Signs of natural selection affecting the behavior of the descendants of a troop of free-ranging monkeys were found in an observation of their social interactions and an analysis of their family trees.
Rhesus macaques who had large, strong networks tended to be descendants of similarly social macaques, according to a Duke University team of researchers.
And their ability to recognize relationships and play nice with others also won them more reproductive success.
"If you are a more social monkey, then you're going to have greater reproductive success, meaning your babies are more likely to survive their first year," post-doctoral research fellow Lauren Brent, who led the study said.
"Natural selection appears to be favoring pro-social behavior," he said.
The analysis combined sophisticated social network maps with 75 years of pedigree data and some genetic analysis.
The monkeys are a free-ranging population of macaques descended from a 1938 release of monkeys from India on undeveloped 38-acre Cayo Santiago Island, off the eastern coast of Puerto Rico. They live in a natural setting with little human intervention other than food provisioning.
Field researchers who had learned to identify each of the nearly 90 monkeys on sight carefully logged interactions between individuals in 10-minute episodes over a two-year span.
They compiled four or five hours of data per individual, logging grooming, proximity and aggression.
From that, the team built web-like network maps to analyze pro-social and anti-social interactions.
They also looked at the maps for a measure they called "betweenness"-the shortest paths between individuals-and "eigenvector," a friends-of-friends measure that shows how many friends each friend of an individual has.
"The really 'popular' monkeys would have a high eigenvector, or a really big friends-of-friends network," Brent said.
There were also less-popular outliers who had fewer social interactions and a lower eigenvector.
"They're sort of the dorks," Brent added.
When these measures were then compared to family trees, "a lot of these network measures popped out as having significant heritability," Brent said. That is, the behaviors seemed to run in families.
The study is published in the journal Nature Scientific Reports.