Male chimps who form strong bonds with other males in their troop are more likely to father offspring, a new study finds. There are two paths to doing this, either getting in close with the most powerful figures, or developing a large network of less dominant friends. The findings not only discredit some simplistic models of animal, and human, mating dynamics, they also shed light on the forces that allow social primates to flourish.
Animal sex lives are often portrayed as a world of pure competition, where males fight for access to females, and only the strongest or trickiest succeed. There are plenty of alternative examples, however, at least in social animals, now including our nearest relatives.
Dr Joseph Feldblum of the University of Michigan led a study of chimpanzees in Tanzania. In iScience, he reports that having multiple wing-chimps definitely works to males' advantage.
"One big question that biologists have had for a long time is why you see so many friendly behaviors such as cooperation and alliance in animals," Feldblum said in a statement. "One would expect to see these social bonds—or strong, friendly social relationships—only if they provide some sort of fitness benefit to the individuals. Males wouldn't spend all this time grooming other males and forgoing trying to find females or food unless you get some kind of benefit from it."
Of course, such benefits can come in many forms. Social bonds help unite groups against predators, for example, and may encourage food sharing that ensures larger numbers survive bad times. Nevertheless, building these bonds can involve putting in a lot of work long before any payoff, so incentives need to be strong, and few are stronger than sex.
Despite all the work that has gone into researching chimpanzee social behavior over the last sixty years, the connection between social relationships and offspring has been ignored, at least in males.
"Chimps cooperate frequently, and often in these very dramatic ways: You see things like grooming, all kinds of complex alliance formation and group territorial defense," Feldblum said. "The question is: What do males get out of it and how?"
Feldblum and coauthors investigated the 56 babies with known fathers born in Gombe National Park between 1980 and 2014. They found a male was 50 percent more likely to sire offspring if they had strong social bonds – measured by physical proximity and grooming – than another male of the same age and dominance rank.
As they say, “Apes together strong”.
A strong connection to the alpha male was particularly advantageous. "Sucking up to the boss is nothing new," said co-author Professor Anne Pusey of Duke University. "We show that it's always paid off." Unsurprisingly, actually being the boss proved better still for maximizing paternity. It's good to be the king.
The paper doesn't resolve how bonding works. Feldblum speculates allies may run interference for males during mating, preventing others from breaking up the tryst. Alternatively, the lowered stress levels that come from knowing someone has your back may make a male healthier and able to devote more energy to sex.
It's easy to over-interpret the implications of behavior in any one species for humans. Nevertheless, senior author Dr Ian Gilby of Arizona State University said; “This study suggests that strong bonds among males have deep evolutionary roots and provided the foundation for the more complex relationships that we see in humans." "This research also highlights the value of long-term studies like these, which are essential for understanding the biology of a species that lives for many decades and is slow to reproduce."