Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure (e.g. time, energy, resources) that benefits offspring.Clutton-Brock, T.H. 1991. The Evolution of Parental Care. Princeton, NJ: Princeton U. Press. pg. 9Trivers, R.L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man, 1871-1971 (pp. 136–179). Chicago, IL: Aldine. . Parental investment may be performed by both males and females (called biparental care), females alone (exclusive maternal care) or males alone (exclusive paternal care). Care can be provided at any stage of the offspring's life, from prenatal (e.g. egg guarding and incubation in birds, and placental nourishment in mammals) to postnatal (e.g. food provisioning and protection of offspring).
Parental investment theory, a term coined by Robert Trivers in 1972, predicts that the sex that invests more in its offspring will be more selective when choosing a mate, and the less-invested sex will have intra-sexual competition for access to mates. This theory has been influential in explaining sex differences in sexual selection and mate preferences throughout the animal kingdom and in humans.
History Sexual selection is an evolutionary concept that has been used to explain why in some species males and females behave differently in selecting mates. In 1930, Ronald Fisher wrote The Genetical Theory of Natural Selection, in which he introduced the modern concept of parental investment, introduced the sexy son hypothesis, and introduced Fisher's principle.
In 1948, Angus John Bateman published an influential study of fruit flies in which he concluded that because female gametes are more costly to produce than male gametes, the reproductive success of females was limited by the ability to produce ovum, and the reproductive success of males was limited by access to females. In 1972, Robert Trivers continued this line of thinking with his proposal of parental investment theory, which describes how parental investment affects sexual behavior. He concluded that whichever sex has higher parental investment will be more selective when choosing a mate, while the sex with lower investment will compete intra-sexually for mating opportunities. In 1974, Trivers extended parental investment theory to explain parent–offspring conflict, the conflict between the amount of investment that is optimal from the parent's perspective, versus from the offspring's perspective.
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 parental investment to include costs to any other component of parental fitness.
Male dunnocks (Prunella modularis) tend to not discriminate between their own young and those of another male in polyandrous or polygynandrous systems. 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 indiscriminate parental care by males is also observed in redlip blennies (Ophioblennius atlanticus).
In some insects, male parental investment is given in the form of a nuptial gift. For instance, ornate moth (Utetheisa ornatrix) 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.
In some species, such as humans and many birds, the offspring are altricial and unable to fend for themselves for an extended period of time after birth. In these species, males invest more in their offspring than do the male parents of precocial species, since reproductive success would otherwise suffer.
The benefits of parental investment to the offspring are large and associated with the effects on condition, growth, survival, and ultimately the reproductive success of the offspring. For example, in the blunthead cichlid (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, effectively starving the mother. In doing so, however, the young are larger, heavier, and faster than they would have been without the parental care. 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 require nourishment and protection, but the genetic parents do not actually contribute in the effort to raise their own offspring. For example, in buff-tailed bumblebee (Bombus terrestris), oftentimes sterile female workers will not reproduce on their own, but will raise their mother's brood instead. This is common in social Hymenoptera insects 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 maximize the difference between the benefits and the costs, and parental care will be likely to evolve when the benefits exceed the costs.
Parent-offspring conflict
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, since it enhances their condition, survival, and reproductive success. These differences may lead to parent-offspring conflict. 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.
Parents are equally related to all offspring, and so in order to optimize their fitness and chance of reproducing their genes, they should distribute their investment equally among current and future offspring. However, any single offspring is more related to themselves (they have 100% of their DNA in common with themselves) than they are to their siblings (siblings usually share 50% of their DNA), so it is best for the offspring's fitness if the parent(s) invest more in them. To optimize fitness, a parent would want to invest in each offspring equally, but each offspring would want a larger share of parental investment. The parent is selected to invest in the offspring up until the point at which investing in the current offspring is costlier than investing in future offspring.Barett, L., Dunbar, R. & Lycett, J. (2002). Human Evolutionary Psychology. Palgrave Press.
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 formalized 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".
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. Their parental 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 (Aptenodytes patagonicus). 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.
It shows that these penguins initially exhibit a trade-off of their own health, in hopes of increasing the survivorship of their egg. But there comes the cost for the male penguin costs outweigh successful raising the young. 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.
Maternal-offspring conflict in investment The maternal-offspring conflict has also been studied in animals species and humans. One such case has been documented in the mid-1970s by ethologist Wulf Schiefenhövel. Eipo women of West New Guinea engage in a cultural practice in which they give birth just outside the village. Following the birth of their child, each woman weighed whether or not she should keep the child or leave the child in the nearby brush (leading to the death of the child). Likelihood of survival and availability of resources within the village were factors that played into this decision of whether or not to keep the baby. During one illustrated birth, the mother felt the child was too ill and would not survive, so she wrapped the child up, preparing to leave the child in the brush; however, upon seeing the child moving, the mother unwrapped the child and brought it into the village, demonstrating a shift of life and death. This conflict between the mother and the child resulted in detachment behaviors in Brazil, seen in Scheper-Hughes work as "many Alto babies remain[ed] not only unchristened but unnamed until they begin to walk or talk", or if a medical crisis arose and the baby needed an emergency baptism. This conflict between survival, both emotional and physical, prompted a shift in cultural practices, thus resulting in new forms of investment from the mother towards the child.
Alloparental care