Rapid sperm transport the vas deferens and sperm competition

Given that sexual selection has influenced relative testes sizes and hence sperm production as well as the morphology of individual gametes (sperm mid-piece volume) in mammals, we may further enquire as to whether other components of the male reproductive tract have been likewise affected by sperm competition. Rapid transport of many millions of spermatozoa during copulation is a complex process, and especially so for those species where the numbers of gametes ejaculated and frequencies of ejaculation may be greater due to competition between males for access to a given set of ova. Beyond the cauda epididymis, where sperm are pooled and stored, lays the vas deferens, the most muscular tubular duct in the human body (Figure 3.12). During sexual activity, sperm are transported rapidly through the vas deferens, by peristaltic contractions of its muscular walls. There are usually three muscle layers in the wall of the vas def-erens; these can be seen in a transverse section of

Figure 3.11 Scatter plot showing the relationship between sperm concentrations (in millions per ml.) and the time taken by semen donors to produce a semen sample (by masturbation) within a 30 min period. No. of specimens = 272.

Source: After Pound et al. (2002).

Gambar Vas Deferens
Figure 3.12 Diagram to show the anatomical relationships of the vas deferens. On the right is a transverse section of the vas deferens of a gorilla to demonstrate the powerful longitudinal and circular muscle layers surrounding the central lumen.

the duct (Figure 3.12). An outer layer of longitudinal muscles and an inner longitudinal muscle layer are separated by a central layer of circular muscles. However, thicknesses of these muscle layers may vary along the length of the vas deferens. In the distal region of the vas deferens, as, for example, in the stump-tail monkey, the inner longitudinal muscle layer may be absent. The central lumen of the vas deferens is narrow and is lined with ciliated, secretory epithelium. Some mammals have an expansion of the duct at its distal extremity, which is called the ampulla. The ampulla may be very large in certain species, such as the jackass, zebra and other equids, and absent or virtually so in others. The ampulla is of less concern to us here, as it is relatively small in the human male, but it is interesting to note that its functions still remain largely unknown. The richly folded secretory epithelium and expanded volume of the ampulla indicates that it may be an important glandular structure in some species (e.g. in the common shrew: Suzuki and Racey 1984).

Careful measurements of the length of the vasa deferentia and thicknesses of the various muscle layers in seventy genera of mammals, including Homo, have revealed some significant correlations between morphology, mating systems, and sperm competition

(Anderson, Nyholt, and Dixson 2004). The vas deferens is shorter, in relation to body weight, in mammals where females typically mate with multiple partners (MP systems) rather than with single partners (SP systems) (Figure 3.13). Like other mammals with SP systems, the human vas is relatively long in relation to body size. Muscular thickness of the vas defer-ens also varies in a consistent fashion depending on the mating system. The ratio of muscular thickness of the vas (all three muscle layers combined) to the width of the central lumen is greatest in mammals with MP mating systems. In such cases it averages 9.9, as compared to 6.8 in mammals which have SP mating systems (Figure 3.13). The human male, with a muscle wall/lumen width ratio of 6.3, thus displays an anatomical arrangement typical of mammals in which sperm competition pressure is low, as is the case in monogamous or polygynous mating systems. This finding is confirmed if we examine the thicknesses of the individual muscle layers which comprise the wall of the vas deferens. In mammals which have multiple-partner mating systems, both layers of longitudinal muscles are thicker than expected in relation to body size. The central circular muscle layer is actually thinner than expected, however, and it appears that it has undergone reduction as a result of sexual selection in mammals where sperm competition pressures are greatest. Species with SP mating systems have thinner longitudinal muscles than MP species, but their circular muscles are quite prominent and significantly thicker than in mammals where sperm competition is more marked. Examination of a transverse section of the human vas deferens reveals that it conforms to the pattern typical of SP mating systems among mammals (Figure 3.14). Thus, the longitudinal muscle layers are quite thin, and the central circular muscle layer is more pronounced in the human vas.

Residual vas deferens length **

I I Single partner ^H Multiple partner

Muscle : Lumen ratio *

Figure 3.13 Vas deferens length and thickness (muscle wall/lumen ratio) in mammals where females have single-partner vs. multiple partner mating systems. *P < .05; **P < .01.

