Bateman's principle

Angus John Bateman (1919–1996), an English geneticist, suggested that in most species, variability in reproductive success (or reproductive variance) is greater in males than in females. This can be seen as the result of anisogamy. Bateman suggested that since males are capable of producing millions of sperm cells with little effort, while females invest much higher levels of energy in order to nurture a relatively small number of eggs, the female plays a significantly larger role in their offspring's reproductive success. Thus Bateman’s paradigm views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully.

Typically it is the females who have a relatively larger investment in producing each offspring. Bateman attributed the origin of the unequal investment to the differences in the production of gametes: sperm are cheaper than eggs. A single male can easily fertilize all females' eggs: she will not produce more offspring by mating with more than one male. A male is capable of fathering more offspring if he mates with several females. By and large, a male's potential reproductive success is limited by the number of females he mates with, whereas a female's potential reproductive success is limited by how many eggs she can produce. This results in sexual selection, in which males compete with each other, and females become choosy in which males to mate with. As a result of being anisogamous, males are fundamentally promiscuous, and females are fundamentally selective.

Bateman initially published his review in 1948. He was a botanist, contributing to the literature of sexual selection only once in his lifetime. Bateman initially saw his study on Drosophila to be a test of Charles Darwin’s doctrine of sexual selection. He saw Darwin’s theory of natural selection not as flawed, but as incomplete. He felt that if he were to provide a concrete demonstration of how sexual selection played a role in the reproductive success of certain species, he could explain the gap between Darwin’s ideas and sexual dimorphism.

Although it is common to confuse Bateman's ideas with those of later scientists, his principle can be expressed in three simple statements. The first is that male reproductive success increases with the number of mates they attempt to copulate with, while female reproductive success does not. The second is that male reproductive success will show greater variance than female. The third is that sexual selection will have a greater effect on the sex with greater variance in reproductive success.

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