Parent-offspring conflict
Encyclopedia
Parent–offspring conflict (POC) is a term coined in 1974 by Robert Trivers
. It is used to signify the evolutionary conflict arising from differences in optimal parental investment
(PI) to an offspring from the standpoint of the parent
and the offspring
. Here, PI is any investment by the parent in an individual offspring that decreases the parent's ability to invest in other offspring, while the selected offspring's chance of surviving increases.
POC occurs in sexually reproducing species and is based on a genetic conflict: Parents are equally related to their offspring and are therefore expected to equalize their investment among them. Offspring are only half or less related to their sibling
s (and fully related to themselves) so they try to get more PI than the parents intended to provide even at their siblings' disadvantage.
However, POC is limited by the close genetic relationship between parent and offspring: If an offspring obtains additional PI at the expense of its siblings, it decreases the number of its surviving siblings. Therefore, any gene
in an offspring that leads to additional PI decreases (to some extent) the number of surviving copies of itself located in siblings. Thus, if the costs in siblings are too high, such a gene might be selected against despite the benefit to the offspring.
The problem of specifying how an individual is expected to weigh a relative against itself has been examined by W. D. Hamilton
in 1964 in the context of kin selection
. Hamilton's rule says that altruistic behavior will be positively selected if the benefit to the recipient multiplied by the genetic relatedness of the recipient to the performer is greater than the cost to the performer of a social act. Conversely, selfish behavior can only be favoured when Hamilton's inequality is not satisfied. This leads to the prediction that, other things being equal, POC will be stronger under half siblings (e.g., unrelated males father a female's successive offspring) than under full siblings.
s matured within a single fruit) and seed size. Concerning brood size, the most economic use of maternal resources is achieved by packing as many seeds as possible in one fruit, i.e., minimizing the cost of packing per seed. In contrast, offspring benefeits from low numbers of seeds per fruit, which reduces sibling competition before and after dispersal. Conflict over seed size arises because there usually exists a hyperbolic relationship between seed size and fitness
, that is, the fitness of a seed increases at a diminishing rate with resource investment but the fitness of the maternal parent has an optimum, as demonstrated by Smith and Fretwell (see also marginal value theorem
). However, the optimum resource investment from the offspring's point of view would be the maximum that it can possibly get from the maternal parent.
This conflict about resource allocation is most obviously manifested in the reduction of brood size (i.e. a decrease in the proportion of ovule
s matured into seeds). Such reduction can be assumed to be caused by the offspring: If the maternal parent's interest were to produce as few seeds as observed, selection
would not favour the production of extra ovules that do not mature into seeds. (Although other explanations for this phenomenon exist, such as genetic load
, resource depletion or maternal regulation of offspring quality, they could not be supported by experiments.)
There are several possibilities how the offspring can affect paternal resource allocation to brood members. Evidence exists for siblicide
by dominant embryos: Embryos formed early kill the remaining embryos through an aborting chemical. In oaks, early fertilized ovules prevent the fertilization of other ovules by inhibiting the pollen tube
entry into the embryo sac. In some species, the maternal parent has evolved postfertilization abortion
of few seeded pods. Nevertheless, cheating by the offspring is also possible here, namely by late siblicide, when the postfertilization abortion has ceased.
According to the general POC model, reduction of brood size – if caused by POC – should depend on genetic relatedness between offspring in a fruit. Indeed, abortion of embryos is more common in out-crossing than in self-pollinating plants (seeds in cross-pollinating plants are less related than in self-pollinating plants). Moreover, the level of solicitation of resources by the offspring is also increased in cross-pollinating plants: There are several reports that the average weight of crossed seeds is greater than of seeds produced by self-fertilization.
broods and especially in raptor species. While parent birds often lay two eggs and attempt to raise two or more young, the strongest fledgling takes a greater share of the food brought by parents and will often kill the weaker sibling (siblicide
). Such conflicts have been suggested as a driving force in the evolution of optimal clutch size in birds.
s wean themselves or mothers actively wean their infants. Furthermore, it was discussed whether maternal rejections increase infant independence. It turned out that both mother and infant contribute to infant independence. Maternal rejections can be followed by a short-term increase in infant contact but they eventually result in a long-term decrease of contact as has been shown for several primates: In wild baboon
s infants that are rejected early and frequently spend less time in contact whereas those that are not rejected stay much longer in the proximity of their mother and suckle or ride even in advanced ages. In wild chimpanzee
s an abrupt increase in maternal rejections and a decrease in mother-offspring contact is found when mothers resume estrus and consort with males. In rhesus macaque
s a high probability of conception in the following mating season is associated with a high rate of maternal rejection. Rejection and behavioral conflicts can occur during the first months of an infant's life and when the mother resumes estrus. These findings suggest that the reproduction of the mother is influenced by the interaction with their offspring. So there is a potential for conflicts over PI.
