Social

monogamy Monogamy ( ) is a social relation, relationship of Dyad (sociology), two individuals in which they form a mutual and exclusive intimate Significant other, partnership. Having only one partner at any one time, whether for life or #Serial monogamy ...

in mammals is defined as sexually mature adult organisms living in pairs. While there are many definitions of social monogamy, this social organization can be found in invertebrates, reptiles and amphibians, fish, birds, mammals, and humans. It should not be confused with genetic monogamy, which refers to two individuals who only reproduce with one another. Social monogamy does not describe the sexual interactions or patterns of reproduction between monogamous pairs; rather it strictly refers to the patterns of their living conditions. Rather, sexual and genetic monogamy describe reproductive patterns. It is possible for a species to be both genetically monogamous and socially monogamous but it is more likely for species to practice social monogamy and not genetic monogamy. Social monogamy consists of, but is not limited to: sharing the same territory; obtaining food resources; and raising offspring together. A unique characteristic of monogamy is that unlike in

polygamous Polygamy (from Late Greek , "state of marriage to many spouses") is the practice of marrying multiple spouses. When a man is married to more than one wife at the same time, it is called polygyny. When a woman is married to more than one h ...

species, parents share parenting tasks. Even though their tasks are shared, monogamy does not define the degree of paternal investment in the breeding of the young. Microtus fortis Plzen 02

Only ~3–5% of all mammalian species are socially monogamous, including some species that mate for life and ones that mate for an extended period of time. Monogamy is more common among primates: about 29% of primate species are socially monogamous. Lifelong monogamy is very rare; however, it is exemplified by species such as the Prairie vole ('' Microtus ochrogaster''). A vast majority of monogamous mammals practice serial social monogamy where another male or female is accepted into a new partnership in the case of a partner's death. In addition, there are some species that exhibit short-term monogamy which involves partnership termination while one's partner is still alive; however, it usually lasts for at least one breeding season. Monogamy usually does not occur in groups where there is a high abundance of females, but rather in ones where females occupy small ranges. Socially monogamous mammals live at significantly lower population densities than do solitary species. Additionally, most mammals exhibit male-biased dispersal; however, most monogamous mammalian species display female-biased dispersal. Some socially monogamous species exhibit pair bonds that occur between two sexually mature organisms, have an affective component, be specific to the individual, last longer that one reproductive cycle, and be quantifiable in strength or quality of relationship. Pair bonding can exhibit (but does not have to) sexual behaviors and/or bi-parental care. Pair bonding cannot exhibit, however, organisms that cannot identify one another in a pair, end in the death of a mate or separation from the mate directly after mating, lack of distress when separated from the mate, or lack sociality. Not all socially monogamous species exhibit pair bonding, but all pair bonding animals practice social monogamy. These characteristics aid in identifying a species as being socially monogamous. At the biological level, social monogamy affects the neurobiology of the organism through hormone pathways such as vasopressin and oxytocin. Vasopressin is related to the distress hormone an organism feels when separated from their mate while oxytocin is associated with the affective component of the social interactions between mates. These biological factors give way to a genetic component that evolution could act on via selection to evolve social monogamy in animals.

Types of social monogamy

Facultative monogamy

Facultative monogamy, or Type I monogamy, occurs when the male is not fully committed to one female, but he chooses to stay with her because there are no other mating opportunities available to him. In this type of monogamy, species rarely spend time with their families, and there is a lack of paternal care towards the offspring. Elephant shrews ('' Rhynchocyon chrysopygus'' and '' Elephantulus rufescens''), Agoutis ('' Dasyprocta punctata''), Grey duikers ('' Sylvicapra grimmia''), and Pacaranas ('' Dinomys branickii'') are some of the most common examples of the mammalian species that display Type I monogamy. In addition, these species are characterized to occupy low areas over a large expand of land.

Obligate monogamy

Obligate monogamy, or Type II monogamy, is practiced by species that live in overlapping territories, where females cannot rear their young without the help of their partners. Species such as Indris ('' Indri indri''), Night monkeys ('' Aotus trivirgatus''), African dormice ('' Notomys alexis''), and Hutias ('' Capromys melanurus'') are observed as family groups who live together with a number of generations of their young. There are several factors that are associated with Type II monogamy: *high paternal investment when offspring mature in the family setting *delayed sexual maturation observed in juveniles that remain in the family group *juveniles contributing greatly to the rearing of their siblings when retained in the family group.