Source: Redrawn from Anderson et al. (2004).

Muscle : Lumen ratio *

I I Single partner ^H Multiple partner

Figure 3.13 Vas deferens length and thickness (muscle wall/lumen ratio) in mammals where females have single-partner vs. multiple partner mating systems. *P < .05; **P < .01.

Source: Redrawn from Anderson et al. (2004).

Longitudinal muscle (Outer layer)

Longitudinal muscle (Inner layer)

Lumen

Circular muscle

Longitudinal muscle (Outer layer)

Longitudinal muscle (Inner layer)

Lumen

Circular muscle

Figure 3.14 Transverse section through the human vas deferens, to show the thicknesses of its two longitudinal muscle layers, and the intermediate (circular) muscles. The thickness of the circular muscles (relative to the longitudinal muscles) is typical of those mammals which have monogamous or polygynous mating systems.

Figure 3.14 Transverse section through the human vas deferens, to show the thicknesses of its two longitudinal muscle layers, and the intermediate (circular) muscles. The thickness of the circular muscles (relative to the longitudinal muscles) is typical of those mammals which have monogamous or polygynous mating systems.

Source: Author's drawing, after Bloom and Fawcett (1962).

We do not know exactly how the muscle layers of the vas deferens operate during sperm transport. It may be useful to compare the functions of the vas deferens with those which characterize other tubular organs which exhibit peristaltic movements. One example is the human oesophagus, which contains a single, outer layer of longitudinal muscles, and an inner circular muscle layer. The outer layer of longitudinal muscles typically contracts sequentially in blocks, concentrating the activity of the inner circular muscles as they in turn contract to force a bolus of food down to the stomach. Thickening of the longitudinal muscles in the vas deferens of mammals where sperm competition pressures are greatest may increase the efficiency of the circular muscles in some way and thus make it possible to reduce their bulk. The actions of the inner longitudinal muscle layer of the vas are not fully understood. In this regard, it may be relevant that the vas deferens is capable of transporting sperm in both directions. In the buck rabbit, for example, experiments with a radio-opaque dye (ethiodiol) have shown that material is transported proximally, and returned to the cauda epididymis, from the vas after sexual activity (Prins and Zaneveld 1980). In this way spermatozoa might be recouped for storage in the cauda epididymis and undue wastage of gametes prevented. Likewise, a shorter vas deferens may be significant in terms of reducing the overall distance sperm must be transported during copulation. Even when possible biases in body size are controlled for, it is clear that the vas is significantly shorter in those mammals where females commonly engage in multi-partner matings, and its walls are 45 per cent thicker in relation to width of the central lumen.

In all mammals, many millions of sperm must traverse the vas deferens very rapidly prior to ejaculation. In a species like the rhesus monkey, with large relative testes sizes and multiple-partner matings by females, more than 1000 million sperm are transferred to the female during the ejacula-tory mount. Interestingly, the male rhesus makes a series of mounts and intromissions prior to ejaculation. This specialized pattern of copulation is more commonly found in primates where sperm competition occurs (see Chapter 5, for a discussion of the evolution of copulatory patterns). It is not known whether spermatozoa are moved along the vas in cohorts during each mount of the series, so that numbers in the vas are maximized prior to ejaculation. This possibility requires experimental study, perhaps in a mammal such as the rat where the necessary experiments could be performed to count numbers of spermatozoa in the vas deferens. It may be that the vas deferens (and not just the cauda epididymis) is a temporary repository for gametes. Thus prolonged, or multiple, intromissions, may serve to fill the vas with gametes prior to the final ejaculatory phase. In the shrew (Sorex araneus) the tendency for the vas to store sperm has resulted in a more permanent arrangement, with the distal portion of the duct being distended to contain large reserves of sperm in the same way as the cauda epididymis (Suzuki and Racey 1984).

The possible effects of sexual selection upon the mammalian vas deferens will thus require deeper investigation in future. For the present, it is clear that the human vas deferens is typical of mammals which have monogamous or polygynous mating systems and where sperm competition pressures are low. This conclusion runs counter to Smith's (1984)

hypothesis that the human vas deferens might display adaptations for sperm competition. At the time his paper was published, no comparative morpho-metric studies had been attempted, and the results summarized here were not available to him.

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