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.
Parental investment theory is a branch of life history theory. The earliest consideration of parental investment is given by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection, wherein Fisher argued that parental expenditure on both sexes of offspring should be equal. Clutton-Brock expanded the concept of PI to include costs to any other component of parental fitness.
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 maximize the difference between the benefits and the costs, and parental care will tend to exist when the benefits are substantially greater than the costs.
Penguins are a prime example of a species that drastically sacrifices their own health and well-being in exchange for the survival of their offspring. This behavior, one that does not necessarily benefit the individual, but the genetic code from which the individual arises, can be seen in the King Penguin. Although some animals do exhibit altruistic behaviors towards individuals that are not of direct relation, many of these behaviors appear mostly in parent-offspring relationships. While breeding, males remain in a fasting-period at the breeding site for five weeks, waiting for the female to return for her own incubation shift. However, during this time period, males may decide to abandon their egg if the female is delayed in her return to the breeding grounds. This is an interesting case, as it shows that these penguins initially show a trade-off of their own health, in hopes of increasing the survivorship of their egg, but there comes a point where the male penguin's costs become too high in comparison to the gain of a successful breeding season. In a study, Olof Olsson investigated the correlation between how many experiences in breeding an individual has and the duration an individual will wait until abandoning his egg. He proposed that the more experienced the individual, the better that individual will be at replenishing his exhausted body reserves, allowing him to remain at the egg for a longer period of time. The males' sacrifice of their body weight and possible survivorship, in order to increase their offspring's chance of survival is a trade-off between current reproductive success and the parents' future survival. This trade-off makes sense with other examples of kin-based altruism and is a clear example of the use of altruism in an attempt to increase overall fitness of an individual's genetic material at the expense of the individual's future survival.