No dummy. Although not considered as cognitively advanced as some apes, spider monkeys perform better on a test of behavioral flexibility.

Filippo Aureli

You've Got to Be Flexible to Live in Complex Societies

The social life of humans is seldom dull. Every day, most people hang out with family, work with colleagues, and commute with strangers. Sometimes we get divorced or change jobs. Chimps also shift loyalties and affiliations, called "fission-fusion" behavior, but not all primates do this. A new study suggests the reason isn't just brainpower; rather, it has to do with the flexibility of their behaviors.

To arrive at the findings, animal behavior experts Federica Amici and Filippo Aureli of Liverpool John Moores University in the United Kingdom and Josep Call of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, tested the behavioral flexibility of seven primate species. The team presented the animals with five tasks that required them to exercise restraint when given the chance to grab food that they liked, a proxy measure for behavioral flexibility called "inhibitory control." In one task, the primates could choose between one piece of food right away and three pieces of food if they waited various periods of time. In another, the animals were placed in front of a clear acrylic box with two small swinging doors that opened only toward the inside; a piece of food was placed on one of two small shelves behind the doors, so that it would fall off if the subject tried to get it via the direct route rather than reaching around through the adjacent door.

The researchers found that primates that live in fission-fusion societies, such as chimpanzees, bonobos, orangutans, and spider monkeys, had much greater inhibitory control than gorillas, capuchin monkeys, and long-tailed macaques, which live in more stable, cohesive groups. This correlation held true even for species only distantly related on the evolutionary tree. Thus the gorilla, an ape, did much more poorly than its cousins the chimps, bonobos, and orangutans, while the spider monkey performed much better than the other two monkeys tested, even though apes are often assumed to be more cognitively advanced than monkeys. The findings, reported in the 23 September issue of Current Biology, support the hypothesis that fission-fusion animals have greater behavioral flexibility. That flexibility in turn enables their fluid social life, says Aureli.

Frans de Waal, a primatologist at Emory University in Atlanta, Georgia, calls the study "great work," adding that he was surprised by the spider monkey's high scores because it has a much smaller brain than the apes. But Andrew Whiten, a psychologist at the University of St Andrews in Fife, U.K., says the team hasn't really nailed down the link between behavioral flexibility and the fission-fusion lifestyle. To do that, he says, the researchers need to use tasks that directly test social interactions between the animals, such as competition or cooperation.