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Figure 3.8 Daily sperm production (in millions per gram of testicular parenchyma) in human beings as compared to other mammals. Source: Based upon data from Sharpe (1994).

Figure 3.9 The human seminiferous epithelium, to show relationships between the developing germ cells and Sertoli cells. SG = spermatogonia; SC = primary spermatocytes; SD = spermatids; S = Sertoli cells, containing lipids (L). Specialized inter-Sertoli cell junctions are indicated by heavy lines. Source: After de Kretser and Kerr (1994).

Figure 3.9 The human seminiferous epithelium, to show relationships between the developing germ cells and Sertoli cells. SG = spermatogonia; SC = primary spermatocytes; SD = spermatids; S = Sertoli cells, containing lipids (L). Specialized inter-Sertoli cell junctions are indicated by heavy lines. Source: After de Kretser and Kerr (1994).

of Sertoli cells in the testis and rates of daily sperm production. Figure 3.10 shows the relationship between Sertoli cell numbers and daily sperm production rates in the testes of men (aged 18-71 years) studied in the USA by Johnson et al. (1984).

These authors calculated that each human testis contains approximately 500,000 Sertoli cells, and that their numbers decline significantly with age. However, each human Sertoli cell is associated with the development of smaller numbers of germ

Number of Sertoli cells (millions)

Figure 3.10 Positive relationship between Sertoli cell numbers and daily sperm production by the human testis (y = 18.8 + 0.2 x rho = + 0.7). Source: Redrawn from Johnson et al. (1984).

Number of Sertoli cells (millions)

1000

Figure 3.10 Positive relationship between Sertoli cell numbers and daily sperm production by the human testis (y = 18.8 + 0.2 x rho = + 0.7). Source: Redrawn from Johnson et al. (1984).

cells than is the case for other mammals studied so far. Thus, each human Sertoli cell is associated, on average, with 3.9 mature (elongate) spermatids in the final phase of transition to mature spermatozoa. Comparable figures for other mammals are, for the orangutan: 5.7 (based on a single specimen), stallion: 11.5, rat: 10.3, rabbit: 12.2, and long-tailed macaque: 8.2. Individual differences certainly occur (for example, among long-tailed macaques) but in general it appears that human beings produce less spermatozoa on average, per gram of parenchyma in their testes, in part because each Sertoli cell is associated with a smaller number of developing gametes (Johnson et al. 1984; Russell and Griswold 1993; Sharpe 1994).

Given the comparatively low rates of sperm production achieved per gram of human testis, it is not surprising that total sperm production by human testes is much less than that measured for most other mammals (M0ller 1989; Sharpe 1994). Body size is an important consideration, however, as testes size, and hence the bulk of sperm-producing tissue, is affected by body size. Male rats produce less spermatozoa each day than human males, (86 million vs. 130 million on average), but rat testes weigh only 3.8 g by comparison with the approximately 40 g testes of a European man. However, we have already noted that rats produce a remarkable 24 million sperm per gram of testicular tissue each day, by comparison with just 4.4 million per gram in men (Figure 3.8). Sperm competition is likely to be intense in rats and complex mechanisms exist to maximize efficient placement of copulatory plugs and to enhance sperm transport within the female tract in this species (Adler 1978).

Sperm which leave the testes are transported via the efferent ducts to the epididymis, a complex, highly coiled, tubular organ in which sperm undergo further biochemical changes required for the attainment of motility and fertilizing capacity. Epididymal transit times vary from 1 to 12 days in the human male, and approximately 440 million sperm are stored in the tail (cauda) region (Table 3.2). Human sperm reserves are small by comparison with those of other mammals (M0ller 1989). Consider, for example, the epididymal sperm reserves of the rat (700 million), rabbit (2,200 million), rhesus macaque (13,000 million), or sheep (165,000 million). It is not surprising, therefore, that human sperm counts quickly begin to decrease as a result of repeated ejaculations. Freund (1962, 1963) showed that men's sperm counts decrease by 55 per cent when their frequencies of ejaculation increase from an average of 3.5 to 8.6 times per week. I shall return to this subject in Chapter 5, which deals with the evolution of human copulatory patterns.