Group living

One of the key factors of monogamous pairings is group living. Advantages to living in groups include, but are not limited to: * Susceptibility to predation: animals such as the common dwarf mongoose ('' Helogale parvula'') and tamarin (such as ''

Saguinus oedipus The cotton-top tamarin (''Saguinus oedipus'') is a small New World monkey weighing less than . This New World monkey can live up to 24 years, but most of them die by 13 years. One of the smallest primates, the cotton-top tamarin is easily recogn ...

'') may benefit from such group living by having alarm calls in response to an approaching predator. * Food acquisition: it is considerably easier for animals to hunt in a group rather than by themselves. For this reason, mammals such as dwarf mongooses, marmosets and

tamarins The tamarins are squirrel-sized New World monkeys from the family (biology), family Callitrichidae in the genus ''Saguinus''. They are the first offshoot in the Callitrichidae tree, and therefore are the sister group of a clade formed by the l ...

hunt in groups and share their food among their family members or members of the group. * Localization of resources: in some species, such as Eurasian beaver (''

Castor fiber The Eurasian beaver (''Castor fiber'') or European beaver is a species of beaver widespread across Eurasia, with a rapidly increasing population of at least 1.5 million in 2020. The Eurasian beaver was hunted to near-extinction for both its fur ...

''), localization of an adequate lodge area (a pond or a stream) is more beneficial in a group setting. This group living arrangement gives beavers a better chance to find a high quality place to live by searching for it in a group rather than by one individual. These group living advantages, however, do not describe why monogamy, and not

polygyny Polygyny () is a form of polygamy entailing the marriage of a man to several women. The term polygyny is from Neoclassical Greek πολυγυνία (); . Incidence Polygyny is more widespread in Africa than in any other continent. Some scholar ...

, has evolved in the species mentioned above. Some possible conditions which may account for cases of monogamous behavior in mammalian species may have to do with: * scarce resources available on any given territory so that two or more individuals are needed in order to defend it * physical environment conditions are so unfavorable that multiple individuals are needed to cope with it * early breeding serves as an advantage to the species and is crucial to monogamous species.

Evolution of monogamy

There are several hypotheses for the evolution of mammalian monogamy that have been extensively studied. While some of these hypotheses apply to a majority of monogamous species, other apply to a very limited number of them.

Proximate causes

Hormones and Neurotransmitters

Vasopressin Mammalian vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the ''AVP'' gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It ...

is a

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

that induces a male Prairie vole to mate with one female, form a

pair bond In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of young and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is frequently ...

, and exhibit mate-guarding behavior (i.e. increase the degree of monogamous behavior). The presence of vasopressin receptor 1A (V1aR) in the ventral forebrain is associated with pair bonding, which is necessary for monogamy. Genetic differences in the V1aR gene also play a role in monogamy: voles with long V1aR

allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...

s exhibit more monogamous tendencies by preferring their mate over a stranger of the opposite sex, whereas voles with short V1aR alleles displayed a lesser degree of partner preference. Vasopressin is responsible for forming attachment between male and female prairie voles. Vasopressin also regulates paternal care. Finally, vasopressin activity results in "postmating aggression" that allows prairie voles to protect their mate.

Oxytocin Oxytocin is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. Present in animals since early stages of evolution, in humans it plays roles in behavior that include Human bonding, ...

is a hormone that regulates pair bond formation along with vasopressin. Blocking either oxytocin or vasopressin prevents formation of the pair bond but continues to allow for social behavior. Blocking both hormones resulted in no pair bond and reduced sociality. Oxytocin also attenuates the negative effects of

cortisol Cortisol is a steroid hormone in the glucocorticoid class of hormones and a stress hormone. When used as medication, it is known as hydrocortisone. Cortisol is produced in many animals, mainly by the ''zona fasciculata'' of the adrenal corte ...

