Female Reproductive Variance
The degree to which sexual selection pressures female members of a species. Female reproductive variance is tied to the fertility of the individual and less to their ability to mate and reproduce. Hence, female reproductive variance is quite low relative to male reproductive variance.
Research has found a pattern of reproductive variance for many species in which females have far less variability in their reproductive output than males. Three factors contribute to the pattern of mating behaviors that are typical for females: most females successfully reproduce, the amount of copulations shows a weak relationship with the number of offspring for females, and for many species, females have a greater minimum obligate investment to produce viable offspring.
Female Reproductive Variance
Angus Bateman is among the first to formally study reproductive variance. In 1948, Bateman tracked the reproductive output of male and female fruit flies (Drosophila melanogaster) (Bateman 1948). Bateman made two important observations regarding sex differences. First, very few females failed to successfully mate (about 4%), while a much larger proportion of males failed to reproduce. Second, the reproductive output of females showed a much weaker relationship with the number of copulations than for males. Based on these observations, Angus Bateman articulated what would be known as Bateman’s principle: reproductive variance is greater for males than for females.
This relationship has been found consistently across a wide range of species. Among humans, the majority of women who are able to produce offspring will be able to, assuming they have the desire to do so. Additionally, the maximum amount of offspring a woman can birth in her lifespan is highly limited, practically not much more than 12 (Buss 2014).
According to Robert Trivers’ parental investment theory, females, among most vertebrate species, have a greater minimum obligate investment in the production of offspring (Trivers 1972). This is largely accounted for by the anisogamy of male and female gametes as well as the burden of internal gestation. Due to this higher minimum investment in offspring, Trivers proposed that females adopt a mating strategy based on being highly selective and “choosy” when assessing potential partners and mating opportunities. Put another way, because reproductive output is much more constrained for females, a more viable strategy is to focus on quality of mating opportunities rather than quantity.
It is important to note that there are species that do not conform to this pattern of behavior, however. The phalarope is a genus consisting of three species of shore birds known for having sex-reversed roles (Colwell 1986). These species engage in polyandry in which females are larger with more coloration and compete for mates, while males incubate and provide primary care for the offspring. These exceptions to the rule can be explained by factors such as operational sex ratio (Colwell and Oring 1988).
There is a pattern across many species where the females will adopt a mating strategy of being highly selective of mates. Although this strategy is not found among all species, the prevalence of this strategy can be largely attributed to the lower reproductive variance that often characterizes the females of a species as well as the greater minimum obligate investment that females of most species must endure in order to produce viable offspring.
- Buss, D. M. (2014). Evolutionary psychology: The new science of the mind (5th ed.). New York: Psychology Press.Google Scholar
- Colwell, M. A. (1986). The first documented case of polyandry for Wilson’s phalarope (Phalaropus tricolor). The Auk, 103(3), 611–612.Google Scholar
- Trivers, R. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man (pp. 136–179). New York: Aldine de Gruyter.Google Scholar