Like humans, chimpanzees are social creatures. Very social, we recently discovered. The importance of their social bonds can be read in their stress hormones, with chimps that have their friends around experiencing less stress than when placed in a similar situation without close companions. Moreover, the presence of friendly mates can act as a stress reliever even when no threats are on the horizon.
Within their troop, chimpanzees have special friends, referred to as bond partners, who they frequently groom and share food with.
The stress levels of humans and other animals can be measured by testing concentrations of cortisol, a hormone released by the adrenal gland when things are going badly. Urinary glucocorticoids (uGC) include cortisol itself and other chemicals into which the body converts it.
Dr Roman Wittig of the Max Plank Institute for Evolutionary Anthropology measured uGCs from wild chimpanzees living in Uganda, rather than the artificial environment of a laboratory or zoo. He found uGCs rose when the chimpanzees he was tracking ran into a rival band of chimps. When Wittig's colleagues drummed on trees to replicate the sounds made by enemy chimps, the troops in the study produced similar uGC spikes to encounters with real chimps.
The presence of “bond partners” had a soothing effect, causing chimpanzees to sail through encounters with enemies relatively unscathed.
This much was anticipated, but Wittig hoped to shed light on the debate between two competing theories. The “main effects hypothesis” holds that stress levels are reduced through daily contact with bond partners, even under relaxed conditions. The “social buffering hypothesis” proposes that it's good to have those you've bonded with around during stressful times, but they don't provide the same benefit during grooming or other everyday interactions.
Wittig's results, published in Nature Communications, are a clear win for advocates of the main effects hypothesis. When the chimpanzees were groomed by a regular partner, their urine subsequently showed lower uGC levels than after simply laying around resting. Being groomed by a relatively unfamiliar troop member provided no such benefits.
It didn't seem to matter whether bond partners were of the same or opposite sex, or if they were particularly closely related. The mysterious processes of friendship formation turned a bond partner into a form of anxiety control.
By sampling frequently, and comparing results before and after stressful events, Wittig's group achieved much more fine-grained measures of stress levels than studies that relied on long-term averages. This required attaching plastic bags to forked sticks and standing beneath trees in which chimps were relieving themselves, proving once again that scientists' dedication to their work knows no bounds.
As the authors note, “stress is a major cause of poor health and mortality in humans and other social animals.” Just how applicable the research is to more complex human social dynamics remains to be seen. Nevertheless, it seems our nearest relatives get by with a little help from their friends.
