It is in Bonobos I think. I used to think the adults, which often look bald too, plucked themselves due to high stress levels, but I've been told its more to do with social over-grooming, a product of captive life, and that would apply to the babies too. All the young I've seen in the Twycross group have looked like this to some degree.
It is obvious, that especially handraised females and males show this social over-grooming or self-over-grooming. I myself corroborate the theory that handraised bonobos, affected by keepers in the past, try to imitate human visual nature unconsciously.
Bonobos, raised in a social bonobo group or in groups of apes, for example "Bonnie" and "Clyde" at Cologne, who live together with young gorillas and orang-utans in their infancy, do not show this special behaviour.
