Fossils, Teeth, and Sex: New Perspectives on Human Evolution

By Charles E. Oxnard | Go to book overview

CHAPTER 2
The Present Day: Sexual Dimorphisms in the Teeth of Living Apes and Humans

Sexual Dimorphism or Sexual Dimorphisms?
The Dimensions of Ape and Human Teeth -- Canines -- Incisors
Premolars -- Molars -- Measures taken one by one
Sexual Dimorphism in Multivariate Studies of Ape and Human Teeth
Canonical Variates Analyses -- High Dimensional Displays
Some Implications for Evolution

Abstract: In this chapter we study sexual dimorphism in the teeth of the living large hominoids: humans, chimpanzees, gorillas and orang-utans. The discussion of Chapter 1 suggests that we must look for many sexual dimorphisms not one, for complex sexually dimorphic patterns rather than simple, and for arrangements evident in studies of measures taken altogether as well as studies of measures taken individually.

In this chapter we examine the measures of the teeth using simple statistical tools, such as means and variances, and frequency histograms and normal distributions. We also study them using the multivariate statistical methods of canonical variates analysis and high-dimensional displays.

It turns out that there is not a single sexual dimorphism common to all these species. There are many, and they are certainly highly complex. The findings have implications not only for the evolution of the species but also for the evolution of their sexual differences. And they have especial significance for the study of the fossils examined in comparable ways in later chapters.


Sexual dimorphism or sexual dimorphisms?

The larger problem of the nature and meaning of sexual dimorphism in general can be elucidated through study of the smaller problem of sexual differences in tooth dimensions. But we should consider first two very obvious possibilities.

First, since the teeth are primarily a food processing device we must ask: could differences in tooth sizes be specifically adaptive to diet? More specifically, could sexual dimorphisms in tooth sizes be related to differences in diet in males and females? Certainly this is the case in food processing apparatuses in some species -- bird bills, for example ( Selander, 1966). But there is no compelling case from present evidence that this is a major factor for any ape or monkey group (where the greatest differences between the sexes are to be found).

There are well-known sex differences in canine size and form in many non-human primates. But these seem to be unrelated to diet and have no parallels in the human dentition. Nevertheless there are some differences in diet between the sexes and this is a matter to which we shall return.

Second, many studies imply, both directly and by default, that sexual differences in tooth size are reflections of differences in body size. This has never been fully studied in the non-human primates, for the necessary basic data from large enough populations do not exist. Here we have to be careful for many studies have shown that this relationship only holds within a particular population ( Brace and Ryan, 1980). In the one instance within humans. where body size and tooth size are available within known family groups 'taller parents do have children with systematically larger... dental dimensions' ( Garn, Lewis and Walenga, 1968). And genetic links of jaw shape with sexual dimorphism are known (e.g. in rats: Bailey, 1984).

How do these ideas stand up against the data presented in the last chapter?

The studies of chapter one on overall bodily proportions (see also Oxnard, 1983a, c, 1984) must have already raised doubts in our minds about such conventional concepts of sexual dimorphism in primates, especially in apes and humans. The examination, first, of body variables taken one by

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