Enduring Sperm Competition

Abstract

continued with the trip. On getting home late that night I put the corpse in the fridge and the next morning, some 20 hours after I'd found the bird, I dissected out its testes and seminal glomera a sperm store at the end of the vas deferens which forms the cloacal protuberance in male passerines. To count the total numbers of spermatozoa I diluted the contents of the seminal glomera in saline and examined a sample under the microscope. To my amazement the spermatozoa started to swim vigorously across the field of view! Their capacity for independent life, long after their vehicle, the bird itself had died, convincingly reconfirmed the old idea that sperm were little animals in their own right spermato zoa. Was their enduring nature unusual? On checking in the library and with several colleagues I discovered that next to nothing was known about spermatozoa in passerine birds, let alone how they might compete to fertilise eggs. Sperm competition has changed the way we view the world, although it is slightly embarrassing how long following Geoff Parker's (1970) seminal paper it has taken. But a change has certainly occurred. Scientific revolutions follow fairly predictable trajectories a new way of looking at something results in a surge of interest, followed by period of consolidation, and then finally a demise. Within the field of behavioural ecology there have been several subdisciplines, such as optimal foraging, where this pattern has occurred. It will certainly happen to the field we currently call sperm competition too, but and this is the point I wish to discuss not for a while. In fact, studies of sperm competition are still in the surge phase, so what is it that has sustained the continuing interest in sperm competition? In a word, the answer is: diversity. Sperm competition impinges on so many aspects of biology (e.g. evolution, behaviour, reproductive physiology and molecular biology), and has provided explanations for so many phenomena (e.g. interspecific differences in copulation behaviour, aspects of cooperative breeding and reproductive anatomy) that its growth is not surprising. There is an additional reason for the sustained interest in sperm competition which is that reproduction is something close to all our hearts, and unlike foraging, the consequences of reproduction have a much more direct effect on fitness. In terms of the sex-dinner principle, a fox chasing a hare is running only for his dinner, but a fox that runs after a female and copulates with her gets his genes into the next generation. One consequence of this diversity is that the meaning of the term 'sperm competition' has changed. It was used initially in a narrow sense to describe the events taking place in a female's reproductive tract following insemination by two or more males. But subsequently the term has been used to encompass all the behaviours associated with copulation, including multiple mating and paternity guards. From its inception 'sperm competition' has had male connotations (but see Parker 1979, 1984), but following the lead of Moller (1988) and Smith (1988), female-driven phenomena have now received more attention (Madsen et al. 1993, Birkhead et al. 1993a, Sheldon 1994, Gowaty 1994), including the idea that females control which sperm fertilise their eggs. This indicates that the term 'sperm competition' is no longer strictly accurate but, just as photocopiers continue to be called 'Xerox' machines, it is probably too late to change the name of this field. In future if we use the term 'sperm competition' to encompass male and female-driven phenomena then we need to say so explicitly. What has sustained sperm competition? First and foremost, sperm competition, like other aspects of behavioural ecology, rests firmly on a body of theory (e.g. Parker 1970, Trivers 1972, Westneat et al. 1990), and theoretical ideas continue to be important in providing hypotheses for empiricists to test (e.g. Parker 1984, Westneat and Sherman 1993). Darwin's concept of sexual selection is the foundation (although Darwin himself chose not to think, or at least not to write about the nitty gritty of animal reproduction), but it was Trivers' (1972) development of Darwin's ideas that really provided the turning point. Trivers used selection thinking to show that because of the fundamental differences between males and females, the interests of individuals of each sex differ, even within socially monogamous pairs. The consequence of this is that individuals of each sex should attempt to maximise their fitness. We have come a long way since then, but sexual selection continues to be the