The accessory reproductive glands and sperm competition

Only 5 per cent of the volume of human semen is made up of fluids contributed by the testes and epidi-dymis. Spermatozoa which leave the epididymis are transported rapidly through the vas deferens, and then mixed with the secretions of various accessory reproductive glands which provide the bulk of the ejaculate. Adult male mammals possess an impressive variety of these glands, including the seminal vesicles, prostate, and ampullary and bulbourethral (Cowper's) glands (Eckstein and Zuckerman 1956; Price and Williams-Ashman 1961; Hamilton 1990). Most relevant to the present enquiry are the seminal vesicles and prostate gland (Figure 3.15). The human seminal vesicles are tubular glands, each measuring approximately 15 cm when straightened, but normally configured as compact coiled structures measuring about 5 x 2.5 cm. The seminal vesicles produce approximately 60 per cent of the seminal fluid in the human ejaculate. The human prostate is a chestnut-shaped gland, approximately 4 cm in diameter, although it may be enlarged in older men. Prostatic secretions constitute approximately 30 per cent of human semen. The anatomical relationships of the vas deferens, seminal vesicles, and prostate gland are shown in Figure 3.15, which also shows the much smaller bulbourethral glands (approximately 10 mm in diameter in the human male).

A prostate gland is present in all mammals. It is represented by rudimentary ducts in the duckbilled platypus (a monotreme) and by much more complex, compartmentalized structures in the marsupials and eutherians. Seminal vesicles are present in the majority of eutherians, but they are absent in marsupials, in which the Cowper's glands may be greatly enlarged. Some eutherians retain only atrophic seminal vesicles, and the glands are absent in the majority of carnivores and in the cetaceans (Price and Williams-Ashman 1961).

Pubic bone

Corpus cavernosa Corpus spongiosum

Urethra Vas deferens

Penis

Corona Coronal sulcus Glans Prepuce

Shaft

Epididymis Seminiferous tubules

Scrotal skin

Dartos muscle

Figure 3.15 Diagram of the structure of the adult male reproductive tract in H. sapiens. Source: After Piñón (2002).

Shaft

Corona Coronal sulcus Glans Prepuce

Bladder Vas deferens

Seminal vesicle

Prostate

Bulbourethral gland

Epididymis Seminiferous tubules

Testis

Scrotal skin

Dartos muscle

Figure 3.15 Diagram of the structure of the adult male reproductive tract in H. sapiens. Source: After Piñón (2002).

The huge morphological diversity displayed by the seminal vesicles of mammals is intriguing, in view of the possible functions of seminal vesicular secretions in reproduction and sperm competition. As well as producing the bulk of the fluid in which human sperm are transported at ejaculation, the seminal vesicles also provide an alkaline secretion (pH 7.2-7.8) which may assist spermatozoa to survive in the more hostile acidic environment of the human vagina (Masters and Johnston 1966; Fox, Meldrum, and Watson 1973). Seminal vesicular secretions also contain an impressive variety of chemical constituents including sugars (notably fructose), prostaglandins (19-hydroxylated pros-taglandins in human males), and proteins (Mann 1964; Mann and Lutwak-Mann 1981). Proteins are especially important for the coagulation of semen which occurs after ejaculation in many mammals. In primates, two proteins (semenogelin 1 and seme-nogelin 2) are involved in this process of coagulation, which is catalysed by an enzyme (vesiculase) produced by the cranial lobe of the prostate gland.

Gertrude van Wagenen first described the effects of prostatic secretions upon seminal coagulation in the rhesus monkey, in a paper published in 1936. Coagulation produces a soft, whitish gelatinous material, but in some primates a more solid, rubbery copulatory plug is formed (e.g. in the chimpanzee, Figure 3.16). The functions of mammalian copula-tory plugs have been much debated. Although it is tempting to speculate that copulatory plugs might act as physical (or chemical) barriers to the sperm of rival males, where mammals are concerned there is little experimental support for this view (Har-tung and Dewsbury 1978). It seems more likely that copulatory plugs or coagulated semen may facilitate sperm retention and survival within the female reproductive tract (e.g. in the rat: Blandau 1945; in the rhesus macaque: Settlage and Hendrickx 1974). Copulatory plugs do not occur in human beings. For the moment, it is sufficient to note that in some primates and other mammals, one function of the seminal vesicles may be to assist coagulation of semen and to enhance male fertility.

Lemurfulvus Lemur catta

Lemurfulvus Lemur catta

Loris tardigradus Pan troglodytes

Figure 3.16 Primate copulatory plugs. The cervical end of the plug is shown on the right hand side in each case. Note that Lemur fulvus is now classified as Eulemur fulvus. Source: After Dixson (1998a).

