Primates are amongst the most social of mammals. They tend to gather together in bands or aggregations, often based on female kinship, and with a relatively small number of dominant males, born outside the group. The details vary considerably between species, as we might expect, but one feature that is relatively common is the way that such social bonds are formed and cemented. A particularly common way that primates maintain these bonds within a group is through grooming.
In grooming, the animals pick through a partners hair, removing lice, ticks, and other parasites, and generally keeping each other clean. This has obvious health benefits, and, in fact, these may be wider than is clear at first glance, since there
is evidence that grooming causes the release of endorphins, hormones that, in humans, reduce pain and generate a feeling of well-being. Probably as a result of this, in at least some species, monkeys with better social relationships apparently
live longer and have
healthier offspring than those who feel more isolated.
Such benefits generally apply to the recipient. What does the animal performing the grooming from the practice? The most apparent and immediate benefit is that the other animal may groom you back, so that everyone benefits. Indeed, it's entirely possible that we humans don't cement social bonds this way simply because we haven't got the fur to make it worthwhile. But, given the complex, and non-egalitarian, social lives of most primates, there might be other benefits as well, which could explain why it's not simply the case that every monkey in a troop grooms every other one equally.
