Nepotism is known to be important in chimpanzee society, but male chimps' ability to cooperate extends beyond family connections, new research reveals.
This extra level of sophistication is yet another way in which the social behaviour of chimps parallels that of humans.
Kevin Langergraber, an anthropologist at the University of Michigan in Ann Arbor, US, and colleagues recorded alliances, meat-sharing and other cooperative behaviour among 41 male chimps in Kibale National Park, Uganda.
The team also genotyped each animal to measure how closely they were related. Over a period of seven years, and over 5000 hours of observations, they observed 753 aggressive coalitions - where they cooperated to fight enemies – and 421 instances of meat sharing.
Chimps who shared a mother were far more likely to cooperate with each other. In contrast, there was no evidence that the same applied to chimps with a shared father. This is probably because fathers do not stay with their offspring, so a chimp has no easy way to recognise his paternal brothers.
However, since maternal brothers were rare in this population, most of the cooperating pairs were unrelated or only distantly related.
Extensive cooperation among non-relatives suggests that chimps do it for selfish reasons, with the expectation that favours will be reciprocated, says Langergraber. Human societies use cooperation with similar motives – another behaviour shared with our primate cousins.