Many mammal species live solitary lives, at least outside of the mating season. For those, there would be little opportunity for social grooming and no wider benefit to be gained from it if there was. Self-grooming or "autogrooming" - such as licking one's own fur - may well be sufficient for them. But, of course, primates are not the only social mammals, and many of those other animals have fur in which parasites could hide, so it's reasonable to ask if allogrooming really is unique to primates.
It takes little observation to confirm that the answer to this is clearly 'no'. So, while primates have received the most attention from behavioural scientists when it comes to grooming, some have chosen to look further afield. One obvious place to look is among hoofed herd animals, because that's a description that covers multiple related sociable species some of which are of considerable economic importance to humans. (In contrast, most carnivores are solitary, although there are exceptions such as lions, wolves, and meerkats).
Hoofed animals, quite obviously, can't pluck at parasites with their hands. Instead, they use their mouths, licking at one another's fur and perhaps gently nipping with their teeth. But that's still grooming and anyone familiar with horses, cows, and other domestic ungulates will know that it's a common behaviour. With the help of a recently published survey on the subject, we can take a look at how much we know about it, and how it may resemble, or differ from, similar behaviour in primates.
Social grooming, so far as we can tell, initially evolved as a means of removing biting insects, ticks, and so on from the fur. Looking for evidence that hoofed mammals benefit from this in the same way that primates do is therefore a good place to start. A study on impalas back in the 1990s, for example, showed that if you don't let them groom properly, they will get covered in ticks, and, moreover, if you then kill all the ticks chemically and let the animals free, they don't bother to groom as much as they normally would.
That study was really looking at self-grooming rather than the social sort, although it's a start in demonstrating that that's why they're doing it. If social grooming is doing the same thing by more than mere coincidence, however, it should be concentrated on the parts of the body that an animal can't reach by itself. And this is what we see; horses lick each other mainly on the head, neck, and chest, cattle on the head and neck, and red deer on the head, neck, and back.
Similarly, we see that white-tailed deer practice social grooming more frequently when they are in wooded habitats where biting parasites get onto them from bushes and trees than when they are out in the open. Wild horses groom one another more often at times of the year when parasites are most common, although domestic horses apparently don't - possibly because there's less need if their owners are caring for them properly.
So much for the basics. But, in primates, we know that social grooming can also serve other purposes beyond the purely functional, helping to boost social bonds. While it may have started as a means of keeping down parasites, it is evidently a pleasurable experience for non-human primates, not just because it can get rid of an annoying itch, but because if the animal didn't evolve to find it enjoyable in itself, they probably wouldn't let their neighbour do it.
While we don't have specific evidence for this in hoofed mammals, we can say that dairy cattle are motivated to seek out mechanical brushing, and they show physiological signs of relaxation when licked by another cow. Similarly, domestic horses seem to enjoy humans brushing them - something that's likely unsurprising to anyone who has owned one.
However, this doesn't prove that grooming has a social role, as it does in primates, not least because neither of the previous examples involves another member of the same species. For that, we'd have to show that the animal performing the grooming received something in return, or that it somehow benefits the larger herd - for example, by reducing conflict.
At the most basic level, what the grooming animal can expect is that the recipient will return the favour - an almost literal "you scratch my back, I'll scratch yours". We know that this happens, for example, in horses and impalas, where a pair of animals will groom each other. It has generally been assumed that this must be more limited among hoofed mammals than primates, because only the latter are intelligent enough, or have sufficient social memory, to recall which specific individual groomed them in the past. Hoofed mammals, the reasoning goes, would have to reciprocate straight away, or not at all.
Just last year, however, a study was published showing that Himalayan tahr (a species of wild goat) do appear able to remember which of their fellows has groomed them most in the past, returning the favour well after the fact. So at least some hoofed mammals may have greater cognitive capacity than we have given them credit for.
While it's less clear than it is in primates, hoofed animals could be hoping for other benefits beyond mere tit-for-tat. Support for this idea comes from the fact that cows of low status within the herd are more likely to groom those which are dominant. Exactly what they expect in return is harder to determine, but it could be either to placate the dominant cow, encourage future support against a rival, or if they are a different sex, obtain mating opportunities.
Regardless, it does appear that grooming helps to develop and maintain social bonds between herd members, and to benefit those taking part in it. The foals of feral horses that engage in regular grooming with their fellows and that are otherwise more socially active are more likely to survive. The exact reason is unclear, but there's obviously some sort of social benefit here. Like many primate species, domestic goats use mutual grooming as a means to "make up" after an aggressive encounter; similar behaviour has been observed in pigs, where a third party may use it to offer reassurance to the loser of such a fight.
More generally, social grooming can be a sign of what we might call "friendship", in that two animals that are familiar with one another are more likely to take part, helping to maintain existing social bonds. Female zebras will intervene if they see one of their friends being sexually molested by a stallion, in the same way that they would if she were a close relative but not an unfamiliar animal. Some even become jealous, breaking up social grooming bouts involving one of their friends and a stranger.
Much of this sort of study has been undertaken on domestic animals, because those are easier to observe. There's also an animal welfare issue here in that grooming among domesticated animals is thought to be unlikely if they are stressed or ill... although if they also use it as a stress-reduction mechanism, its mere presence doesn't prove that the animals are comfortable.
The survey I mentioned at the beginning, and which forms the basis of this post, concludes with the usual caveat that there's a lot we still don't know, especially about wild animals such as deer or antelopes. Primates have been the primary subjects of studies on social grooming, both because they have a lot to tell us about how our own sociality evolved and because their behaviour is generally more complex. For instance, while we have underestimated how good some herd animals are at holding information about past social relationships, it's plausible that there genuinely are some limitations here, at least among some species.
But even so, for those hoofed animals that live in herds, domesticated or otherwise, social grooming is an important part of their lives. And not just because it helps to get rid of those pesky biting insects.
[Photo by Jim Champion, from Wikimedia Commons.]
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