Males have greater reproductive success if they spend more time taking care of kids, and not necessarily only their own, according to a study conducted in gorillas.
Researchers have previously found that wild male mountain gorillas living in Rwanda do something that is quite unusual for a mammal -- they help take care of all of the kids that live in their social group, regardless of whether they are the father.
The goal of the new study, published in the journal Scientific Reports, was to figure out why.
"Mountain gorillas and humans are the only great apes in which males regularly develop strong social bonds with kids, so learning about what mountain gorillas do and why helps us understand how human males may have started down the path to our more involved form of fatherhood," said Stacy Rosenbaum, a post-doctoral fellow at Northwestern University in the US.
The findings run counter to how we typically think of male mountain gorillas -- huge, competitive and with reproduction in the group dominated by a single alpha male.
"Males are spending a lot of time with groups of kids -- and those who groom and rest more with them end up having more reproductive opportunities," said Christopher Kuzawa, a professor at Northwestern University.
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"One likely interpretation is that females are choosing to mate with males based upon these interactions," Kuzawa said.
"We've known for a long time that male mountain gorillas compete with one another to gain access to females and mating opportunities, but these new data suggest that they may have a more diverse strategy," said Rosenbaum.
"Even after multiple controls for dominance ranks, age and the number of reproductive chances they get, males who have these bonds with kids are much more successful," he said.
This research suggests an alternative route by which fathering behaviours might have evolved in our own species, Rosenbaum said.
"We traditionally have believed that male caretaking is reliant on a specific social structure, monogamy, because it helps ensure that males are taking care of their own kids.
"Our data suggest that there is an alternative pathway by which evolution can generate this behaviour, even when males may not know who their offspring are," Rosenbaum said.
This raises the possibility that similar behaviours could have been important in the initial establishment of fathering behaviours in distant human ancestors, researchers said.
They are currently investigating whether hormones might play a role in helping facilitate these male behaviours, as they do in humans.
"In human males, testosterone declines as men become fathers, and this is believed to help focus their attention on the needs of the newborn," said Kuzawa.