Sexual dimorphisms in the overall proportions of primates.

Morphometric analysis of Professor A.H. Schultz's data on the overall proportions of primates reveals differences between the sexes. Univariate examinations of these data confirm the existence of the spectrum of sexual dimorphism already well known. This spectrum relates mainly to differences in the proportions of the trunk. It has a differential expression with largest differences between the sexes in species such as orangutans and boboons, and smallest in species such as spider monkeys and douroucoulis. Multivariate statistical study of these same data reveal, however, further unsuspected sexual dimorphisms. Although differences between the sexes are only small when measures of the relative lengths of bodily parts are examined, they are big when bodily breadths are studied. Investigation of breadths alone reveals that the primates display two major patterns of sexual dimorphisms and seven unique sexual dimorphisms among the 18 genera examined. Such findings mean that sexual dimorphism of bodily structure is not a single phenomenon with differential expression, a concept widely noted in the literature and most recently associated with social organization. There are several different sexual dimorphisms and this suggests that their causation is likely to be multifactorial with multiple complex interactions among the factors. Some of the sexual dimorphisms must have evolved in parallel a number of times, and, given that chimpanzees, gorillas, and humans each display a different sexual dimorphism, at least some of the evolutionary changes in different sexual dimorphisms must be very recent. The findings even imply the possibility of further unique patterns of sexual dimorphism in some fossils. By further extension, the findings may have some implications for our understanding of nonstructural dimorphisms in humans.