Male chimpanzees can be quite violent creatures, often going to war over territory, or resorting to murder to take out a rival. It’s also known that they can form lasting friendships with other male chimpanzees, which appears to be linked to a longer lifespan.

New research, as reported by New Scientist, has revealed that male chimpanzee "bromances" may also calm them down as they enter an aggressive confrontation with an opponent. The study's team, led by the Max Planck Institute for Evolutionary Anthropology in Leipzig, presented their findings at the Ethological Society annual meeting in Germany last week.

Friendships between chimpanzees are already understood to be a key aspect of their societies: Trusted bonds strongly alter how resources are shared within and between groups. Chimpanzee male-male friendships, for example, involve cooperative behaviors, including sharing food. Curiously, some have even been seen to engage in the grooming of each others' intimate areas – but why?

In order to find out, the researchers tracked wild chimpanzees as they headed out on defensive, territorial patrols around Uganda and the Ivory Coast, watching carefully as several males engaged in aggressive displays or combat. By taking samples of the chimpanzees’ urine, they were able to monitor the levels of stress hormones pre- and post-encounter.

Ultimately, lower levels of stress hormones were found in chimpanzees who patrolled or fought alongside a regular male grooming partner. This suggests that male friendships act as a “social buffer,” potentially preventing the patrollers becoming dangerously stressed during each hostile encounter.

The stress response of primates is controlled by three glands comprising the nervous system's hypothalamic-pituitary-adrenal (HPA) axis. When chimpanzees experience trauma or chronic stress, this axis can malfunction, causing impairment to their immune and reproductive systems, as well as disturbances to their mood and cognitive functions. Chronic stress in humans is also acknowledged to have similar effects; in the long-term, too much stress can lead to a decrease in bone density, muscle mass, and can even cause wounds to heal far slower.

Although the activation of these glands during aggressive encounters helps chimpanzees become more alert, the presence of another male friend appears to lower their overall stress levels – thereby mitigating the potential negative health consequences.

[H/T: New Scientist]