It’s good to be social. Male chimpanzees with more friends are more likely to father offspring – and there are at least three ways this can occur.
“Animals with more social bonds or stronger social bonds have higher reproductive success,” says Joseph Feldblum at the University of Michigan. “We tried to find out how that might happen.”
His team focused on chimpanzees, our closest living relatives. They live in groups of around 25 individuals, which are dominated by a powerful alpha male. The alpha sires most of the offspring, while subordinate males struggle to mate at all.
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Feldblum and his colleagues tracked 32 males and 26 females in the Kasekela chimpanzee community in Gombe National Park, Tanzania. The animals have been studied since 1973, and the researchers had access to detailed behavioural and genetic data collected between 1986 and 2014.
They first confirmed, in line with previous studies, that males with more male friends and allies were more likely to father offspring. Then they dug into the data to figure out why. “There are multiple independent routes by which male social bonds seem to help them achieve higher reproductive success,” says Feldblum.
One pattern that stood out was that males that formed close relationships with the alpha were more likely to mate, compared with those that didn’t. This may be because the alpha is a little more generous with them, permitting them to mate with receptive females. “We think we understand why forming a strong bond with the alpha male matters,” says Feldblum.
Independently of this, males that formed a lot of strong male friendships were more likely to rise in rank and thus become the alpha. Feldblum says this is more of a long-term advantage, but the reproductive pay-off of becoming alpha is enormous.
Finally, males that formed a lot of strong ties with other males were more likely to sire offspring, regardless of the rank of anyone involved. This suggests there is an advantage to being part of a friendship group, even if it doesn’t lead to an increase in rank.
Feldblum says it is “less clear” why simply having lots of friends, even if no one in the group rises to alpha or close to it, would be helpful. But the team did find that having a big network of strong friendships was correlated with forming aggressive coalitions.
“It seems coalition formation is the behaviour that links bond formation with siring success,” he says. “It could be that if you have a big network of strong ties, you’re less likely to be the target of aggression because your friends protect you,” says Feldblum.
Groups of males might also become aggressive when females are in oestrus, and force matings against the wishes of the alpha male. Perhaps in line with that, when the groups are small the alpha male maintains total control over mating, and it is only in larger groups that more complex dynamics emerge.
Feldblum says there could also be another social dynamic that his team hasn’t thought of.
Male chimps’ ability to form complex alliances may date back to the common ancestor we share with them, which lived at least 6 million years ago. Chimps and humans are unusual because both species are good at cooperating with individuals who aren’t blood relatives.
It may be that this ability to cooperate with non-kin was driven, at least in part, by the selection pressures driving males to cooperate to secure matings. Later, as our ancestors became more human, this non-kin cooperation was supercharged. “What we know is that human cooperation exists on a scale and in ways more complex than other species,” says Feldblum.
iScience
