Social 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, and mammals, including 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 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.
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) 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 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), 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, 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 is a hormone that induces a male Prairie vole to mate with one female, form a pair bond, 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 alleles 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 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, a hormone related to stress, so that monogamy helps produce positive health effects. Male marmosets that received an oxytocin antagonist had increased 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, a neurotransmitter, produces pleasurable effects that reinforce monogamous behavior. Haloperidol, a dopamine antagonist, prevented partner preference but did not disrupt mating while apomorphine, a dopamine agonist, 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. 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 and lactation, 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.