Promiscuity: An Evolutionary History of Sperm Competition

Males are promiscuous and ferociously competitive. Females--both human and of other species--are naturally monogamous. That at least is what the study of sexual behavior after Darwin assumed, perhaps because it was written by men. Only in recent years has this version of events been challenged. Females, it has become clear, are remarkably promiscuous and have evolved an astonishing array of strategies, employed both before and after copulation, to determine exactly who will father their offspring.

Tim Birkhead reveals a wonderful world in which males and females vie with each other as they strive to maximize their reproductive success. Both sexes have evolved staggeringly sophisticated ways to get what they want--often at the expense of the other. He introduces us to fish whose first encounter locks them together for life in a perpetual sexual embrace; hermaphrodites who "joust" with their reproductive organs, each trying to inseminate the other without being inseminated; and tiny flies whose seminal fluid is so toxic that it not only destroys the sperm of rival males but eventually kills the female. He explores the long and tortuous road leading to our current state of knowledge, from Aristotle's observations on chickens, to the first successful artificial insemination in the seventeenth century, to today's ingenious molecular markers for assigning paternity. And he shows how much human behavior--from the wife-sharing habits of Inuit hunters to Charlie Chaplin's paternity case--is influenced by sperm competition.

Lucidly written and lavishly illustrated, with a wealth of fascinating detail and vivid examples, Promiscuity is the ultimate guide to the battle of the sexes.

Biologists have long been aware that sex poses different problems for male and female animals. Males typically compete for sexual access to females, and females are typically choosier. There are exceptions to this rule, but they are rare. In the vast majority of animal species, males fight among themselves to mate with females, and females choose from the victors.

In the past few decades, biologists have realised that male competition and female choice rarely stop at the moment of copulation. Unless the species is completely monogamous, females may sometimes mate with more than one male during the same fertile period. Whenever this happens, the competition between males may continue after copulation, via a process known as sperm competition. Sperm from one male encounter sperm from another male inside the female's reproductive tract, and they do battle, continuing the competition that their originators engaged in prior to copulation. Similarly, the female continues to exercise her power of choice by selecting from the rival sperm she finds inside her.

Incredible as this may seem, the phenomena of sperm competition and sperm choice, as they are called, have now been well documented in dozens of species. In a new book for Faber and Faber, Tim Birkhead gives an accessible and comprehensive overview of this research. Birkhead is Professor of Behavioural Ecology at the University of Sheffield, and has spent many years researching the reproductive behaviour of birds, but his book covers many other kinds of animals too. From shrimp and water bugs to hamsters and hyenas, Birkhead takes the reader on a guided tour of infidelity and its consequences. Promiscuity is perhaps misleading as a title, as it suggests that females are being less choosy in mating with multiple males, when it is often precisely the opposite. The main lesson of the book is that multiple mating by females is often a way of being more discriminating, because it allows the female to compare the quality of rival sperm directly.

This is a fascinating and well-written introduction to one of the most interesting areas of research in contemporary biology.--Dylan Evans