Bateman's principle

In biology, Bateman's principle is the theory that the sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete.

Typically it is the females who have a relatively larger investment in producing each offspring. A single male can easily fertilize all a female's eggs: she will not produce more offspring by mating with more than one male. A male is capable of fathering more offspring than any female can bear, if he mates with several females. By and large, a male's reproductive success increases with each female he mates with, whereas a female's reproductive success is not increased nearly as much by mating with more males. This results in sexual selection, in which males compete with each other, and females become choosy in which males to mate with.

Bateman's observations came from his empirical work on mating behaviour in fruit flies. He attributed the origin of the unequal investment to the differences in the production of gametes, sperm are cheaper than eggs. Animals are therefore fundamentally polygynous, as a result of being anisogamous.

"A male can easily produce sperm in excess of what it would take to fertilize all the females that could conceivably be available [...] Hence the development of the masculine emphasis on courtship and territoriality or other forms of conflict with competing males." --Williams, 1966.

"in most animals the fertility of the female is limited by egg production which causes a severe strain on their nutrition. In mammals the corresponding limiting factors are uterine nutrition and milk production, which together may be termed the capacity for rearing young. In the male, however, fertility is seldom likely to be limited by sperm production but rather by the number of inseminations or the number of females available to him... In general, then, the fertility of an individual female will be much more limited than the fertility of a male... This would explain why in unisexual organisms there is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females." --Bateman, 1948.

"among polygynous species, the variance in male reproductive success is likely to be greater than the variance in female reproductive success." --Huxley, 1938.

"The female, with the rarest exceptions, is less eager than the male... she is coy, and may often be seen endeavouring for a long time to escape." --Darwin, 1871.

Sex-role reversal
The most well known exceptions to Bateman's principle are the existence of sex-role reversed species such as pipefish (seahorses), phalaropes and jacanas in which the males perform the majority of the parental care, and are cryptic while the females are highly ornamented and territorially aggressive (Emlen & Oring, 1977; Knowlton 1982; Berglund et al. 2005).

Other examples of violations to Bateman's Principle
Some modern evolutionary biologists believe Bateman's principle is often not correct. Olivia Judson argues that the formulation of Bateman's principle was limited by such things as short observation time in his experiments. Tim Birkhead has also documented extensive examples of exceptions to Bateman's principle, with a focus on sperm competition.

Observation of many species, from rabbits to fruit flies, has shown that females have more children if they have sex with more males. This is in contradiction to Bateman's theory that "a female's reproductive success is not increased by mating with more males."

Research has also shown some species in which males will guard one female and mate only with her, attempting to prevent her from mating with any other males. Examples include stick insects and Idaho ground squirrels. These observations seem to challenge Bateman's theory that "a male's reproductive success increases with each female he mates."

The assumption that females have a relatively larger investment in producing offspring is also often false. In animals that spawn into the sea, for example, each sex's investment is approximately equal. In animals with internal fertilization, many sperm must be produced for every egg - so even though it takes less energy to create one sperm than one egg, males of many species spend more energy making gametes than do females. The statement that the sex that invests the most in producing offspring will become a limiting resource is not always true. In flowers, for example, the female part of the flower invests more energy into making seeds than the male part of the flower does. The reproduction of most flowering plants, however, is limited by delivery of the male gamete - pollen - not by production of the female gamete.

Bateman's statement "there is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females" and his assumption that anisogamous species would be polygynous is false. Actually, females of most species mate with several males.