, a hormone related to stress, so that monogamy helps produce positive health effects. Male

marmoset The marmosets (), also known as zaris or sagoin, are twenty-two New World monkey species of the genera '' Callithrix'', '' Cebuella'', '' Callibella'', and ''Mico''. All four genera are part of the biological family Callitrichidae. The term ...

s that received an oxytocin

antagonist An antagonist is a character in a story who is presented as the main enemy or rival of the protagonist and is often depicted as a villain.HPA-axis activity in response to a stressor than when treated with a control, showing the oxytocin associated with the pair bond lessens the physiological responses to stress. Also, marmosets who previously had elevated cortisol levels spent more time in close proximity to their mate than marmosets with previously normal cortisol levels.

Dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. It is an amine synthesized ...

, a

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

, produces pleasurable effects that reinforce monogamous behavior.

Haloperidol Haloperidol, sold under the brand name Haldol among others, is a typical antipsychotic medication. Haloperidol is used in the treatment of schizophrenia, tics in Tourette syndrome, mania in bipolar disorder, delirium, agitation, acute psychos ...

, a

dopamine antagonist A dopamine antagonist, also known as an anti-dopaminergic and a dopamine receptor antagonist (DRA), is a type of drug which blocks dopamine receptors by receptor antagonism. Most antipsychotics are dopamine antagonists, and have been used in t ...

, prevented partner preference but did not disrupt

mating In biology, mating is the pairing of either opposite-sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. '' Copulation'' is the union of the sex organs of two sexually repr ...

while apomorphine, a

dopamine agonist A dopamine agonist is a compound that activates dopamine receptors. There are two families of dopamine receptors, D1-like and D2-like. They are all G protein-coupled receptors. D1- and D5-receptors belong to the D1-like family and the D2-like ...

, induced pair bonding without mating, showing dopamine is necessary for the formation of the pair bond in prairie voles. In addition, mating induced a 33% increase in turnover of dopamine in the

nucleus accumbens The nucleus accumbens (NAc or NAcc; also known as the accumbens nucleus, or formerly as the ''nucleus accumbens septi'', Latin for ' nucleus adjacent to the septum') is a region in the basal forebrain rostral to the preoptic area of the hypo ...

. While this result was not statistically significant, it may indicate that mating can induce pair bond formation via the dopaminergic reward system. Elevated testosterone levels are associated with decreased paternal behavior and decreased testosterone levels are associated with decreased rates of infanticide. Experienced Marmoset fathers had decreased testosterone levels after exposure to their 2-week-old infant's scent but not their 3-month-old infant's or a stranger infant's, suggesting offspring-specific olfactory signals can regulate testosterone and induce paternal behavior.

Ultimate causes

Female distribution

Female distribution seems to be one of the best predictors of the evolution of monogamy in some species of mammals. It is possible that monogamy evolved due to a low female availability or high female dispersion where males were unable to monopolize more than one mate over a period of time. In species such as Kirk's dik-dik ('' Madoqua kirkii'') and Rufous elephant shrew ('' Elephantulus rufescens''), biparental care is not very common. These species do, however, exhibit monogamous mating systems presumably due to high dispersal rates. Komers and Brotherton (1997) indicated that there is a significant correlation between mating systems and grouping patterns in these species. Furthermore, monogamous mating system and female dispersion are found to be closely related. Some of the main conclusions of the occurrence of monogamy in mammals include: * Monogamy occurs when males are unable to monopolize more than one female * Monogamy should be more likely if female under-dispersion occurs * Female home range is larger for monogamous species * When females are solitary and occupy large ranges This phenomenon is not common for all species, but species such as the Japanese serow ('' Capricornis crispus'') exhibits this behavior, for example.

Bi-parental care

It is believed that bi-parental care had an important role in the evolution of monogamy. Because mammalian females undergo periods of

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...

and

lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The process ...