Loris tardigradus Pan troglodytes

Figure 3.16 Primate copulatory plugs. The cervical end of the plug is shown on the right hand side in each case. Note that Lemur fulvus is now classified as Eulemur fulvus. Source: After Dixson (1998a).

Given the diverse functions of the seminal vesicles, it is significant that these structures are largest in those primate species which have multi-partner mating systems and an increased likelihood of sperm competition (Dixson 1998a, 1998b). This is the case, for example, in the chimpanzee and bonobo, in the Old World macaques, mandrills and most baboon species, as well as in the spider monkeys and woolly spider monkeys (muriquis) of South America. By contrast, those primates which have primarily monogamous or polygynous mating systems also have relatively small seminal vesicles in relation to body size (Figure 3.17). Examples include the smaller apes (gibbons), the gorilla and gelada, as well as various New World monkeys which live in small family groups (e.g. the marmosets, owl monkeys, and titi monkeys). Reduction of the seminal vesicles may have occurred in some of these cases, because these structures are physiologically costly to maintain and are less advantageous in those mating systems where sperm competition pressures are low. The seminal vesicles are of medium size in Homo sapiens, not as large as in those primates which have multi-male/multi-female mating systems, but somewhat larger than in the typically monogamous species.

These findings are reinforced by the results of work to examine seminal coagulation and copulatory plug formation in relation to primate mating systems (Dixson and Anderson 2002). Using a four-point scale (from 1 = no coagulation, up to 4 = copulatory plug formation), ratings of seminal coagulation were obtained for twenty-six primate genera, including the apes and human beings. Coagulation ratings were greatest (mean = 3.64) for those genera in which females commonly mate with multiple partners, and lowest (mean 2.09) in monogamous and polygynous forms. This result was highly significant (P < . 001) and the coagulation rating for Homo sapiens (2.0) was the same as those for the gorilla, and various gibbons and marmosets, as summarized in Figure 3.17. None of the primate species for which adequate information exists exhibits complete absence of coagulation. It appears that some degree of biochemical interaction between seminal vesicular and prostatic secretions is common to all primates. Based upon this common heritage, a more marked degree of seminal coagulation or copulatory plug formation may have evolved due to some reproductive advantage (e.g. in sperm competition). A coagu-lum, such as occurs in many macaques, baboons, and mandrills, may promote sperm survival by retaining gametes in an alkaline environment, rather than exposing them directly to the hostile, acidic conditions in the vagina. Under conditions

Seminal vesicle sizes

Seminal coagulation

Multiple partner

Single partner

Multiple partner

Multiple partner

Single partner

Multiple partner

N = 27 genera including Homo N = 26 genera including Homo

Figure 3.17 Seminal vesicle sizes and seminal coagulation ratings for primates in which females mate primarily with single partners, or with multiple partners, during the fertile period. ***P < .001.

Source: Based on data from Dixson (1998b), and Dixson and Anderson (2002).

of intense sperm competition, further specializations might have occurred to enhance coagulation and plug formation, thus maintaining a sperm-rich fraction of the ejaculate in close contact with the os cervix and facilitating sperm transport into the uterus. Such specializations are absent in Homo sapiens because sexual selection did not favour their evolution in the ancestral forms which gave rise to human beings.

Subsequent to the reports described above, molecular studies have shown that a relationship exists between sexual selection and evolution of the seme-nogelin genes in primates (Jensen-Seaman and Li 2003; Dorus et al. 2004). Dorus and colleagues compared rates of evolution of the semenogelin 2 gene in twelve primate species (including the great apes and human beings). They found that rates of evolution were accelerated in those species where sperm competition was most pronounced, as reflected by larger relative testes sizes, higher semen coagulation ratings, and more frequent multi-partner matings by females (Figure 3.18). The rate of semenogelin 2 gene evolution in Homo sapiens is comparable to that measured in other primates which are monogamous or polygynous, and notably less than that of macaques and chimpanzees, which have multi-male/multi-female mating systems.

Less information is available on the sizes and functions of mammalian prostate glands in relation to sperm competition, but recent work on rodents (Ramm, Parker, and Stockley 2005) and on a variety of other mammals (Anderson and Dixson, in press) confirms that prostate weights as well as weights of seminal vesicles are correlated with relative testes sizes and multi-partner mating systems. Both seminal vesicular weight and prostate weight in human beings are comparatively small in relation to body weight. By comparison with other mammals, the accessory reproductive glands are of modest size and do not provide evidence for significant effects of sexual selection during human evolution.

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