Monogamy in animals

Natural history of mating systems in which species pair bond to raise offspring
pairing for animals in non human species

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.

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. 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. Female voles have shown no difference in fecundity with genetic monogamy, but it may be enforced by males in some instances. 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. The introduction of anisogamy has caused males and females to tend to have different optimal mating strategies. Monogamy is suggested to limit fitness differences, as males and females will mate in pairs. This theory has not garnered much support, however, critiqued by several authors including Lukas and Clutton Brock and Dixson.

Monogamous mating may also be caused simply by enforcement through tactics such as mate guarding. Females are limited in fitness by their fecundity, so multiple mating does not affect their fitness to the same extent. 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 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. Sexual dimorphism is often caused through evolution in response to male male competition and female choice. 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. Typically the sperm of the highest quality are selected. This list is not complete. Other factors may also contribute to the evolution of social monogamy. Moreover, different sets of factors may explain the evolution of social monogamy in different species. There is no one size fits all explanation of why different species evolved monogamous mating systems.

In species where the young are particularly vulnerable and may benefit from protection by both parents, monogamy may be an optimal strategy. Monogamy tends to also occur when populations are small and dispersed. This is not conductive to polygamous behavior as the male would spend far more time searching for another mate. The monogamous behavior allows the male to have a mate consistently, without having to waste energy searching for other females. Furthermore, there is an apparent connection between the time a male invests in their offspring and their monogamous behavior. A male which is required to care for the offspring to ensure their survival is much more likely to exhibit monogamous behavior over one that does not. The selection factors in favor of different mating strategies for a species of animal, however, may potentially operate on a large number of factors throughout that animal's life cycle. For instance, with many species of bear, the female will often drive a male off soon after mating, and will later guard her cubs from him. It is thought that this may be due to the fact that too many bears close to one another may deplete the food available to the relatively small but growing cubs. Monogamy may be social but rarely genetic. For example, in the cichlid species Variabilichromis moorii, a monogamous pair will care for their eggs and young but the eggs are not all fertilized by the same male. Thierry Lodé argued that monogamy should result from conflict of interest between the sexes called sexual conflict.

There are species which have adopted monogamy with great success. For instance, the male prairie vole will mate exclusively with the first female he ever mates with. The vole is extremely loyal and will go as far as to even attack other females that may approach him. This type of behavior has been linked to the hormone vasopressin. This hormone is released when a male mates and cares for young. Due to this hormone's rewarding effects, the male experiences a positive feeling when they maintain a monogamous relationship. To further test this theory, the receptors that control vasopressin were placed into another species of vole that is promiscuous. After this addition, the originally unfaithful voles became monogamous with their selected partner. These very same receptors can be found in human brain, and have been found to vary at the individual level—which could explain why some human males tend to be more loyal than others. Black vultures stay together as it is more beneficial for their young to be taken care of by both parents. They take turns incubating the eggs, and then supplying their fledglings with food. Black vultures will also attack other vultures that are participating in extra pair copulation, this is an attempt to increase monogamy and decrease promiscuous behavior. Similarly, emperor penguins also stay together to care for their young. This is due to the harshness of the Antarctic weather, predators and the scarcity of food. One parent will protect the chick, while the other finds food. However, these penguins only remain monogamous until the chick is able to go off on their own. After the chick no longer needs their care, approximately 85% of parents will part ways and typically find a new partner every breeding season. Hornbills are a socially monogamous bird species that usually only have one mate throughout their lives, much like the prairie vole. The female will close herself up in a nest cavity, sealed with a nest plug, for two months. At this time, she will lay eggs and will be cared for by her mate. The males are willing to work to support himself, his mate, and his offspring in order for survival; however, unlike the emperor penguin, the hornbills do not find new partners each season. It is relatively uncommon to find monogamous relationships in fish, amphibians and reptiles; however, the red backed salamander as well as the Caribbean cleaner goby practice monogamy as well. However, the male Caribbean cleaner goby fish has been found to separate from the female suddenly, leaving her abandoned. In a study conducted by Oregon State University, it was found that this fish practices not true monogamy, but serial monogamy. This essentially means that the goby will have multiple monogamous relationships throughout its life – but only be in one relationship at a time. The red backed salamander exhibited signs of social monogamy, which is the idea that animals form pairs to mate and raise offspring, but still will partake in extra pair copulation with various males or females in order to increase their biological fitness. This is a relatively new concept in salamanders, and has not been seen frequently – it is also concerning that the act of monogamy may inhibit the salamanders reproductive rates and biological success. However, the study which was conducted in cooperation by the University of Louisiana, Lafayette, and the University of Virginia showed that the salamanders are not inhibited by this monogamy if they show alternative strategies with other mates. Azara's night monkeys are another species that proved to be monogamous. In an 18 year study conducted by the University of Pennsylvania, these monkeys proved to be entirely monogamous, exhibiting no genetic information or visual information that could lead to the assumption that extra pair copulation was occurring. This explained the question as to why the male owl monkey invested so much time in protecting and raising their own offspring. Because monogamy is often referred to as "placing all your eggs in one basket" the male wants to ensure his young survive, and thus pass on his genes. The desert grass spider, Agelenopsis aperta, is mostly monogamous as well. Male size is the determining factor in fights over a female, with the larger male emerging as the winner since their size signifies success in future offspring. Other monogamous species include wolves, certain species of fox, otters, a few hooved animals, some bats, and the Eurasian beaver. This beaver is particularly interesting, as it is practicing monogamy in its reintroduction to certain parts of Europe; however, its American counterpart is not monogamous at all and often partakes in promiscuous behavior. The two species are quite similar in ecology, but American beavers tend to be less aggressive than European beavers. In this instance, the scarcity of the European beavers' population could drive its monogamous behavior; moreover, it lowers the risk of parasite transmission which is correlated with biological fitness. Monogamy is proving to be very efficient for this beaver, as their population is climbing.