were likely to steal a piece of food if a human wasn't watching them.

The three-phase experiment involved introducing a lemur to a room where two people sat near plates of food. Phase one presented the lemur with a human facing toward it and the food, and a human with his back turned to the food. In the second phase, the testers sat in profile, facing towards or away from the plate. Lastly, the testers were both placed directly in front of the plate, with one tester blindfolded and another with a black band covering their mouth.

Evan MacLean, the research leader from Duke's department of evolutionary anthropology, said that as the lemurs jumped onto the table where the plates were and decided which bit of food to go for, the lemurs from large social groups, like the ringtailed lemur, were apparently more sensitive to social cues that eyes were watching them, whereas lemurs living in small social groups, such as the mongoose lemur, were less sensitive to the humans' orientation and whether or not someone was watching. Researchers concluded that lemurs living in large social groups are more likely to steal from behind your back than in front of your face. 

The significance of the blindfold was poorly understood by most of the lemurs, the researchers reported.

McLean said the study is the first to test the relationship between group size and social intelligence across multiple species. The results of the experiment, McLean said, are in support of the "social intelligence hypothesis," which suggests that living in large social networks drove the evolution of complex social cognition in primates, including humans.

The behavioral experiment supports the notion that brain size is not the sole indicator of advanced intelligence. McLean noted that his study found that some lemur species had evolved social smarts without increasing the size of their brains.

Another recent study of Moroccan Barbary macaques found that monkeys with a larger network of social companions were more likely to survive throughout extreme weather conditions, another indicator of advanced social intelligence. 

Results of the lemur study are published in the journal PLOS ONE