Parental investment
Encyclopedia
In evolutionary biology, parental investment (PI) is any parental expenditure (time, energy etc.) that benefits one offspring
at a cost to parent
s' 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. What is the difference?
Parental investment theory is a branch of life history theory
. The earliest consideration of parental investment is given by Fisher (1930), Fisher's principle
, wherein Fisher argued that parental expenditure on both sexes of offspring should be equal. 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 ectotherm
ic (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.
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 (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. However, these 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.
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.
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
at a cost to parent
Parent
A parent is a caretaker of the offspring in their own species. In humans, a parent is of a child . Children can have one or more parents, but they must have two biological parents. Biological parents consist of the male who sired the child and the female who gave birth to the child...
s' ability to invest in other components of fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
(Clutton-Brock 1991: 9; Trivers 1972). Components of fitness (Beatty 1992) include the wellbeing of existing offspring, parents' future reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
, and inclusive fitness
Inclusive fitness
In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness and the number of equivalents of its own offspring it can add to the population by supporting others...
through aid to kin (Hamilton
W. D. Hamilton
William Donald Hamilton FRS was a British evolutionary biologist, widely recognised as one of the greatest evolutionary theorists of the 20th century....
, 1964). Parental investment is sometimes incorrectly equated with parental care or parental effort. What is the difference?
Parental investment theory is a branch of life history theory
Life history theory
Life history theory posits that the schedule and duration of key events in an organism's lifetime are shaped by natural selection to produce the largest possible number of surviving offspring...
. The earliest consideration of parental investment is given by Fisher (1930), Fisher's principle
Fisher's principle
Fisher's principle explains why the sex ratio of most species is approximately 1:1. It was famously outlined by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection . Nevertheless, A. W. F. Edwards has remarked that it is "probably the most celebrated argument in evolutionary...
, wherein Fisher argued that parental expenditure on both sexes of offspring should be equal. Clutton-Brock (1991: 9) expanded the concept of PI to include costs to any other component of parental fitness.
Robert Trivers
Robert Trivers
Robert L. Trivers is an American evolutionary biologist and sociobiologist and Professor of Anthropology and Biological Sciences at Rutgers University. Trivers is most noted for proposing the theories of reciprocal altruism , parental investment , facultative sex ratio determination , and...
' theory of parental investment predicts that the sex
Sex
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...
making the largest investment in lactation
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
, nurturing, and protecting offspring will be more discriminating in mating
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
; and that the sex that invests less in offspring will compete for access to the higher-investing sex (see Bateman's principle
Bateman's principle
In biology, Bateman's principle is the theory that females almost always invest more energy into producing offspring than males invest, and therefore in most species females are a limiting resource over which the other sex will compete...
). Sex differences in parental effort are important in determining the strength of sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural 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
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
(female uniparental care), the male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
(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 ectotherm
Ectotherm
An ectotherm, from the Greek εκτός "outside" and θερμός "hot", refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...
ic (invertebrates, fish, amphibians and reptiles), and endothermic
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
(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.
In iteroparous
Semelparity and iteroparity
Semelparity and Iteroparity refer to the reproductive strategy of an organism. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. Some...
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 (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
Reproductive success
Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass those genes on. In practice, this is often a tally of the number of offspring produced by an individual. A more correct definition, which incorporates inclusive fitness, is the...
) 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. However, these 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.
Overall, parents are selected
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
to maximise the difference between the benefits and the costs, and parental care will be likely to evolve when the benefits exceed the costs.
Further reading
- Geary, D. C.David C. GearyDavid C. Geary is a notable United States cognitive developmental and evolutionary psychologist with interests in mathematical learning and sex differences...
(2005). Evolution of paternal investment. In D. M. BussDavid BussDavid M. Buss is a professor of psychology at The University of Texas at Austin, known for his evolutionary psychology research on human sex differences in mate selection.-Biography:...
(Ed.), The handbook of evolutionary psychology (pp. 483–505). Hoboken, NJ: John Wiley & Sons. Full text