Two gelada males threaten an intruder while grooming with an infant. (Image: Clay Wilton.)

Being the top dog—or in this case, the top gelada monkey—is even better if the alpha male is willing to concede at times to subordinates, a new study indicates.

Alpha male geladas who allowed subordinate competitors into their group had a longer tenure as leader, resulting in an average of three more offspring over his lifetime.

The findings, collected from data during a five-year period ending in January 2011 through the University of Michigan Gelada Research Project, appear in the current issue of the Proceedings of the Royal Society.

Cooperation is surprisingly common among wild animals, the researchers say. While it makes evolutionary sense for animals to help their kin, it is harder to explain cases where competitors—especially unrelated adult males—join forces. This conundrum is particularly hard to explain because mating is generally a zero-sum game in which males can only reproduce by stealing mating opportunities from one other.

Thus, as a male, why would you want to hang out with other males who inevitably decrease the probability you will pass on your own genes? Yet in many species, males do.

A follower male sits with an offspring that he may have sired. (Image: Noah Snyder-Mackler.)

“In some species unrelated males will sometimes tolerate the presence of one another and in rare cases, form bonds and even appear to cooperate,” says Noah Snyder-Mackler, a doctoral graduate of psychology at the University of Pennsylvania and the study’s lead author.

To understand why potential rivals might team up, researchers compared the fitness consequences for dominant male gelada monkeys living in single or multi-male groups. They found that, although subordinate males father some of the offspring in multi-male groups, dominant males gain a lifetime fitness benefit because the subordinate aids in defense of the group from other males, thus extending the dominant male’s reproductive career.

Even more tantalizing is evidence that subordinate males who are allowed to mate stay around in the group for much longer. This suggests the alpha males may allow the subordinate to reproduce as a “staying incentive” for defending the group—a payment for their services, says Thore Bergman, U-M assistant professor of psychology.

While it is not yet clear such an exchange is occurring (subordinate males may simply steal some reproduction), the evidence that subordinates benefit the leader is strong.

“These findings demonstrate a benefit of forming multi-male groups in a predominantly single-male system—an important step in the evolution of sociality among unrelated competitors,” Bergman says.

A leader and follower fend off a bachelor. (Image: Noah Snyder-Mackler.)

In studying wild geladas in the Simien Mountains National Park in Ethiopia, researchers identified the leader and follower males, noted the numbers of females in all units, tracked individuals involved in unit takeovers, and noted all new infant births. They used non-invasively collected genetic samples to conduct paternity analysis to determine the identity of the father.

Researchers saw the dominant and subordinate males cooperatively defended their females by fending off unattached bachelor males intent on taking their females—the likely mechanism leading to the increased tenure of the dominants.

Even though geladas primarily form single-male groups, researchers showed a benefit to forming multi-male groups. Multi-male units had fewer takeovers (a rate of 0.27) per year from other competitors and longer tenure (3.7 years as leader) compared with single-male units (0.35 takeovers/year and 2.86 years as leader).

“Overall, this means that just because animals appear to be in direct competition for a limited resource, they may still benefit from the relationship overall,” Snyder-Mackler says.

Thus, cooperation can evolve among competitors through a variety of mechanisms. In the study, researchers chronicled how it can evolve in the context of reproductive sharing.

“More comparative research on other species will give us a better understanding of how and under what circumstances cooperation among unrelated individuals may have evolved,” Snyder-Mackler says.