Cock and testes display and training. The external genitali..

Cock and testes display and training. The external genitalia (the pudendal pad of the female and the male’s scrotum) are richly supplied with cutaneous scent-marking glands. Both sexes also employ urine in marking displays, and olfactory cues play an integral role in social dominance as well as in sexual contexts (Epple 1980; Epple et al . 1993). The genital display may thus derive from behavioural patterns important in olfactory communication. Some other New World monkeys also engage in same-sex and oppo- site-sex genital displays (e.g. in howlers ( Alouatta) and in Capuchin ( Cebus )), although these are still poorly documented (see Dixson 1998 for a review). Stereotyped genital displays do not occur in the Old World anthropoids, except for the use of penile erection as part of pre-copulatory invitational behaviour by male chimpanzees and bonobos (Goodall 1986; Kano 1992) and orang-utans (Nadler 1988). However, genital touching, manipulation and oro-genital contacts do occur during homosexual and heterosexual interactions in a variety of monkey and ape species. Touching, or handling, of the external genitalia has been recorded as part of dominance, greeting and reassurance behaviour between male macaques, baboons, mandrills, and chimpanzees, and probably occurs in other species as well. Stimulation of the penis may also be more prolonged and purposeful, as for example during interactions between male bonobos. Genital contacts may also occur between female monkeys, as for example during mutual lateral embracing in Celebes (Sulawesian) macaques ( Macaca nigra ), stump-tails and grey-cheeked mangabeys (Dixson 1998). Whether such contacts constitute mutual genital inspection or active genital stimulation is unclear. In M. nigra , for example, females may sniff and briefly touch the partner’s vulval area during mutual embraces. Bagemihl (1999) presents a drawing of these macaques engaging in what he describes as mutual (manual) masturbation during the embrace. This was not reported in the original paper (Dixson 1977) cited by Bagemihl, however, and I include the original (unmodified) drawing here (Fig. 9.6). The requirement for caution in interpreting same-sex embracing as sexual behaviour also applies to the New World monkeys. Members of some New World genera (e.g. Ateles , Brachyteles , and Lagothrix ) engage in face-to-face embraces. In Ateles , males or females often sniff the neck region of the partner during such embraces, and there are specialised cutaneous glands situated in this area. In such cases, embracing appears to be some form of greeting, or affiliative behaviour, rather than having a primarily sexual function, and it may derive from behavioural patterns associated with mutual olfactory inspections by conspecifics. In the next section I examine the possible functions and derivation of homosexual patterns of behaviour in monkeys and apes in more detail. Does homosexual behaviour function for social communication, for example, and are there consistent relationships between social structure, and the expression of homosexual interactions, between males and between females? In 1914, the great evolutionary biologist Julian Huxley published an account of the courtship behaviour of the great crested grebe ( Podiceps cristatus ). His paper is widely recognised as the fore- runner of later developments in ethology and, as such, it introduced the concept of ritualisation as a contributory mechanism underlying the evolution of courtship displays. Huxley noted that grebes exhibited behavioural patterns derived from non- sexual activities, but which had become ritualised and incorporated into their complex courtship sequences (Figure 9.7). Almost two decades later, when Zuckerman (1932) studied the sexual and social behaviour of hamadryas baboons ( Papio hamadryas ) on ‘Monkey Hill’ at London Zoo, he noted that both sexes displayed patterns of sexual behaviour, such as hindquarter presentations and mounts, in non-reproductive contexts. These presentations and mounts often occurred in situations where submissiveness and dominance were com- municated, rather than any intent to engage in copulation. He noted that amongst male baboons ‘one assumes the feminine role and is mounted by the other’ and that, likewise, female baboons sometimes present to, or mount, other members of the same sex. Further studies of primate behaviour, involving a much broader range of species, revealed that same-sex (i.e. homosexual) interactions occurred in a variety of contexts, as a social ‘greeting’ and prior to affiliative interactions (such as grooming), during ‘reconciliation’ after aggression and so forth. Wolfgang Wickler (1967) subsequently developed the concept of socio-sexual mimicry among non- human primates. He proposed that socio-sexual behaviour had facilitated the evolution of morpho- logical traits in the male sex, to resemble those present in females. He cited the example of the hamadryas baboon, in which the male’s red rump resembles the female’s sexual skin swelling. Wickler proposed that the male’s colourful rump might serve to reduce the likelihood of inter-male aggression, especially when displayed as a mimic of female sexual skin, during male–male presentations. This ingenious idea has received little support, however, as more evidence has become available over the years. Thus, in many species of monkeys and apes which engage in homosexual presentation and mounting, males lack homologues of the female’s sexual skin. In talapoins, for example, the adult male has a blue perineum and scrotum, in contrast to the female’s pink swelling. In baboons other than the hamadryas, males do not have perineal structures resembling those of females. Nor do male chimpanzees or bonobos. The brilliant coloration of the male mandrill’s face, rump and genitalia does not closely correspond with the swelling of the female. Moreover, because the physiological mechanisms that produce cutaneous colouration are limited in the primates (i.e. vascular changes causing reddening and dermal melanin causing blue and greenish hues) it is inevitable that sexual skin, when present, may sometimes be similar in the two sexes (Grubb 1973). Setting aside Wickler’s hypothesis of socio-sexual mimicry, evidence that patterns of sexual behaviour are incorporated into same-sex activities has grown steadily over the years as well as the realisation that such patterns may be expressed in yung animals, well before puberty. Thus, infant or juvenile male macaques frequently mount each other as well as engaging in ‘rough and tumble play’. The expression of such mounting behaviour is important for the development of social relationships and for the successful expression of sexual patterns during later life (Harlow 1971).