1. Primates, like other mammals, exhibit large inter-specific variations in their testes sizes, in relation to body weight. The evolutionary significance of these variations only became apparent after Parker (1970) had developed the theory of sexual selection by sperm competition. Larger testes are required to contain the increased volume of seminiferous tissue necessary to produce higher sperm counts. Short (1979) and Harcourt et al. (1981, 1995) showed that relative testes sizes in primates correlate with their mating systems. The multi-male/ multi-female mating systems of baboons, macaques, and chimpanzees are associated with large relative testes sizes. Females commonly mate with multiple partners during the peri-ovulatory period and sperm competition is pronounced in such species. By contrast, multi-partner matings are much less frequent in polygynous primates (e.g. the gorilla) or in pair-living (monogamous) forms such as gibbons. Relative testes sizes are significantly smaller in these cases.

2. Human relative testes sizes were shown to lie between these two extremes, being larger than those of the gorilla, but much smaller than the chimpanzee.

Some refinements to these conclusions emerge from the larger scale comparisons of mammalian relative testes sizes presented here. Thus, some Asiatic human populations have testes which are not larger, in relation to body weight, than those of orangutans or mountain gorillas. Yet, it remains the case that studies of testes size alone are unlikely to resolve debates about the likely importance of polygyny or monogamy in the evolution of human sexual behaviour.

3. Ethnic differences occur in human testes size. However, a consistent finding is that the right tes-tis is, on average, 5 per cent larger than the left tes-tis; this is confirmed here using comparative data from eight countries. Combined testes weights are smaller in men from Asiatic populations (China, Japan) than in their European and African counterparts. However, relatively few populations have been sampled and some data are subject to errors. This qualification applies particularly to volumetric measurements made using orchidometers as these often overestimate testes sizes.

4. Differences in testes sizes between human populations may be associated with genetic mechanisms which relate to gonadal function and fertility in both sexes (Short 1984). In mice and sheep, higher ovulation rates in females are associated with increased testes size in males of the same genetic strain. In human beings there is significant correlation between dizygotic twinning frequencies and testes sizes (data from nine countries). However, the caveat concerning possible errors of measurement of human testes sizes also applies to this finding.

5. Extrapair copulations and paternities occur in human beings but their current frequencies may tell us very little about selective pressures which shaped the evolution of human reproductive biology during the emergence of Homo sapiens from its African precursor, more than 195,000 years ago. It is noted, however, that frequencies of extrapair paternities are low in modern human populations (median value = 1.82 per cent for studies in six countries: Simmons et al. 2004).

6. A number of authors have overestimated the importance of relatively modest data on human testes size and frequencies of extrapair copulation. Their conclusion that sperm competition played a significant role in the evolution of human reproduction is not justified on current evidence.

The next chapter examines the effects of sexual selection upon the evolution of a variety of reproductive traits, including sperm morphology, the sizes and functions of the accessory sexual organs, and phallic morphology. This broad approach, involving comparisons of many mammalian species with Homo sapiens, offers much deeper insights into the likely origins of human sexuality.

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