Copulatory Pelvic Thrusting In Apes

Variable as human copulatory positions may be (due to cultural factors) they are fundamentally the same as those which occur in non-human primates. The, ventro-ventral, or 'missionary position' and female superior copulatory positions of humans are homologous with patterns described for the great apes. Elsewhere among the primates, distinctive copulatory postures are restricted to particular phylogenetic groupings. Examples discussed in this chapter include the inverted copulatory positions of the lorisines of Africa and Asia, and the leg-lock mating postures of some of the ateline monkeys in South America.

The evolution of face-to-face copulatory positions in hominoids was probably facilitated by several factors, including increased brain size and intellectual capacities, as well as the communicatory advantages of continued eye contact between partners during mating. However, another possibility discussed in this chapter is that anatomical specializations of the shoulder joint and suspensory patterns of arboreal locomotion in ancestral apes may have facilitated the emergence of face-to-face copulatory postures. These patterns may then have been retained and modified among the progressively more terrestrial australopithecines which gave rise to the genus Homo. An alternative hypothesis, which ascribes the evolution of ventro-ventral copulation in humans to enhancement of clitoral stimulation and facilitation of female orgasm, is examined and rejected on the basis of information discussed in Chapter 4.

Sexual selection has affected patterns of intromission and pelvic thrusting during copulation in nonhuman primates. Specialized copulatory patterns, involving multiple brief intromissions or a single prolonged intromission, occur most frequently in primate species where males exhibit large relative testes sizes and females engage in multi-partner matings. Sperm competition and cryptic female choice may have moulded the evolution of these specialized copulatory patterns, as well as complex genital morphologies in such species. However, there is very little evidence that sexual selection has influenced the evolution of human copulatory patterns. Although prolonged intercourse may be facilitated by learning and encouraged by cultural preferences, the fundamental human pattern is mostly likely to have been relatively brief, involving a single intromission with pelvic thrusting to achieve ejaculation.

Frequencies of copulation are also greatest in those primate species in which females mate with multiple partners during the fertile period. Although data on ejaculatory frequencies in monkeys and apes are limited, they are sufficient to show that males in multi-male/multi-female groups copulate much more frequently than those which have polygynous/monogamous mating systems. Large-scale surveys of human sexual behaviour conducted in North America, Europe and China confirm that for the majority of couples, frequencies of intercourse are commensurate with those that occur in polygynous or monogamous primates. Moreover, experimental studies that require men to ejaculate at artificially high daily frequencies clearly show that human extra-gonadal sperm reserves rapidly become depleted under such conditions (see Figure 5.12). Men, unlike males of multi-male/ multi-female species such as chimpanzees or macaques, are not physiologically adapted to sustain optimal sperm counts under conditions of sperm competition.

The comparative studies of sexual behaviour discussed in this chapter support the conclusions reached in Chapters 2, 3, and 4 on the basis of comparative analyses of mammalian reproductive anatomy and physiology. Humans have evolved from forms which had primarily polygynous or monogamous mating systems. Chapter 7 will address the relative importance of polygyny versus monogamy in ancestral hominids. First, however, the (often vexed) question of oestrus in primates, and the 'loss of oestrus' during human evolution will be examined in Chapter 6.

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