Monogamy in animals
Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.
This article is about pairing for animals in non-human species. For monogamy in humans, see monogamy.Monogamous mating[edit]
Monogamy is defined as a pair bond between two adult animals of the same species. This pair may cohabitate in an area or territory for some duration of time, and in some cases may copulate and reproduce with only each other. Monogamy may either be short-term, lasting one to a few seasons or long-term, lasting many seasons and in extreme cases, life-long. Monogamy can be partitioned into two categories, social monogamy and genetic monogamy which may occur together in some combination, or completely independently of one another.[1] As an example, in the cichlid species Variabilichromis moorii, a monogamous pair will care for eggs and young together, but the eggs may not all be fertilized by the male giving the care.[2] Monogamy in mammals is rather rare, only occurring in 3–9% of these species.[3][4][5] A larger percentage of avian species are known to have monogamous relationships (about 90%),[6] but most avian species practice social but not genetic monogamy in contrast to what was previously assumed by researchers.[7] Monogamy is quite rare in fish and amphibians, but not unheard of, appearing in a select few species.[8][9][10]
Social monogamy[edit]
Social monogamy refers to the cohabitation of one male and one female. The two individuals may cooperate in search of resources such as food and shelter and/or in caring for young.[1][11] Paternal care in monogamous species is commonly displayed through carrying, feeding, defending, and socializing offspring.[3][12] With social monogamy there may not be an expected sexual fidelity between the males and the females.[1][13] The existence of purely social monogamy is a polygamous or polyandrous social pair with extra pair coupling.[14] Social monogamy has been shown to increase fitness in prairie voles. It has been shown that female prairie voles live longer when paired with males in a social monogamous relationship. This could be because of the shared energy expenditure by the males and females lower each individual's input.[1] In largemouth bass, females are sometimes seen to exhibit cuckoo behavior by laying some of their eggs in another female's nest, thus "stealing" fertilizations from other females.[15] Sexual conflicts that have been proposed to arise from social monogamy include infidelity and parental investment. The proposed conflict is derived from the conflict-centric differential allocation hypothesis, which states that there is a tradeoff between investment and attractiveness.[13]
Genetic monogamy[edit]
Genetic monogamy refers to a mating system in which fidelity of the bonding pair is exhibited.[1] Though individual pairs may be genetically monogamous, no one species has been identified as fully genetically monogamous.
In some species, genetic monogamy has been enforced.[16] Female voles have shown no difference in fecundity with genetic monogamy, but it may be enforced by males in some instances.[1] Mate guarding is a typical tactic in monogamous species.[13][15][17] It is present in many animal species and can sometimes be expressed in lieu of parental care by males. This may be for many reasons, including paternity assurance.[16][17]
Enforcement[edit]
Monogamous mating may also be caused simply by enforcement through tactics such as mate guarding.[13][15] In these species, the males will prevent other males from copulating with their chosen female or vice versa.[16] Males will help to fend off other aggressive males, and keep their mate for themselves. This is not seen in all species, such as some primates, in which the female may be more dominant than the male and may not need help to avoid unwanted mating; the pair may still benefit from some form of mate assistance, however, and therefore monogamy may be enforced to ensure the assistance of males. Bi-parental care is not seen in all monogamous species, however, so this may not be the only cause of female enforcement.[14]
Mate assistance and territory defense[edit]
In species where mate guarding is not needed, there may still be a need for the pair to protect each other. An example of this would be sentinel behavior in avian species.[13] The main advantage of sentinel behavior is that many survival tactics are improved. As stated, the male or female will act as a sentinel and signal to their mate if a predator is present. This can lead to an increase in survivorship, foraging, and incubation of eggs.[13]
Male care for offspring is rather rare in some taxa of species. This is because males may increase their fitness by searching for multiple mates.[16][23] Females are limited in fitness by their fecundity, so multiple mating does not affect their fitness to the same extent.[19] Males have the opportunity to find a new mate earlier than females when there is internal fertilization or the females exhibit the majority of the care for the offspring.[23] When males are shown to care for offspring as well as females, it is referred to as bi-parental care.
Bi-parental care may occur when there is a lower chance of survival of the offspring without male care. The evolution of this care has been associated with energetically expensive offspring.[16] Bi-parental care is exhibited in many avian species.[13] In these cases, the male has a greater chance to increase his own fitness by seeing that his offspring live long enough to reproduce. If the male is not present in these populations, the survivorship of the offspring is drastically lowered and there is a lowering in male fitness.[13][23] Without monogamy, bi-parental care is less common and there is an increased chance of infanticide.[1] Infanticide with monogamous pairing would lead to a lowered fitness for socially monogamous males and is not seen to a wide extent.[16]
Consequences of monogamous mating[edit]
Monogamy as a mating system in animals has been thought to lower levels of some pre and post copulatory competition methods.[15][17][24] Because of this reduction in competition in some instances the regulation of certain morphological characteristics may be lowered. This would result in a vast variety of morphological and physiological differences such as sexual dimorphism and sperm quality.
Sexual dimorphism[edit]
Sexual dimorphism denotes the differences in males and females of the same species. Even in animals with seemingly no morphological sexual dimorphism visible there is still dimorphism in the gametes. Among mammals, males have the smaller gametes and females have the larger gametes. As soon as the two sexes emerge the dimorphism in the gamete structures and sizes may lead to further dimorphism in the species.[25] Sexual dimorphism is often caused through evolution in response to male male competition and female choice.[15] In polygamous species there is a noted sexual dimorphism. The sexual dimorphism is seen typically in sexual signaling aspects of morphology. Males typically exhibit these dimorphic traits and they are typically traits which help in signaling to females or male male competition.[17][26] In monogamous species sexual conflict is thought to be lessened, and typically little to no sexual dimorphism is noted as there is less ornamentation and armor. This is because there is a relaxation of sexual selection.[17] This may have something to do with a feedback loop caused by a low population density. If sexual selection is too strenuous in a population where there is a low density the population will shrink. In the continuing generations sexual selection will become less and less relevant as mating becomes more random.[17] A similar feedback loop is thought to occur for the sperm quality in genetically monogamous pairs.
Sperm quality[edit]
Once anisogamy has emerged in a species due to gamete dimorphism there is an inherent level of competition. This could be seen as sperm competition in the very least. Sperm competition is defined as a post copulatory mode of sexual selection which causes the diversity of sperm across species.[24] As soon as sperm and egg are the predominant mating types there is an increase in the need for the male gametes. This is because there will be a large number of unsuccessful sperm which will cost a certain level of expenditure on energy without a benefit from the individual sperm. Sperm in polygamous sexual encounters have evolved for size, speed, structure, and quantity.[24] This competition causes selection for competitive traits which can be pre or post copulatory.[25] In species where cryptic female choice is one of the main sources of competition females are able to choose sperm from among various male suitors.[24][27][28] Typically the sperm of the highest quality are selected.[24]
In genetically monogamous species it can be expected that sperm competition is absent or otherwise severely limited. There is no selection for the highest quality sperm amongst the sperm of multiple males, and copulation is more random than it is in polygamous situations. Therefore, sperm quality for monogamous species has a higher variation and lower quality sperm have been noted in several species. The lack of sperm competition is not advantageous for sperm quality. An example of this is in the Eurasian bullfinch which exhibits relaxed selection and sperm competition. The sperm of these males have a lower velocity than other closely related but polygamous passerine bird species and the amount of abnormalities in sperm structure, length, and count when compared to similar bird families is increased.[24]