Outside of humans, chimpanzees are one of the only species known to cooperate with unrelated individuals to achieve a common goal, like defending territory from mutual rivals.

What motivated chimps to work together? At least one possibility, new research suggests, is that conflict has a negative impact on wild female chimpanzees' reproductive success.

Scientists linked between-group competition and reproductive success after analyzing decades-long studies of four neighboring chimpanzee communities.

"We developed a new index of neighbor pressure that reflects the danger of intrusion by neighboring groups into one's territory," Sylvain Lemoine, researcher with the Max Planck Institute for Evolutionary Anthropology, said in a news release. "We show that high neighbor pressure during the time when females are supposed to resume reproduction is associated with a delay in reproduction, leading to longer intervals between births. We also show that having many males in a group is advantageous and speeds up reproduction."

The findings, detailed this week in the journal Current Biology, offer one possible explanation for the origins of cooperation. Groups that diminished the chance of conflict by cooperating likely produced more offspring.

Researchers also found that heightened conflicts with neighboring groups during gestation periods had a negative effect on the odds of offspring survival. Authors of the new study suspect conflict makes it more difficult for females to access vital food resources, causing nutritional deficiency. Conflict also likely elicits a stress response in females.

"These physiological mechanisms remain to be examined, as well as the potential efficiency of in-group cooperation to reduce the received pressure from neighbors, such as cooperative border patrols regularly observed in wild chimpanzees," said Catherine Crockford, a researcher with the Max Planck Institute.

The same factors that motivated chimpanzees, our closest relatives, to cooperate, may have also motivated the earliest humans to work together.

"For highly territorial species, including humans, these findings shed light on how between-group competition could have acted as a selective pressure favoring the evolution of particular traits, such as group-level cooperation with non-kin, and how this could have shaped our ancestors," Max Planck Institute Roman Wittig said.

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