, they are well adapted to take care of their young for a long period of time, as opposed to their male partners who do not necessarily contribute to this rearing process. Such differences in parental contribution could be a result of the male's drive to seek other females in order to increase their reproductive success, which may prevent them from spending extra time helping raise their offspring. Helping a female in young rearing could potentially jeopardize a male's fitness and result in the loss of mating opportunities. There are some monogamous species that exhibit this type of care mainly to improve their offspring's survivorship; however it does not occur in more than 5% of all mammals. Bi-parental care has been extensively studied in the California deermouse ('' Peromyscus californicus''). This species of mice is known to be strictly monogamous; mates pair for a long period of time, and the level of extra-pair paternity is considerably low. It has been shown that in the event of female removal, it is the male that takes direct care of the offspring and acts as the primary hope for the survival of his young. Females who attempt to raise their young in cases where their mate is removed often do not succeed due to high maintenance costs that have to do with raising an offspring. With the presence of males, the survival of the offspring is much more probable; thus, it is in the best interest for both parents to contribute. This concept also applies to other species, ilike the Fat-tailed dwarf lemurs ('' Cheirogaleus medius''), where females were also not successful at raising their offspring without paternal help. Lastly, in a study performed by Wynne-Edwards (1987), 95% of Campbell's dwarf hamsters ('' Phodopus campbelli'') survived in the presence of both parents, but only 47% survived if the father was removed. There are several key factors that may affect the extent to which males care for their young: * Intrinsic ability to aid offspring: the male's ability to exhibit parental care. * ''Sociality'': male paternal behavior shaped by permanent group living. There is a closer association between the male and his offspring in small groups that are often composed of individuals that are genetically related. Common examples include

mongoose A mongoose is a small terrestrial carnivorous mammal belonging to the family Herpestidae. This family has two subfamilies, the Herpestinae and the Mungotinae. The Herpestinae comprises 23 living species that are native to southern Europe, A ...

wolves The wolf (''Canis lupus''; : wolves), also known as the grey wolf or gray wolf, is a canine native to Eurasia and North America. More than thirty subspecies of ''Canis lupus'' have been recognized, including the dog and dingo, though gr ...

, and

naked mole-rat The naked mole-rat (''Heterocephalus glaber''), also known as the sand puppy, is a burrowing rodent native to the Horn of Africa and parts of Kenya, notably in Somali regions. It is closely related to the blesmols and is the only species in th ...

s. * ''High costs to polygyny'': some males could evolve to care for their offspring in cases where females were too dispersed over the given territory and the male could not find consistent females to mate with. In those territories, individuals such as elephant shrews, and dasyproctids, stay within their known territories rather than going outside of their limits in order to search for another mate, which would be more costly than staying around his adapted territory. * ''Paternity certainty'': There are cases where males care for offspring that they are not genetically related to especially in groups where cooperative breeding is practiced. However, in some species, males are able to identify their own offspring, especially in threat of infanticide. In these groups, paternity certainty could be a factor deciding about biparental care.

Infanticide

In primates, it is thought that risk of

infanticide Infanticide (or infant homicide) is the intentional killing of infants or offspring. Infanticide was a widespread practice throughout human history that was mainly used to dispose of unwanted children, its main purpose being the prevention of re ...

is the primary driver for the evolution of socially monogamous relationships. Primates are unusual in that 25% of all species are socially monogamous; additionally, this trait has evolved separately in every major clade. Primates also experience higher rates of infanticide than most other animals, with infanticide rates as high as 63% in some species. Opie, Atkinson, Dunbar, & Shutlz (2013) found strong evidence that male infanticide preceded the evolutionary switch to social monogamy in primates rather than bi-parental care or female distribution, suggesting that infanticide is the main cause for the evolution of social monogamy in primates. This is consistent with the findings that indicate that the percentage of infant loss is significantly lower in monogamous than in polyandrous species. Due to the length of gestation and lactation in female mammals, infanticide, the killing of the offspring by adult individuals, is relatively common in this group. Since there is a strong male to male competition for reproduction in species with this behaviour, infanticide could be an adaptative strategy to enhance fitness if: * the male only kills unrelated infants. * the male's chance of siring the next offspring is high. * the female could benefit from killing other female's offspring by reducing future competition for food or shelter. The rates of infanticide are very low in other monogamous groups of larger mammals.

Evolutionary consequences

The forementioned ultimate causes of monogamy in mammals can have phenotypic consequences on the sexual size dimorphism of mammals. In other words, it is thought that in monogamous species males would tend to have a similar or lower body size to the one of females. This is because males from monogamous species do not compete as strongly with each other, hence investing in greater physical abilities would be costlier for males. Comparatively, we can conclude that sexual dimorphism is reduced in long-term

pair bonding In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of young and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is frequently ...

species, by observing that polygynous species tend to have a greater sexual size dimorphism.

References

{{reflist, 3 Monogamy Animal sexuality Mammalian sexuality