Many of the primates spend a lot of time grooming. If you stand and watch lemurs for any length of time, or chimpanzees, or particularly baboons, you are going to see them arranging themselves in groups and reaching out to stroke the fur of whoever is sitting nearby. Grooming helps with the removal of dirt, parasites, dead skin, insects and tangled fur. So that's a big part of the reason you see baboons etc. doing a lot of grooming. Not so with the gorillas at the zoo where I visit baby gorilla brothers Apollo and Bomassa. I have been there just about every week over the past year, but I don't see the grownups grooming each other. Nor have I observed any gorilla grooming at other zoos. None. Yes, the moms will lovingly stroke the fur of their babies every now and then. That's what Jamani is doing below. So you will occasionally see a little mother to child grooming, but that's about it.
You've perhaps seen videos showing grooming among the wild mountain gorillas. That's the species of gorilla made famous when Diane Fossey studied them for many years. One of the zookeepers told me that gorilla grooming in the wild happens with some frequency, though perhaps not as much as with some of the other primates. I wonder if that is the result of gorillas being so big, and being leaf eaters. There is some evidence that grooming behavior, beyond being purely for purposes of hygiene, may also help a lower ranking animal get the support of a higher ranking animal. But in the gorilla world, perhaps the idea of support is not so crucial. The gorilla is king, with no natural predators to worry about, so they don't have to protect one another from predation. The silverback leads the group to a good feeding spot each day. There is not a lot of competition for food, because gorillas eat a diet of leaves, branches and bark, which are omnipresent in the thick forests they call home, so they don't have to work with each other to get better access to food. The smaller primates have a much tougher time protecting themselves and finding food. So it would not be surprising that they would put more energy into building alliances, and to the extent that grooming is part of gaining the good graces of a higher ranking individual, smaller primates are likely to have a very strong reason to engage in grooming behaviors. So that's my theory for why gorillas in the wild groom less frequently than other primates.
But in zoos, there is very little mutual grooming among gorilla adults. So we have another question which is why is there less gorilla grooming in zoos, than in the wild. I have a theory about that. Do you?