Tim Birkhead, May 2005

(extract from ‘The More Deadly of the Species’’)

Female birds do it, female honey bees do it; female mites, crabs, fish, reptiles, even female mammals are at it. They all copulate with several different males each breeding cycle. Biologists are used to the concept of male promiscuity, but promiscuous females are a whole new ball game. Until recently, scientists thought promiscuity was not in the interests of females, but a number of studies over the past few years clearly show that having sex with many different partners can no longer be seen as an exclusively male preserve. Female promiscuity generates a special form of competition between males - competition between their sperm for fertilisations. Charles Darwin and, until about 1970 most of his followers, assumed that competition between males and the act of choosing between male suitors by females - processes which together comprised his ingenious idea of sexual selection - ceased at mate acquisition. The recent discovery of female promiscuity means that sexual selection continues after copulation.

In the currency of evolution, males do not compete for females, they compete for fertilisations. By doing so, they generate what is referred to as sperm competition. The more likely it is that females copulate with different males, or the more males they copulate with, the greater the intensity of competition between sperm in the female's reproductive system.

Let's consider birds. Like us, in most species a male and female rear babies together as an apparently monogamous pair. Yet DNA studies reveal that often the babies a male helps to raise are not his own. In the reed bunting, 55 per cent of all offspring are fathered by other males. In swallows it is 25 per cent, in blue tits and house sparrows the figure is 12 per cent. True sexual monogamy is a rare thing: Shakespeare's model of avian monogamy, the swan, is one of the few birds to have remained unsullied by the molecular revolution.

The evolutionary benefits to males of such promiscuity are obvious: more offspring and more copies of their genes in subsequent generations. But until the mid-1980s it was assumed that females gained little or nothing from being promiscuous. The naive view that females were passive, acquiescent participants in infidelity was shattered when the females of some birds were seen actively seeking illicit matings.

What about humans? Researchers have used three techniques to try and discover the extent to which female promiscuity occurs: questionnaires, paternity studies and anatomical evidence.

In their 1995 book, ’Human Sperm Competition’ (Chapman & Hall, London), biologists Robin Baker & Mark Bellis suggest that females in contemporary Western society are rather promiscuous, basing their findings on a questionnaire placed in Company magazine. Their review of paternity studies also suggested frequent infidelity, with extra-pair paternity running between 1.4 per cent and 30 per cent in different communities. Finally, Baker & Bellis presented some extraordinary anatomical information and proposed that different sperm types had evolved specifically in response to sperm competition generated by female promiscuity.

Their most extreme idea was 'kamikaze sperm' - sperm which, on contacting those of another male, exploded and killed both themselves and their rival. Although this might sound surreal, similar things have been recorded in other animals. Baker & Bellis's evidence for intense sperm competition in humans was ingenious, but unfortunately it has not survived subsequent scientific scrutiny. The best evidence that humans evolved with only a modest degree of sperm competition comes from some additional anatomical studies. Across a range of different non-human animals, males are found to have relatively larger testicles in species with more intense sperm competition. This makes sense - males can swamp the competition by inseminating more sperm, and bigger testicles produce more sperm.

This association between relative testicle size and the intensity of sperm competition is so consistent that we can use it to gauge the basic level of sperm competition in humans. Among gorillas, for instance, female promiscuity and sperm competition are almost unknown: the male's testicles are relatively tiny, weighing in at a mere 30g or 0.03% of body weight. Chimpanzees, on the other hand are highly promiscuous (females may copulate a thousand times for each pregnancy) and, despite their smaller body size males have huge testicles - 119g or 0.3% of their body weight. Human testicles fall in between these two simian extremes (40g or 0.08 per cent of body weight), suggesting only modest levels of female promiscuity in our evolutionary past.