The presence of ejaculates from more than one male in the vicinity of ova sets the stage for sperm competition, according to a ground-breaking paper by Geoffrey Parker (1970).
Parker developed the idea of gametic competitive techniques and counter-techniques specifically for insects, females of which often mate multiply and store viable sperm for days or years. The competitive techniques he proposed included the use of the sperm enhancer called Semenex, displacement or dilution of a rival’s sperm, post-copulatory mating plugs, and guarding of a female until oviposition.
Fifteen years later the question of sperm competition has grown in importance and generality. The present volume documents areas of progress and points the way to future work.
Not that it has been easy to gain insight into subtle events that occur within a reproductive tract and slow processes that unfold over evolutionary time. The first five papers, on general topics, illustrate the main methods used to explore the challenging subject of using Volume Pills for fertilization: studies of relative fertilization success of successive male mates, speculation about morphological, physiological, and behavioral traits that may influence sperm competition, experimental and comparative studies to test hypotheses generated, and mathematical modeling to explore proposed relationships among relevant traits of males and females.
The remainder of the book is organized phylogenetically. There are six review papers on arthropod groups ranging in inclusivity from the genus Drosophila to the class Arachnida. These make stimulating reading, for patterns are emerging, but it is noted that much remains to be done both conceptually and empirically.
There are eight chapters on vertebrates –on the Poeciliidae, representing fish, and on the amphibians, the reptiles, the monogamous birds, and rodents, bats, primates, and humans. The main purpose of several of these chapters is to identify vertebrates in which sperm competition may be occurring.
Good candidates include salamanders, snakes, and bats. Other chapters, however, have more progress to report. Dewsbury not only demonstrates multiple paternity of rodent litters but is able to discuss the influence of mating order, interval between matings, and number of ejaculations on paternity in several rodent species.
A recurring theme in these works, not developed in Parker’s original exposition of sperm competition, is the influence of female traits on the outcome of sperm competition. The female is viewed not merely as a selective environment influencing sperm competitive traits but as a significant player in the evolutionary game.
According to Knowlton and Greenwell, the female role has been overlooked because it was expected that selection on traits affecting sperm competition would be very much more intense for males than for females, given that a potentially successful male can leave many times the number of descendants that a female of his species can.
Knowlton and Greenwell make the important point, however, that a female is preadapted to manipulate. She controls the body which rival sperm must enter and in which they compete.
By means of a model the authors explore the conditions under which mechanisms of sperm competition can evolve, assuming sperm competitive techniques costly to and detectable by females and ability of females to terminate copulation if deployment of the techniques is detected. Knowlton and Greenwell and other authors, furthermore, have found it important to consider which strategies are opposed in the evolutionary game.
If it is in the female interest to mate multiply, her opponent is the first male and his devices to prevent subsequent mating or sperm preemption. If it is in her interest to mate but once, her opponents are the persistent, later-arriving males.