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.
Parental investment and parental care
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 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.
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 and the overall fitness of the population. This altruistic behavior, one that does not necessarily benefit the individual, but the population as a whole, 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 a population at the expense of the individual’s fitness.
A study found that male dunnocks tend to not discriminate between their own young and those of another male in polyandrous or polygynandrous systems. However, they increase their own reproductive success through feeding the offspring in relation to their own access to the female throughout the mating period, which is generally a good predictor of paternity. This indiscriminative parental care by males is also observed in redlip blennies.
In iteroparous species, where individuals may go through several reproductive bouts during their lifetime, a tradeoff may exist between investment in current offspring and future reproduction. Parents need to balance their offspring's demands against their own self-maintenance. This potential negative effect of parental care was explicitly formalised by Trivers in 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.
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. For example, in the cichlid fish Tropheus moorii, a female has very high parental investment in her young because she mouthbroods the young and while mouthbrooding, all nourishment she takes in goes to feed the young and she effectively starves herself. In doing this, her young are larger, heavier, and faster than they would have been without it. These benefits are very advantageous since it lowers their risk of being eaten by predators and size is usually the determining factor in conflicts over resources. However, such benefits can come at the cost of parent's ability to reproduce in the future e.g., through increased risk of injury when defending offspring against predators, loss of mating opportunities whilst rearing offspring, and an increase in the time interval until the next reproduction.
A special case of parental investment is when young do need nourishment and protection, but the genetic parents do not actually contribute in the effort to raise their own offspring. For example, in Bombus terrestris, often times sterile female workers will not reproduce on their own, but will raise their mother's brood instead. This is common in social Hymenoptera due to haplodiploidy, whereby males are haploid and females are diploid. This ensures that sisters are more related to each other than they ever would be to their own offspring, incentivizing them to help raise their mother's young over their own. 
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 exceed the costs.
Specificity to offspring and situations
Parental care only requires behaviors that effectively enhances offspring's chances of survival, it does not require underlying mechanisms to be potentially at a continuum with generalizable empathy appliceable to adults, nor even other situations involving young than the specific reactions, nor does it require the offspring to be altruistic back in any way. Total ungeneralizability to adults has the evolvability advantage of not making parentally caring individuals vulnerable to being exploited by other adults.
Sexual selection and parental investment
In many species, sexual selection is closely linked to parental investment. In theory, a male from such a species can produce a large number of offspring over the course of his life by minimizing parental investment in favor of investing his time instead impregnating any reproductive-age female who is fertile. In contrast, a female from said species typically can have a much smaller number of offspring during her reproductive life, partly due to an obligatory non-nil parental investment (i.e., gestation and delivery). This suggests that females will be more selective ("choosy") of mates than males will be, choosing males with good fitness (e.g., genes, high status, resources, etc.), so as to help offset any lack of direct parental investment from the male, and therefore increase reproductive success. 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).
In species where both sexes invest highly in parental care, mutual choosiness is expected to arise. An example of this is seen in crested auklets, where parents share equal responsibility in incubating their single egg and raising the chick. In crested auklets both sexes are ornamented.
The importance of parental investment can be seen especially in species in which the offspring are altricial (i.e., unable to fend for themselves from earliest ages). In many bird species and in modern humans, this leads to males spending more time caring for their offspring than do the male parents of precocial species, since reproductive success would otherwise suffer. The higher parental investment in humans is the result of an extended childhood required in order to develop the unusually large human brain. During this time, parental investment comes in the form of feeding, teaching, and protection, for example.
However, if human parental investment starts from the point when the sperm fertilizes the egg, then the minimal obligatory parental investment for the male is zero effort, while the minimal obligatory investment for the female is nine months of pregnancy, followed by delivery. This difference of minimal obligatory investment between males and females suggests that the amount of investment and effort put into mating and parenting will differ.
In some insects male parental investment is given in the form of a nuptial gift. For instance, ornate moth females receive a spermatophore containing nutrients, sperm and defensive toxins from the male during copulation. This gift, which can account for up to 10% of the male's body mass, constitutes the total parental investment the male provides.
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- Parental care in birds
- Bateman's principle
- Cinderella effect
- Cost of raising a child
- Kin selection
- r/K selection theory
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