Parental investment

Parental investment (PI), in evolutionary biology and evolutionary psychology, is any parental expenditure (time, energy etc.) that benefits one offspring at a cost to parents' ability to invest in other components of fitness, and is thus a form of sexual selection. Components of fitness include the wellbeing of existing offspring, parents' future sexual reproduction, and inclusive fitness through aid to kin. Parental investment may be performed by both the male and female (biparental care), the mother alone (exclusive maternal care) or the father alone (exclusive paternal care).

Initially introduced in 1930 by the English biologist and statistician Ronald Fisher, parental care is found in a broad range of taxonomic groups, including both ectothermic (invertebrates, fish, amphibians and reptiles), and endothermic (birds and mammals) species. Care can be provided at any stage of the offspring's life: pre-natal care including behaviours such as egg guarding, preparation of nest, brood carrying, incubation, and placental nourishment in mammals; and post-natal care including food provisioning and protection of offspring.

An alternative to Fisher's theory is Trivers' parental investment theory (PIT) proposed in 1972, in which he focuses on how parental investment in humans affects their sexual behaviour. This theory has been influential in explaining sex differences in phenomena such as sexual selection and mate preference. The main premise of this theory is the differential investment of males and females, particularly how in most species females invest more due to the mating effort required. Another aspect of the theory is mate preferences of each sex, for example female choosiness and male promiscuity.

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