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Frequencies of social grooming recorded from 44 species of free-living primates correlate with group size but not body size. This is interpreted as evidence for the social function of grooming and against the purely hygienic function. However, there is some evidence to suggest that body size is a more important determinant of grooming time among platyrrhine primates. This might imply that there has been a shift in the functional system governing grooming during primate evolution.
Robin Dunbar (Thu,) studied this question.