![]() I shall return to this question in the next chapter, which deals with sexual selection by cryptic female choice. Measurements of penile and vaginal lengths in the chimpanzee indicate that the long and filiform penis probably co-evolved in association with the development of the large sexual skin swelling which characterizes this species. troglodytes (Izor, Walchuk, and Wilkins 1981). paniscus, terminating in a 'Y' shaped urethral aperture as compared to the simpler, slit-shaped opening of P. The tip of the penis is distinctive in P. A glans penis is lacking, and distally the penis is filiform and contains a very small baculum (6.9 mm in P. The gorilla's genitalia are remarkably similar (in miniature) to the human condition.Īmong the African apes, the chimpanzee and the bonobo have the most specialized and derived penile morphologies. In Figure 3.23, I have included an illustration of the external genitalia of a sub-adult male gorilla, from a rarely cited paper by Hill and Matthews (1949). sapiens and the gorilla which, despite having a very small penis in relation to its body size, exhibits a distal morphology more similar to the human condition than is the case for other apes. stress that reduced length of the portion of the penile shaft covered by the prepuce (the pars intrapreputialis) in man is unusual, citing this as an adaptation to assist in removing semen from the vagina. A helmet or acorn-shaped glans is common amongst Old World monkeys, such as various colobines, macaques, baboons, mangabeys, and guenons, regardless of whether they have polygy-nous or multi-male/multi-female mating systems (see Figure 3.21 for examples). Despite their contention that the large diameter of the human glans and its posterior margin (corona) represent adaptations to displace semen and provide an advantage in sperm competition, I can find no comparative evidence to support this view. They used models of human penes and vaginae to examine the putative effects of copulatory movements upon displacement of previously deposited (artificial) semen. (2003) tested Baker and Bellis's (1995) hypothesis concerning the 'plunger' action of the human glans penis in relation to sperm competition. In some of them, the glans is filiform or plunger-shaped, whilst the baculum may protrude distally, carrying the external opening of the urethra beyond the tip of the glans. Twenty genera score higher than Homo for complexity of the distal penis. Overall distal complexity of the penis (the shape and size of the penile glans or the equivalent if a glans is lacking) is rated as 3.0 for Homo the same score as achieved by twenty-one other genera. However, sixteen nonhuman primate genera also score equally high for this trait. Human beings do achieve a maximum score for one trait ( penile length). Leontopithecus, Callimico, Erythrocebus, Theropithecus) and less than the ratings given to twenty-seven of the forty-eight primate genera included in the study. This is the same rating as scored by a number of putatively monogamous or polygynous primates (e.g. The overall rating for all four traits analysed is 10 for H. Human penile morphology is not exceptional when compared to that of the prosimians, monkeys, and apes. MP mating systems for their various penile traits. Thus, Figure 3.22 shows the overall comparisons between SP vs. Multi-male/multi-female and dispersed mating systems are characterized by occurrences of multi-partner (MP) matings by females, large relative testes sizes, and higher sperm competition pressures among males. Polygyny and monogamy involve predominantly single partner (SP) matings by females and lower relative testes sizes among males. sapiens might equally be included with the monogamous primate genera for statistical purposes. The same rationale was applied to comparative analyses of relative testes sizes in human beings and non-human primates in Chapter 2. Homo sapiens has been included in the comparative analysis as a polygynous primate species, on the basis that the majority of recent human cultures are at least partially polygynous (Ford and Beach 1951). Reflect a phylogenetic tendency among prosim-ians to exhibit this trait, and not just the effects of sexual selection (Harcourt and Gardiner 1994).
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