In evolutionary biology, parental investment (PI) is any parental expenditure (time, energy etc.) that benefits one offspring at a cost to parents' ability to invest in other components of fitness (Clutton-Brock 1991: 9; Trivers 1972). Components of fitness (Beatty 1992) include the wellbeing of existing offspring, parents' future reproduction, and inclusive fitness through aid to kin (Hamilton, 1964). Parental investment is sometimes incorrectly equated with parental care or parental effort. Parental investment theory is a branch of life history theory. This potential negative effect of parental care was explicitly formalised by Trivers (1972) who originally defined the term parental investment to mean any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring. Clutton-Brock (1991: 9) expanded the concept of PI to include costs to any other component of parental fitness.
Robert Trivers' theory of parental investment predicts that the sex making the largest investment in lactation, nurturing and protecting offspring will be more discriminating in mating and that the sex that invests less in offspring will compete for access to the higher investing sex (see Bateman's principle). Sex differences in parental effort are important in determining the strength of sexual selection.
Reproduction is costly. Individuals are limited in the degree to which they can devote time and resources to producing and raising their young, and such expenditure may also be detrimental to their future condition, survival and further reproductive output. However, such expenditure is typically beneficial to the offspring, enhancing their condition, survival and reproductive success. These differences may lead to parent-offspring conflict. Parental investment can be provided by the female (female uniparental care), the male (male uniparental care), or both (biparental care). Parents are naturally selected to maximise the difference between the benefits and the costs, and parental care will tend to exist when the benefits are substantially greater than the costs.
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 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, protection of offspring.
Since both males and females go through several reproductive bouts during their lifetime, it is expected that parents trade-off the benefits of investing in current offspring against the costs to future reproduction. In particular, parents need to balance their offspring demands against their own self-maintenance. The benefits of parental investment to the offspring are large and are associated with the effects on condition, growth, survival and ultimately, on reproductive success of the offspring. However, these benefits can come at the cost of parent's ability to reproduce in the future e.g. through the increased risk of injury when defending offspring against predators, the loss of mating opportunities whilst rearing offspring and an increase in the time to the next reproduction. Overall, parents are selected to maximise the difference between the benefits and the costs, and parental care will be likely to evolve when the benefits are higher than the costs.