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.

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.

In addition to spermatozoa, the products of a number of androgen-dependent accessory reproductive glands are transferred to the female during copulation. These include secretions of the vesicular glands and the prostate (which includes the coagulating glands in some taxa), the Cowper’s, or bulbourethral, glands and ampullary glands. The accessory reproductive glands differ in their taxonomic distribution, sizes and secretory functions. These differences are discussed here, in relation to possible evolutionary effects of sexual selection at copulatory and post-copulatory levels. Although we are currently far from understanding the physiological significance of most of their secretions, some functions of the accessory glands are becoming clearer. This is particularly true of the processes that control seminal coagulation, and the production of ‘copulatory plugs’. Seminal coagulation and plug formation play important roles in sperm transport and survival. Much of this chapter focuses on these topics.

Representatives of the Class Mammalia are to be found on every continent, and in every ocean of the world. They comprise a handful of extraordinary egg-laying species (the monotremes), as well as 7 orders of marsupials and 19 orders of placental mammals. Phylogeny, and modes of life, as well as sexual selection (via sperm competition and cryptic female choice) have all profoundly influenced mammalian reproductive biology. The reproductive adaptations of these animals have been forged over vast spans of time, beginning some 297–252 million years ago, during the Permian Period, when one branch of the synapsid reptiles (the Therapsida) gave rise to the furry, warm-blooded forerunners of the Class Mammalia (Kemp, 2005). Some therapsids survived a major extinction event that occurred at the close of the Permian; their modern descendants include the echidnas, kangaroos and possums, elephants, whales, rodents, bats and primates, as well as a host of other taxa.

Evidence concerning occurrences of multiple-partner matings by female mammals was reviewed in Chapter 4. There, the conclusion reached was that sperm competition is widespread among extant members of the Monotremata, Marsupialia and Placentalia. Because sperm competition arises when the gametes of two or more males are situated in contention for access to a given set of ova, males that are able to produce and store larger numbers of sperm prior to mating may then gain a reproductive advantage via post-copulatory sexual selection. Greater sperm production may be achieved by increasing the mass of gamete-producing (seminiferous) tissue in the testes, or by increasing the rate of spermatogenesis. Evidence regarding both these mechanisms is discussed in this chapter. When sperm leave the testis, they pass via the excurrent ducts into the epididymis and are stored in its terminal region (cauda) prior to copulation. Transit times and numbers of sperm stored in the cauda are discussed here, as they may also be affected by post-copulatory sexual selection.

So great is mammalian ecological and anatomical diversity that mating can take place under widely differing conditions depending upon which species is considered (e.g. in the water, on land, in the rainforest canopy, in burrows and on ice floes). This chapter focuses on the phylogenetic distribution of copulatory and associated behavioural traits throughout the Mammalia. The goal of this exercise is to determine, as far as possible, the degree of homology or convergent evolution that might exist between the various taxa, regarding their patterns of copulatory behaviour. As part of this exercise, it is important to consider the extent to which copulatory traits might have been moulded by ecological factors and by natural selection. This will pave the way for the discussions of the role played by sexual selection, in the next chapter.

In most marsupials and placental mammals such as the primates, perissodactyls and elephants, as well as many rodents and artiodactyls, females ovulate ‘spontaneously’, meaning that ovulation proceeds in the absence of copulation. However, in certain species, such as rabbits, cats, and ferrets, stimulation provided by the male partner during mating is required to trigger the pre-ovulatory LH surge and thus to induce ovulation. Often, tactile stimulation during mating triggers ovulation, but in certain cases chemical cues, present in the semen, stimulate the LH surge. Examples of this latter type occur in members of the Family Camelidae, which includes the llamas and alpacas, as well as the dromedary and the bactrian camel.

The last 25 years have seen significant developments in the fields of mammalian classification and evolutionary biology. Molecular genetic techniques have increasingly been applied to test traditional classifications that were based upon fossil evidence and comparative anatomical studies of extant taxa. As well as confirming many of the established tenets of mammalian classification, some fresh insights have emerged as a result of these endeavours. The following brief review of the phylogeny and basic biology of the 27 extant orders of mammals is designed to provide the reader with a sound basis for the discussions of copulatory behaviour, reproductive biology and evolution that unfold in the ensuing chapters.

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.