It was also observed in rhesus macaques that the number of contacts made by offspring is significantly higher than the number of contacts made by mother during a mating season, whereas the opposite holds for the number of broken contacts. This fact suggests that the mother resists offspring's demands for contact, whereas offspring is apparently more interested in spending time in contact. At three months of infant age a shift from mother to infant in responsibility for maintaining contact takes place. So when the infant becomes more independent, its effort to maintain proximity to its mother increases. This might sound paradoxical but becomes clear when one takes into account that POC increases during the period of PI. In summary, all these findings are consistent with POC-theory.
Now one might object that time in contact is not a reasonable measure for PI and that, for example, time for milk transfer (lactation
) would be a better one. Here one can argue that mother and infant have different thermoregulatory
needs due to the fact that they have different surface-to-volume ratios resulting in more rapid loss of heat in infants compared to adults. So infants may be more sensitive to low temperatures than their mothers. Infant might try to compensate that by increased contact time to their mother, which could initiate a behavioral conflict over time. Consistency of this hypothesis was shown for Japanese macaque
s where decreasing temperatures result in higher maternal rejections and increased number of contacts made by infants.
genes would be selected to draw more resources from the mother than it would be optimal for the mother to give, a hypothesis that has received empirical support. The placenta
, for example, secretes allocrine hormone
s that decrease the sensitivity of the mother to insulin
and thus make a larger supply of blood sugar available to the fetus. The mother responds by increasing the level of insulin in her bloodstream, and to counteract this effect the placenta has insulin receptors that stimulate the production of insulin-degrading enzyme
s.
About 30 percent of human conceptions
do not progress to full term (22 percent before becoming clinical pregnancies) and this creates a second arena for conflict between the mother and the fetus, because the fetus will have a lower quality cut off point for spontaneous abortion than the mother. The mother's quality cut-off point should also decline as she nears the end of her reproductive life and it may be significant that the offspring of older mothers have a higher incidence of genetic defects. Initially, the maintenance of pregnancy is controlled by the maternal hormone progesterone
, but in later stages it is controlled by the fetal human chorionic gonadotrophin released into the maternal bloodstream, which causes the release of maternal progesterone. There is also conflict over blood supply to the placenta, with the fetus being prepared to demand a larger blood supply than is optimal for the mother (or even for itself, since high birth weight is a risk factor). This results in hypertension
and, significantly, high birth weight
is positively correlated with maternal blood pressure.
After birth the young infant may demand more nutritional resources than the lactating mother is prepared to provide, and the presence of benzodiazepine
s in breast milk
may function to keep this in check, although its primary function is to calm the infant.
lines may survive in women even decades after giving birth. These cells may serve the adaptive interest of the mother, however, they may also serve conflicting interests of the fetus or those of the father. This mixture of shared or contradicting interests has been hypothesized as giving rise to diseases like autoimmune diseases, infertility, and habitual abortion in humans.
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...
. It is used to signify the evolutionary conflict arising from differences in optimal parental investment
Parental investment
In evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...
(PI) to an offspring from the standpoint of the 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...
and the offspring
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...
. Here, PI is any investment by the parent in an individual offspring that decreases the parent's ability to invest in other offspring, while the selected offspring's chance of surviving increases.
POC occurs in sexually reproducing species and is based on a genetic conflict: Parents are equally related to their offspring and are therefore expected to equalize their investment among them. Offspring are only half or less related to their sibling
Sibling
Siblings are people who share at least one parent. A male sibling is called a brother; and a female sibling is called a sister. In most societies throughout the world, siblings usually grow up together and spend a good deal of their childhood socializing with one another...
s (and fully related to themselves) so they try to get more PI than the parents intended to provide even at their siblings' disadvantage.
However, POC is limited by the close genetic relationship between parent and offspring: If an offspring obtains additional PI at the expense of its siblings, it decreases the number of its surviving siblings. Therefore, any gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
in an offspring that leads to additional PI decreases (to some extent) the number of surviving copies of itself located in siblings. Thus, if the costs in siblings are too high, such a gene might be selected against despite the benefit to the offspring.
