Monogamy in animals

Some animal species have a monogamous mating system, in which pairs bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.^{[citation needed]}

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. 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. Monogamy in mammals is rather rare, only occurring in 3–9% of these species. A larger percentage of avian species are known to have monogamous relationships (about 90%), but most avian species practice social but not genetic monogamy in contrast to what was previously assumed by researchers. Monogamy is quite rare in fish and amphibians, but not unheard of, appearing in a select few species.

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. Paternal care in monogamous species is commonly displayed through carrying, feeding, defending, and socializing offspring. With social monogamy there may not be an expected sexual fidelity between the males and the females. The existence of purely social monogamy is a polygamous or polyandrous social pair with extra pair coupling. 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. 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. 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.

Genetic monogamy refers to a mating system in which fidelity of the bonding pair is exhibited. 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. Female voles have shown no difference in fecundity with genetic monogamy, but it may be enforced by males in some instances. Mate guarding is a typical tactic in monogamous species. 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.

While the evolution of monogamy in animals cannot be broadly ascertained, there are several theories as to how monogamy may have evolved.

Anisogamy is a form of sexual reproduction which involves the fusion of two unequally-sized gametes. In many animals, there are two sexes: the male, in which the gamete is small, motile, usually plentiful, and less energetically expensive, and the female, in which the gamete is larger, more energetically expensive, made at a lower rate, and largely immobile. Anisogamy is thought to have evolved from isogamy, the fusion of similar gametes, multiple times in many different species.

The introduction of anisogamy has caused males and females to tend to have different optimal mating strategies. This is because males may increase their fitness by mating with many females, whereas females are limited by their own fecundity. Females are therefore typically more likely to be selective in choosing mates. Monogamy is suggested to limit fitness differences, as males and females will mate in pairs. This would seem to be non-beneficial to males, but may not be in all cases. Several behaviors and ecological concerns may have led to the evolution of monogamy as a relevant mating strategy. Partner and resource availability, enforcement, mate assistance, and territory defense may be some of the most prevalent factors affecting animal behavior.