Male chimpanzees in Uganda’s notorious Ngogo clan are now known to patrol the borders of their territory, actively seeking out members of rival groups to fight and even kill.
The insight comes as part of ongoing analyses of 20 years’ worth of data on the 200-member-strong community, which has dramatically increased its size over the last decade.
While other animals tend to get into territory fights as a result of chance encounters, the Ngogo chimps appear to deliberately search for enemies on the edges of their land, in what researchers say is a remarkable example of group-level cooperation.
According to the new research, led by a scientist at Arizona State University, these patrols often involve multiple individuals who travel to the boundaries of their territory and exhibit hyper-vigilant behaviour.
Sometimes, they even travel deep into their neighbours’ territory.
Patrolling may give the Ngogo chimps in Kibale National Park a leg up on the competition, the researchers explain, as this group has been found to benefit from long life expectancies, massive territory, and unusually bountiful food supply.
This behaviour, though, is dangerous, and often leads to violent spats with members of other communities.
‘The Ngogo chimpanzees patrol and kill neighbours more frequently than any other chimpanzee group,’ said John Mitani of the University of Michigan, who has studied the group for 22 years.
Patrolling also requires a lot of energy – and, it takes away from time the males could be mating, the researchers note.
While the exact benefits of these patrols remain unclear, the researchers uncovered several patterns.
Males varied how often they patrolled, and high-ranking individuals in good physical condition were found to partake more frequently.
And, males who had more offspring in the group patrolled more often as well.
But, patrols were even observed in groups that had no offspring or relatives to protect.
The observations suggest many of the gains are long-term, rather than immediate.
Those with no relatives to protect are still likely to reproduce in the future, they explain, and will benefit from protecting the group as a whole.
‘We know that humans have means ranging from gossip to drastic punishment to aid cooperation in group settings,’ said Kevin Langergraber of ASU’s School of Human Evolution and Social Change and Institute of Human Origins.
‘The puzzle has been to explain cooperation in animal societies, where shirking would seem an attractive option.’
While most studies have focused on short term benefits of this type of behaviour, Langergraber says the new work ‘shows the benefit of long-term data collection, and also that we still have a lot to learn from these chimpanzees.’
Over the last few months, more and more information has steadily emerged on the remarkable lives of the Ngogo chimps, who are said to be the most brutal troop in the world.