The problem of specifying how an individual is expected to weigh a relative against itself has been examined by W. D. 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....
in 1964 in the context of kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
. Hamilton's rule says that altruistic behavior will be positively selected if the benefit to the recipient multiplied by the genetic relatedness of the recipient to the performer is greater than the cost to the performer of a social act. Conversely, selfish behavior can only be favoured when Hamilton's inequality is not satisfied. This leads to the prediction that, other things being equal, POC will be stronger under half siblings (e.g., unrelated males father a female's successive offspring) than under full siblings.
In plants
In plants, POC over the allocation of resources to the brood members may affect both brood size (number of seedSeed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s matured within a single fruit) and seed size. Concerning brood size, the most economic use of maternal resources is achieved by packing as many seeds as possible in one fruit, i.e., minimizing the cost of packing per seed. In contrast, offspring benefeits from low numbers of seeds per fruit, which reduces sibling competition before and after dispersal. Conflict over seed size arises because there usually exists a hyperbolic relationship between seed size and 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...
, that is, the fitness of a seed increases at a diminishing rate with resource investment but the fitness of the maternal parent has an optimum, as demonstrated by Smith and Fretwell (see also marginal value theorem
Marginal value theorem
In behavioral ecology, the marginal value theorem considers an optimally foraging animal exploiting resources distributed in patches and that must decide when to leave a patch to start searching for a fresh one. The animal is assumed to have evolved to optimize a cost/benefit ratio: searching for...
). However, the optimum resource investment from the offspring's point of view would be the maximum that it can possibly get from the maternal parent.
This conflict about resource allocation is most obviously manifested in the reduction of brood size (i.e. a decrease in the proportion of ovule
Ovule
Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
s matured into seeds). Such reduction can be assumed to be caused by the offspring: If the maternal parent's interest were to produce as few seeds as observed, selection
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...
would not favour the production of extra ovules that do not mature into seeds. (Although other explanations for this phenomenon exist, such as genetic load
Genetic load
In population genetics, genetic load or genetic burden is a measure of the cost of lost alleles due to selection or mutation...
, resource depletion or maternal regulation of offspring quality, they could not be supported by experiments.)
There are several possibilities how the offspring can affect paternal resource allocation to brood members. Evidence exists for siblicide
Siblicide
Siblicide is the killing of an infant individual by its close relatives . It may occur directly between siblings or be mediated by the parents. The evolutionary drivers may be either indirect benefits for the genetic viability of a population or direct benefits for the perpetrators...
by dominant embryos: Embryos formed early kill the remaining embryos through an aborting chemical. In oaks, early fertilized ovules prevent the fertilization of other ovules by inhibiting the pollen tube
Pollen tube
The pollen tubes is the male gametophyte of seed plants that acts as a conduit to transport the male sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms .After pollination, the pollen tube...
entry into the embryo sac. In some species, the maternal parent has evolved postfertilization abortion
Abortion
Abortion is defined as the termination of pregnancy by the removal or expulsion from the uterus of a fetus or embryo prior to viability. An abortion can occur spontaneously, in which case it is usually called a miscarriage, or it can be purposely induced...
of few seeded pods. Nevertheless, cheating by the offspring is also possible here, namely by late siblicide, when the postfertilization abortion has ceased.
According to the general POC model, reduction of brood size – if caused by POC – should depend on genetic relatedness between offspring in a fruit. Indeed, abortion of embryos is more common in out-crossing than in self-pollinating plants (seeds in cross-pollinating plants are less related than in self-pollinating plants). Moreover, the level of solicitation of resources by the offspring is also increased in cross-pollinating plants: There are several reports that the average weight of crossed seeds is greater than of seeds produced by self-fertilization.
In birds
Some of the earliest examples of parent-offspring conflict were seen in birdBird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
broods and especially in raptor species. While parent birds often lay two eggs and attempt to raise two or more young, the strongest fledgling takes a greater share of the food brought by parents and will often kill the weaker sibling (siblicide
Siblicide
Siblicide is the killing of an infant individual by its close relatives . It may occur directly between siblings or be mediated by the parents. The evolutionary drivers may be either indirect benefits for the genetic viability of a population or direct benefits for the perpetrators...
). Such conflicts have been suggested as a driving force in the evolution of optimal clutch size in birds.
