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.
Monogamous mating
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, 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
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
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.
Evolution of monogamy in animals
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.
Facultative monogamy
First introduced by Kleiman, facultative monogamy occurs when females are widely dispersed. This can either occur because females in a species tend to be solitary or because the distribution of resources available cause females to thrive when separated into distinct territories. In these instances, there is less of a chance for a given male to find multiple females to mate with. In such a case, it becomes more advantageous for a male to remain with a female, rather than seeking out another and risking not finding another female and or not being able to fight off another male from interfering with his offspring by mating with the female or through infanticide. In these situations, male-to-male competition is reduced and female choice is limited. The end result is that the mate choice is more random than in a more dense population, which has a number of effects including limiting dimorphism and sexual selection.With resource availability limited, mating with multiple mates may be harder because the density of individuals is lowered. The habitat cannot sustain multiple mates, so monogamy may be more prevalent. This is because resources may be found more easily for the pair than for the individual. The argument for resource availability has been shown in many species, but in several species, once resource availability increases, monogamy is still apparent.
With increased resource availability, males may be offsetting the restriction of their fitness through several means. In instances of social monogamy, males may offset any lowered fitness through extra pair coupling. Extra pair coupling refers to male and females mating with several mates but only raising offspring with one mate. The male may not be related to all of the offspring of his main mate, but some offspring are being raised in other broods by other males and females, thereby offsetting any limitation of monogamy. Males are cuckolds, but because they have other female sexual partners, they cuckold other males and increase their own fitness. Males exhibit parental care habits in order to be an acceptable mate to the female. Any males that do not exhibit parental care would not be accepted as a sexual partner for socially monogamous females in an enforcement pattern.
Obligate monogamy
Kleiman also offered a second theory. In obligate monogamy, the driving force behind monogamy is a greater need for paternal investment. This theory assumes that without biparental care fitness level of offspring would be greatly reduced. This paternal care may or may not be equal to that of the maternal care.Related to paternal care, some researchers have argued that infanticide is the true cause of monogamy. This theory has not garnered much support, however, and has been critiqued by several authors including Lukas and Clutton-Brock and Dixson.
Enforcement
Monogamous mating may also be caused simply by enforcement through tactics such as mate guarding. In these species, the males will prevent other males from copulating with their chosen female or vice versa. 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.Mate assistance and territory defense
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. 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.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. Females are limited in fitness by their fecundity, so multiple mating does not affect their fitness to the same extent. 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. 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. Bi-parental care is exhibited in many avian species. 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. Without monogamy, bi-parental care is less common and there is an increased chance of infanticide. Infanticide with monogamous pairing would lead to a lowered fitness for socially monogamous males and is not seen to a wide extent.