Parent–offspring conflict
Parent–offspring conflict (POC) is an expression coined in 1974 by
POC occurs in sexually reproducing species and is based on a genetic conflict: Parents are equally related to each of their offspring and are therefore expected to equalize their investment among them. Offspring are only half or less related to their siblings (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 that may be 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
Occurrence
In plants
In
This conflict about resource allocation is most obviously manifested in the reduction of brood size (i.e. a decrease in the proportion of
There are several possibilities how the offspring can affect paternal resource allocation to brood members. Evidence exists for
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
In birds
Some of the earliest examples of parent-offspring conflict were seen in
In the blue-footed booby, parent-offspring conflict results in times of food scarcity. When there is less food available in a given year, the older, dominant chick will often kill the younger chick by either attacking directly, or by driving it from the nest. Parents try to prevent siblicide by building nests with steeper sides [7] and by laying heavier second eggs.[8]
In mammals
Even before POC theory arose, debates took place over whether
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. An infant might try to compensate by increased contact time with their mother, which could initiate a behavioral conflict over time. Consistency of this hypothesis has been shown for Japanese macaques where decreasing temperatures result in higher maternal rejections and increased number of contacts made by infants.[9]
In social insects
In eusocial species, the parent-offspring conflict takes on a unique role because of haplodiploidy and the prevalence of sterile workers. Sisters are more related to each other (0.75) than to their mothers (0.5) or brothers (0.25). In most cases, this drives female workers to try and obtain a sex ratio of 3:1 (females to males) in the colony. However, queens are equally related to both sons and daughters, so they prefer a sex ratio of 1:1. The conflict in social insects is about the level investment the queen should provide for each sex for current and future offspring. It is generally thought that workers will win this conflict and the sex ratio will be closer to 3:1, however there are examples, like in Bombus terrestris, where the queen has considerable control in forcing a 1:1 ratio.[10]
In amphibians
Many species of frogs and salamanders display complex social behavior with highly involved parental care that includes egg attendance, tadpole transport, and tadpole feeding.
Energy expenditure
Both males and females of the strawberry poison-dart frog care for their offspring, however, females invest in more costly ways.[11] Females of certain poison frog species produce unfertilized, non-developing trophic eggs which provide nutrition to her tadpoles. The tadpoles vibrate vigorously against mother frogs to solicit nutritious eggs. These maternal trophic eggs are beneficial for offspring, positively influencing larval survival, size at metamorphosis, and post metamorphic survival.[12]
In the neotropical, foam-nesting pointedbelly frog (Leptodactylus podicipinus), females providing parental care to tadpoles have reduced body condition and food ingestion. Females that are attending to her offspring have significantly lower body mass, ovary mass, and stomach volume. This indicates that the cost of parental care in the pointedbelly frog has the potential to affect future reproduction of females due to the reaction in body condition and food intake.[13]
In the Puerto Rican common coqui, parental care is performed exclusively by males and consists of attending to the eggs and tadpoles at an oviposition site. When brooding, males have a higher frequency of empty stomachs and lose a significant portion of their initial body mass during parental care. Abdominal fat bodies of brooding males during the middle of parental care were significantly smaller than those of non-brooding males. Another major behavioral component of parental care is nest defense against conspecific egg cannibals. This defense behavior includes aggressive calling, sustained biting, wrestling, and blocking directed against the nest intruder.[14]
Females of the Allegheny Mountain dusky salamander exhibit less activity and become associated with the nest site well in advance of oviposition in preparation for the reproductive season. This results in a reduced food intake and a decrease in body weight over the brooding period. Females either stop or greatly reduce their foraging activities and instead will eat opportunistically following oviposition. Since nutritional intake is reduced, there is a decrease in body weight in females.[15] Females of the red-backed salamander make a substantial parental investment in terms of clutch size and brooding behavior. When brooding, females usually do not leave their eggs to forage but rather rely upon their fat reserves and any resources they encounter at their oviposition site. In addition, females could experience metabolic costs while safeguarding their offspring from desiccation, intruders, and predators.[16]
Time investment
The plasticity of tadpoles may play a role in the weaning conflict in egg-feeding frogs, in which the offspring prefer to devote resources to growth, while the mother prefers nutrients to help her young become independent. A similar conflict happens in direct-developing frogs that care for clutches, with protected tadpoles having the advantage of a slower, safer development, but they need to be ready to reach independence rapidly due to the risks of predation or desiccation.[12]
In the neotropical Zimmerman’s poison frog, the males provide a specific parental care in the form of transportation. The tadpoles are cannibalistic, hence why the males typically separate them from their siblings after hatching by transporting them to small bodies of water. However, in some cases parents do not transport their tadpoles but let them all hatch into the same pool. In order to escape their cannibalistic siblings, the tadpoles will actively seek transportative parental care. When a male frog approaches the water body in which the tadpoles had been deposited in, tadpoles will almost “jump” on the back of the adult, mimicking an attack, while adults would not assist with this movement. While this is an obvious example of sibling conflict, the one-sided interaction between tadpoles and frogs could be seen as a form of parent-offspring conflict, in which the offspring attempts to extract more from the interaction than the parent is willing to provide. In this scenario, a tadpole climbing onto an unwilling frog— who enters the pool for reasons other than tadpole transportation, such as egg deposition, cooling off, or sleeping— might be analogous to mammalian offspring seeking to nurse after weaning. In times of danger, the tadpoles of Zimmerman’s poison frog don't passively await parental assistance but instead exhibit an almost aggressive approach in mounting the adult frogs.[17]
Trade-offs with mating
Reproductive attempts in strawberry poison-dart frog such as courtship activity, significantly decreases or will entirely cease in tadpole-rearing females compared to non-rearing females.[12] Most brooding males of the common coqui cease calling during parental care while gravid females are still available and known to mate, hence why non-calling males miss potential opportunities to reproduce.[18] Caring for tadpoles comes at the cost of other current reproductive opportunities for females, leading to the hypothesis that frequent reproduction is associated with reduced survival in frogs.[12]
In humans
An important illustration of POC within humans is provided by David Haig’s (1993) work on genetic conflicts in pregnancy.[19] Haig argued that fetal genes would be selected to draw more resources from the mother than would be optimal for the mother to give. The placenta, for example, secretes allocrine hormones 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 enzymes.[19]
About 30 percent of human
Initially, the maintenance of pregnancy is controlled by the maternal hormone
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 lymphocyte lines may survive in women even decades after giving birth.[citation needed]
See also
- Intrauterine cannibalism
- The kinship theory of genomic imprinting
- Intragenomic and intrauterine conflict in humans