In mammals
Even before POC theory arose, debates took place over whether infantInfant
A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...
s wean themselves or mothers actively wean their infants. Furthermore, it was discussed whether maternal rejections increase infant independence. It turned out that both mother and infant contribute to infant independence. Maternal rejections can be followed by a short-term increase in infant contact but they eventually result in a long-term decrease of contact as has been shown for several primates: In wild baboon
Baboon
Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger...
s infants that are rejected early and frequently spend less time in contact whereas those that are not rejected stay much longer in the proximity of their mother and suckle or ride even in advanced ages. In wild chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
s an abrupt increase in maternal rejections and a decrease in mother-offspring contact is found when mothers resume estrus and consort with males. In rhesus macaque
Rhesus Macaque
The Rhesus macaque , also called the Rhesus monkey, is one of the best-known species of Old World monkeys. It is listed as Least Concern in the IUCN Red List of Threatened Species in view of its wide distribution, presumed large population, and its tolerance of a broad range of habitats...
s a high probability of conception in the following mating season is associated with a high rate of maternal rejection. Rejection and behavioral conflicts can occur during the first months of an infant's life and when the mother resumes estrus. These findings suggest that the reproduction of the mother is influenced by the interaction with their offspring. So there is a potential for conflicts over PI.
It was also observed in rhesus macaques that the number of contacts made by offspring is significantly higher than the number of contacts made by mother during a mating season, whereas the opposite holds for the number of broken contacts. This fact suggests that the mother resists offspring's demands for contact, whereas offspring is apparently more interested in spending time in contact. At three months of infant age a shift from mother to infant in responsibility for maintaining contact takes place. So when the infant becomes more independent, its effort to maintain proximity to its mother increases. This might sound paradoxical but becomes clear when one takes into account that POC increases during the period of PI. In summary, all these findings are consistent with POC-theory.
Now one might object that time in contact is not a reasonable measure for PI and that, for example, time for milk transfer (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...
) would be a better one. Here one can argue that mother and infant have different thermoregulatory
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
needs due to the fact that they have different surface-to-volume ratios resulting in more rapid loss of heat in infants compared to adults. So infants may be more sensitive to low temperatures than their mothers. Infant might try to compensate that by increased contact time to their mother, which could initiate a behavioral conflict over time. Consistency of this hypothesis was shown for Japanese macaque
Japanese Macaque
The Japanese macaque , historically known as saru , but now known as Nihonzaru to distinguish it from other primates, is a terrestrial Old World monkey species native to Japan....
s where decreasing temperatures result in higher maternal rejections and increased number of contacts made by infants.
In humans
An important illustration of such conflict is provided by David Haig’s (1993) work on genetic conflicts in pregnancy. Haig has argued that fetalFetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
genes would be selected to draw more resources from the mother than it would be optimal for the mother to give, a hypothesis that has received empirical support. The placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
, for example, secretes allocrine hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s that decrease the sensitivity of the mother to insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
and thus make a larger supply of blood sugar available to the fetus. The mother responds by increasing the level of insulin in her bloodstream, and to counteract this effect the placenta has insulin receptors that stimulate the production of insulin-degrading enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s.
About 30 percent of human conceptions
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
do not progress to full term (22 percent before becoming clinical pregnancies) and this creates a second arena for conflict between the mother and the fetus, because the fetus will have a lower quality cut off point for spontaneous abortion than the mother. The mother's quality cut-off point should also decline as she nears the end of her reproductive life and it may be significant that the offspring of older mothers have a higher incidence of genetic defects. Initially, the maintenance of pregnancy is controlled by the maternal hormone progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
, but in later stages it is controlled by the fetal human chorionic gonadotrophin released into the maternal bloodstream, which causes the release of maternal progesterone. There is also conflict over blood supply to the placenta, with the fetus being prepared to demand a larger blood supply than is optimal for the mother (or even for itself, since high birth weight is a risk factor). This results in hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
and, significantly, high birth weight
Birth weight
Birth weight is the body weight of a baby at its birth.There have been numerous studies that have attempted, with varying degrees of success, to show links between birth weight and later-life conditions, including diabetes, obesity, tobacco smoking and intelligence.-Determinants:There are...
is positively correlated with maternal blood pressure.
After birth the young infant may demand more nutritional resources than the lactating mother is prepared to provide, and the presence of benzodiazepine
Benzodiazepine
A benzodiazepine is a psychoactive drug whose core chemical structure is the fusion of a benzene ring and a diazepine ring...
s in breast milk
Breast milk
Breast milk, more specifically human milk, is the milk produced by the breasts of a human female for her infant offspring...
may function to keep this in check, although its primary function is to calm the infant.
A tripartite (fetus—mother—father) immune conflict in humans and other placentals
During pregnancy, there is a two-way traffic of immunologically active cell lines through the placenta. Fetal lymphocyteLymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...
lines may survive in women even decades after giving birth. These cells may serve the adaptive interest of the mother, however, they may also serve conflicting interests of the fetus or those of the father. This mixture of shared or contradicting interests has been hypothesized as giving rise to diseases like autoimmune diseases, infertility, and habitual abortion in humans.