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.
In the 1970s, psychologist Robert Seyfarth (here talking about communication in vervet monkeys) proposed a model as to how this might work. Focussing on female monkeys, so that issues of sexuality don't enter into it, he suggested that the animals doing the grooming might receive various rewards in return. Essentially, by regularly grooming another monkey, that monkey becomes your friend, and that can have certain advantages. For one, they're less likely to beat you up, and monkeys can get quite aggressive with strangers, so that's certainly handy. But they might also allow you access to food resources that they might otherwise hoard, or back you up should you get into a fight with someone else.
If this is true, it explains why monkeys don't just groom each other at random. If you can gain a benefit from grooming, then it makes sense to groom monkeys of higher status than yourself. These will be the bigger, tougher, more dominant individuals, the ones who you most want on your side, and the ones who are most able to stop you getting at the food you want if the don't like you. Why bother to spend a lot of time grooming somebody weaker and less important than yourself when they aren't going to be much use to you in return?
At the same time, and for the same reason, the high status monkeys are busy grooming each other. So, if you're at the bottom of the heap, you just won't get the chance to groom the monkeys at the top, and have to make do with those lower down. The end result is that, while we would expect monkeys to try and groom those of higher status than themselves, they will generally succeed only on those who aren't much higher.
Well, that all makes sense, but how true is it? There are over 250 different species of monkey, and it's a bit much to expect them all to behave in the same way. So it's perhaps unsurprising that attempts to confirm Seyfarth's theory have produced mixed results. It seems to hold up reasonably well as a general rule, but there do appear to be some exceptions.
A recent study published by Barbara Tiddi, of the German Primate Centre, and co-workers, is apparently the first to truly test the hypothesis on New World, rather than Asian or African, monkeys. It's been clear for some time that at least some New World monkeys do follow the rule in terms of who they choose to groom (although howler monkeys, for instance, apparently don't), but not whether Seyfarth was actually right about their reasons for doing so.
Black Capuchin Other Tufted Golden-bellied
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Black capuchins (Sapajus nigritus) are a species of monkey found in the forests of southern Brazil, where they primarily feed on fruit, and live in social groups with several females. There is some dispute as to whether they even are a species, and which genus they belong to if they are one. I'm going to regard them as a species here (the authors of the study in question don't, considering them to be a subspecies of tufted capuchin) since that seems to be the general consensus, and in the genus Sapajus because the genetic evidence for that looks good enough to me.
Keeping a careful watch on a group of nine adult black capuchins and their offspring for eight hours a day over the course of several months, the researchers were able to demonstrate that the female monkeys followed the genera prediction. That is, they tended to groom individuals higher in status than themselves, but the lowest ranking females had little opportunity to groom those at the top. That much wasn't a surprise; similar behaviour has been documented before in closely related species.
But what benefits did they gain? If the model is correct, we would expect dominant females to be less aggressive towards those that groomed them. But that didn't seem to be the case. The monkeys in question aren't particularly violent in the first place, but when they did fight, they really didn't seem to care whether they were attacking somebody who habitually groomed them or not. They did tend to avoid fighting with their own close relatives, but that's a different issue entirely.
Previous studies have tended to focus on these sorts of aggressive interactions, perhaps because many of them have been on species such as baboons, which are relatively violent, making it an easy thing to look out for. Of course, the monkeys are all members of the same group, and the aggression we're talking about is more like children squabbling than real physical violence, but it's clearly a significant factor in the behaviour of some of these Old World species.
Capuchins are generally less argumentative than baboons, and there's also a suggestion in the paper that the dominant males may have been relevant here. Since the males in this species keep a close watch on their harem, the researchers argue that the females may have been relying on him to break up fights - there's little use trying to get a dominant female on your side if the male is the only one who can protect you when things get rough. So far as I can see, they didn't test this possibility, so that remains just a guess, but it does illustrate why we can't simply assume that all primates will behave in the same way.
However, the researchers also looked at tolerance more generally. When it came to simply allowing other females to much food nearby without trying to pinch it off them, or generally be a nuisance, the dominant females were more tolerant of those who had groomed them. This time, kinship - how closely they were related - didn't make a difference. So that suggests that there is still a degree of social bonding going on here, and that's why the dominant females received more grooming - they're the ones with the 'authority' to be a problem if they're not on your side.
[Picture by Jose Reynaldo da Fonseca, from Wikimedia Commons. Cladogram adapted from Lynch-Alfaro et al. 2011].