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This final chapter echoes this sentiment. First, it discusses insights and considers some of the many unresolved questions arising from the preceding chapters. Then, there is a discussion of how broad-based, comparative studies of mammals can inform debates concerning post-copulatory sexual selection and the evolution of human reproduction. Finally, some thoughts are offered concerning the consequences of uncontrolled human population growth and the ongoing extinction of our fellow mammals.

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When Arthur Walton wrote these words, which appeared in the third edition of Marshall’s Physiology of Reproduction, he was unable to account for the evolutionary basis of phallic diversity in mammals, some examples of which are shown in Figure 5.1. Indeed, prior to the application of sexual selection theory to this problem (Eberhard, 1985, 1996; Parker, 1970), the presence of so much variability among male invertebrates, and among the vertebrates as a whole, had remained a mystery. The purpose of this chapter is to explore the various reasons for phallic morphological diversity, including the role that copulatory and post-copulatory sexual selection may have played in this context.

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The last chapter considered the act of mating primarily from the perspective of male mammals, and discussed the effects of phylogeny and modes of life upon masculine patterns of copulatory behaviour. By contrast, this chapter addresses the interplay between the sexes that occurs during copulation. In many mammals, females mate with multiple partners during the fertile period. Under these conditions, the stage is set for sexual selection, via cryptic female choice, as well as sperm competition, to influence the fate of gametes that are deposited in the female reproductive tract by rival males. Thus, in what follows, the behaviour and physiological responses of both sexes will be discussed in relation to events that take place during and after copulation. Figure 4.1 shows, in diagrammatic form, relationships between sperm competition and cryptic female choice. On the left-hand side of the diagram, sperm from several males are depicted as ‘competitors in a race’, as they vie to to gain access to an ovum. The female’s reproductive tract is a ‘level playing field’ in which this contest takes place. The right-hand side of the diagram introduces a note of reality into this androcentric vision. Sperm do not have direct access to ova; the vagina, cervix, uterus, uterotubal junction and oviduct all present challenges to the survival and onward progression of spermatozoa. The female’s reproductive anatomy and physiology play crucial roles in transporting sperm, and also in controlling the temporary storage of gametes. Thus, although vast numbers of spermatozoa are released at ejaculation, few of them ever gain proximity to an ovum.

Ronald Hunter (1995) summarized the many hazards that mammalian spermatozoa encounter during their passage through the female reproductive tract (Figure 8.1). Most of the sperm perish, being engulfed by an army of leucocytes, or entrapped by mucus, so that they advance no further than the cervix, uterus or uterotubal junction. Some of them fail to undergo physiological changes, triggered by the female’s physiology, that are essential if they are to fertilize ova (e.g. capacitation, hyperactivation of sperm motility and the acrosome reaction). Hence, only a small fraction of male gametes that enter the female tract will reach her oviducts and, of these, only one will unite with each ovum to bring about fertilization (Table 8.1).

Previous articleNext article No AccessAnatomy, Physiology, and DevelopmentReproduction in Mammals: The Female Perspective. By Virginia Hayssen and Teri Orr. Baltimore (Maryland): Johns Hopkins University Press. $69.95. xv + 352 p.; ill.; common name, scientific name, and subject indexes. ISBN: 9781421423159 (hc); 9781421423166 (eb). 2017.Alan DixsonAlan DixsonSchool of Biological Sciences, Victoria University of Wellington, Wellington, New Zealand Search for more articles by this author School of Biological Sciences, Victoria University of Wellington, Wellington, New ZealandPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Quarterly Review of Biology Volume 93, Number 3September 2018 Published in association with Stony Brook University Article DOIhttps://doi.org/10.1086/699456 Views: 56Total views on this site For permission to reuse, please contact [email protected]PDF download Crossref reports no articles citing this article.

An all-male band of golden snub-nosed monkeys (Rhinopithecus roxellana) was observed for 3 months in the Qinling Mountains of China, in order to collect data on the frequencies and contextual significance of male-male mounting behaviour. Mounts occurred in a variety of affiliative, dominance-related and sexual contexts, which differed depending upon the ages of the males involved. Mounting behaviour in this group was mainly initiated by adults. Juveniles mounted each other in affiliative contexts (during play and prior to grooming). Adult males mounted subadult and juvenile partners in a greater variety of sociosexual contexts (dominance/rank-related interactions; reconciliation following agonistic encounters, and sometimes as a prelude to receiving grooming). However, subadults and juveniles were never observed to mount adults. In one dyad, involving an adult male and a subadult partner, mounting was more frequent and prolonged, and included bouts of deep pelvic thrusting. Two mounts resulted in anal intromissions and, in 1 case, the subadult partner exhibited seminal emission. Given that the study took place during the annual mating peak period of R. roxellana, it is possible that this unusual male-male sexual activity was related to the absence of mating opportunities for those adults that were excluded from 1-male units.

The woods are lovely, dark and deep,But I have promises to keep,And miles to go before I sleep,And miles to go before I sleep.Robert Frost

Women's breast morphology is thought to have evolved via sexual selection as a signal of maturity, health, and fecundity. While research demonstrates that breast morphology is important in men's judgments of women's attractiveness, it remains to be determined how perceptions might differ when considering a larger suite of mate relevant attributes. Here, we tested how variation in breast size and areolar pigmentation affected perceptions of women's sexual attractiveness, reproductive health, sexual maturity, maternal nurturing abilities, and age. Participants (100 men; 100 women) rated images of female torsos modeled to vary in breast size (very small, small, medium, and large) and areolar pigmentation (light, medium, and dark) for each of the five attributes listed above. Sexual attractiveness ratings increased linearly with breast size, but large breasts were not judged to be significantly more attractive than medium-sized breasts. Small and medium-sized breasts were rated as most attractive if they included light or medium colored areolae, whereas large breasts were more attractive if they had medium or dark areolae. Ratings for perceived age, sexual maturity, and nurturing ability also increased with breast size. Darkening the areolae reduced ratings of the reproductive health of medium and small breasts, whereas it increased ratings for large breasts. There were no significant sex differences in ratings of any of the perceptual measures. These results demonstrate that breast size and areolar pigmentation interact to determine ratings for a suite of sociosexual attributes, each of which may be relevant to mate choice in men and intra-sexual competition in women.