While many animals cooperate with their kin, competition remains a fact of life in the wild. Unrelated individuals, usually males, constantly battle it out in life-or-death struggles for resources and mates.

Human society, however, is based on some of the most complex and intricate systems of cooperation the world has ever known. 

Figuring out when and how our ancestors made this transition from competition to cooperation is a major question in evolutionary psychology, and a recent study from Penn, Duke, and the University of Michigan seeks to figure out the answer.

While a graduate student in the Department of Psychology in Penn’s School of Arts and Sciences, Noah Snyder-Mackler and his colleagues travelled to Ethiopia to study gelada monkeys. By careful observation and non-invasive genetic sampling, they pieced together details of the monkeys’ social dynamics.   

Individual geladas live in small groups, and many groups live in close proximity within a herd. Most groups have a single male that gets to exclusively mate with the group’s females, but some groups have multiple males. In these multi-male groups, the “leader” male still does most of the mating, but as the researchers discovered, his “followers” do some mating, too.  

Given that male geladas routinely fight or kill one another in battles for a group’s females, why should leaders tolerate competition from followers at all? Snyder-Mackler and his colleagues found that these “takeover” battles were at the heart of the answer.

“What we found was that these followers are helping to actively defend the unit and deter immigrant males from coming in or taking over,” says Snyder-Mackler. “Leaders who have a follower in their unit are leaders 30 percent longer than those who don’t.” 

The fact that only some gelada groups had multiple males—not all or none, as with most other primate groups—allowed the researchers to observe this difference in action. While it was not clear whether the leaders willingly allowed the followers to mate as a reward, or simply declined to punish them for “stealing,” the data showed that cooperating ultimately nets the leader more mating chances and offspring due to his extended reign.

“Overall, this means that, even if animals appear to be in direct competition for a limited resource, they may, in fact, be benefiting one another in some other way,” Snyder-Mackler says.