Fi

Orang Chimp Bonobo

Likelihood of sperm competition

Figure 3.7 Percentages of morphologically abnormal (pleiomorphic) sperm in the ejaculates of man and the great apes. Abnormal sperm occur most frequently in man and in the gorilla. By contrast, species which are engaged in sperm competition (e.g. the chimpanzee and bonobo) have the lowest percentages of sperm abnormalities. Source: Based upon data from Seuanez (1980).

Gorilla Man large, in relation to adult body weight, they are larger than those of gorillas and orangutans (at least in men originating from Europe and Africa: see Table 2.2). It can always be argued that sperm competition may have played a significant role in the evolution of human reproduction, although it has been more important in chimpanzees or bono-bos. However, given that comparisons of relative testes size provide only a relatively crude index of differences in sperm production, it is relevant to ask how efficient the human testes are at producing sperm, by comparison with the testes of other mammals? How many sperm are produced each day by each gram of seminiferous tissue (parenchyma) in the testes? After leaving the testis, spermatozoa must pass through the epididymis and be stored in its tail (cauda). How long are epididymal transit times and how many sperm are stored in the cauda epididymis in various mammals? How many sperm are contained in human ejaculates, and what effects do repeated copulations have upon sperm counts? If sperm competition has played a significant role in human evolution then we might expect to encounter the same kinds of specializations that occur in other mammals which engage in sperm competition.

M0ller made valuable contributions to this field in several of his papers which deal with comparative studies of mammalian ejaculate quality and relative testes size (M0ller 1988; 1989; 1991). He showed that a variety of measures of ejaculate quality tend to be correlated and proposed that they 'have been improved simultaneously, apparently by common mechanisms' (M0ller 1991). Species with larger testes, in relation to body weight, produce larger numbers of spermatozoa in the ejaculate, and maintain larger reserves of sperm in the epididymis. They also produce a higher proportion of morphologically normal and motile sperm than mammals which have small testes in relation to body size. Interestingly, the number of spermatozoa produced per gram of parenchyma in mammalian testes does not correlate with relative testes sizes, and nor do epididymal transit times. Although M0ller was able to assemble only a limited data set (nine species of mammals including Homo sapiens) his findings were statistically significant. Additional information has been published in the intervening years, so that now we are in an even stronger position to assess how sperm production and ejaculate quality in human beings compare with those of other mammals.

Table 3.2 provides data on rates of human sperm production, storage, and sperm numbers in the ejaculate. The total time required to produce a human spermatozoan is 74-76 days. Spermatogenesis takes longer in H. sapiens than in almost every other mammal for which measurements have been made. Sharpe (1994), in his detailed review of the regulation of mammalian spermatogenesis, lists durations in several species, as follows: 33.8 days (guinea pig), 35 days (mouse), 42 days (longtailed macaque), 44 days (rhesus and stump-tailed macaques), 48 days (rabbit and blue fox), 54 days (bull and coyote), and 57 days (olive baboon). The ultimate (i.e. evolutionary) mechanisms which may underly these species differences in the time required for the mitotic and

Table 3.2 Human spermatogenesis: Sperm production, epididymal transit time, storage, and numbers per ejaculate

Measurement Duration/Number Source

Table 3.2 Human spermatogenesis: Sperm production, epididymal transit time, storage, and numbers per ejaculate

Measurement Duration/Number Source

1. Duration of spermatogenesis

74-76 days

Sharpe (1994)

2. Daily sperm production/g

4.4 million

Sharpe (1994)

3. Total daily sperm production

20-270 million; mean 130 million

Johnson, Petty, and Neaves (1984);

Sharpe (1994)

4. Sertoli cells/g

33-49 million

Johnson et al. (1984)

5. Spermatids/Sertoli

3.9 ± 0.5

Sharpe (1994)

6. Epididymal transit

1-12 days (mean 5.5)

Orgebin-Crist and Olson (1984)

7. Epididymal sperm reserve

440 million

Moller (1989); Amann and Howards

(1980)

8. Sperm per ejaculate

236.1 ± 124.1 million

Pound et al. (2002)

meiotic cellular divisions and transition of spermatids to completed spermatozoa remain unclear. What is clear is that human males take longer to complete spermatogenesis than most mammals (Hochereau de Riviers et al. 1990; Sharpe 1994).

M0ller (1989, 1991) pointed out that rates of sperm production per gram of parenchyma (seminiferous tissue) are not correlated with relative testes sizes in mammals. It is not the case that a species with large testes in relation to body weight, and a significant degree of sperm competition within its mating system, necessarily produces more gametes per gram of testis. There appear to be physiological constraints limiting the speed of sperm production within a given species; hence sexual selection has led to the evolution of larger testes, containing a greater volume of sperm-producing tissue. Nonetheless, it is intriguing that mammals vary considerably in their daily sperm production (DSP) per gram of parenchyma. Human males have the lowest recorded DSP among mammals (4.4 million/g). This compares to 13 million/g in the bull, 23 million/g in the rhesus monkey, and 25 million/g in the rabbit

(Figure 3.8). Thus, if sperm competition has played a significant role in human evolution it is surprising that DSP rates are not higher in human males, or at least commensurate with those of other mammals.

Why, at the proximate level, are rates of sperm production per gram of testicular tissue so low in human males? One reason may relate to the functions of the Sertoli cells in the testis. Sertoli cells make up a substantial volume (11-40%) of the seminiferous epithelium in mammals (Russell 1998). First described in 1865 by Enrico Sertoli, when he was just 23 years old, these cells have been implicated ever since his discovery in the processes that control sperm production. These are very large and complex cells, each of which makes contact with the various germ cell types as they undergo their divisions and transition from spermatogonia, to spermatocytes, spermatids, and completed spermatozoa (Figure 3.9). Sertoli cells contain receptors for a variety of hormones and they orchestrate and co-ordinate the development of cohorts of spermatozoa (Bardin et al. 1994). It is not surprising, therefore, that there is a marked positive correlation between the numbers

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