In Table 3.2, a figure of 236 million is cited as the average number of spermatozoa in a human ejaculate. However, human sperm counts are exceedingly variable and it may be helpful to consider at least some of the factors involved. The vast majority of sperm samples used for clinical studies are obtained by masturbation. Men are asked to abstain from sexual activity prior to giving a semen sample, as sperm counts are intended to represent the maximum possible in a 'sexually rested' state. It is interesting that sperm numbers and sperm motili-ties are significantly greater in ejaculates collected by condoms as a result of copulation (Zavos 1985, 1988). Even those samples collected by masturbation contain larger numbers of sperm if the time taken to produce the sample is extended, and sexual arousal is prolonged (Pound et al. 2002). There are also considerable geographical variations in human sperm counts (Fisch and Goluboff 1996), and (controversially) declines in sperm counts worldwide have been reported, possibly as a result of environmental toxins (Carlsen et al. 1992). A host of factors including developmental, nutritional, and climatic variables may influence sperm counts in human populations round the world. J0r-gensen et al. (2001), for example, recorded higher sperm counts in winter, than in summer, for men in Denmark, Finland, France, and the UK. Table 3.3 provides data on human sperm counts (per ml. of semen) in twelve countries worldwide. The various studies are arranged in chronological order, given concerns about possible declines in human sperm counts during the last 50 years or so.

Some useful general conclusions may be drawn with reference to Table 3.3. Just as human beings do not have large testes in relation to body weight, so sperm numbers in the human ejaculate are modest by comparison with most mammals. The sperm count in men from Hong Kong averages 83 million/ ml, by comparison with 54.7 million/ml in Nigeria, and 66.9 million/ml in Tanzania. It will be recalled that Hong Kong Chinese have the smallest, and

Table 3.3 Geographic and temporal variations in average human sperm counts, per millilitre of the ejaculate

Location and year

USA: New York (1938)

USA: New York (1945)

USA: New York (1950)

USA: New York (1951)

USA: Washington State (1963)

Germany (1971)

USA: New York (1975)

Brazil (1979)

USA: Texas (1982)

France (1983)

Libya (1983)

Australia (1984)

Greece (1984)

Hong Kong (1985)

Thailand (1986)

Nigeria (1986)

Tanzania (1987)

United Kingdom (1989)

France (1989)

Sperm: millions

No. of men

per ml

200

120.6

100

134

100

100.7

1,000

107

100

110

100

74.4

386

48

1,300

79

185

67.6

4,435

66

809

102.9

1,500

65

119

83.9

114

72

1,239

83

307

52.9

100

54.7

120

66.9

104

91.3

1,222

77.7

Source: From Fisch and Goluboff (1996); data from Carlsen et al. (1992).

Nigerian males the largest testes sizes recorded so far (see Table 2.2). Higher sperm counts have been recorded in certain parts of the USA (e.g. New York: 100.7-120.6 million/ml). The same is true of some European studies, especially for samples collected during the winter months (e.g. Finland: 132 million/ml; Scotland: 119 million/ml; J0rgensen et al. 2001). However, setting aside the many and as yet unresolved variables affecting these sperm counts, none of them approach those recorded in species such as the chimpanzee, bonobo, rhesus macaque, or many other mammals.

Even though men do not produce large numbers of spermatozoa in their ejaculates, it is possible that they might 'allocate' sperm in differing numbers, depending upon the possible risks of sperm competition. Pound, Shackelford, and Goetz (2006) have reviewed evidence relating to this question, and I shall return to it in Chapter 5. For the moment, it is important to note that sperm counts may vary considerably among individual men, as well as between subjects in various studies. As an example,

Pound et al. (2002) found that among twenty-five regular semen donors (aged 22-44 years) concentrations of spermatozoa ranged from 12 to 156 million/ml (mean = 70.6 million/ml). Ejaculate volumes averaged 3.5 ml in these subjects, but again a considerable range was possible (0.9-9.0 ml) and total sperm counts in the ejaculate varied accordingly (from 26.4 to 834.4 million; mean = 236.1 million). All donors reported that they had maintained at least 3 days of sexual abstinence prior to giving their semen samples. Interestingly, there was a weak positive correlation between the time taken to produce a sample, sperm numbers per millilitre, and motile sperm concentrations. These effects applied to samples produced in less than 30 min (Figure 3.11). Pound et al. (2002) attribute their findings to heightened effects of sexual arousal and more prolonged penile stimulation in these subjects. Such behaviour may increase the strength and duration of smooth muscle contractions in the cauda epididymis and vas deferens, thus resulting in the transport of greater numbers of spermatozoa prior to ejaculation. This possibility raises the question of whether sexual selection has resulted in any measurable effect upon the muscular systems which transport sperm. If so, how does the control of sperm transport in human males compare with homologous mechanisms in other mammals and in relation to sperm competition pressures? These questions are explored in the next section.

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