Social monogamy in mammalian species


Monogamous pairing refers to a general relationship between an adult male and an adult female for the purpose of sexual reproduction. It is particularly common in birds, but there are examples of this occurrence in reptiles, invertebrates, fish, amphibians, and mammals.

Monogamy in mammals

Social monogamy in mammals is defined as a long term or sequential living arrangement between an adult male and an adult female. 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. This arrangement 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. 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.

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, Agoutis, Grey duikers, and Pacaranas 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, Night monkeys, African dormice, and Hutias 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:
One of the key factors of monogamous pairings is group living. Advantages to living in groups include, but are not limited to:
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:
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

is a hormone that induces a male prairie vole to mate with one female, form a pair bond, and exhibit mate-guarding behavior. The presence of vasopressin receptor 1A 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 didn’t 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 and elephant shrew, biparental care is not very common. These species do, however, exhibit monogamous mating systems presumably due to high dispersal rates. Komers and Brotherton 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:
This phenomenon is not common for all species, but species such as the Japanese serow 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.
Bi-parental care has been extensively studied in the California mouse. 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, including dwarf lemurs, where females were also not successful at raising their offspring without paternal help. Lastly, in a study performed by Wynne-Edwards, 95% of Djungarian hamsters 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:
, the killing of the offspring by adult individuals, has been reported in many mammalian species and it is considered as an adaptive strategy to enhance fitness. Infanticide as the result of male–male competition for reproduction will occur under the following conditions:
Infanticide allows the male perpetrator to have multiple female partners; females could benefit from killing other female's offspring by gaining access to food resources or shelter.
In primates, it is thought that risk of infanticide 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 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.
The rates of infanticide are very low in other monogamous groups of larger mammals, which can be explained by the fact that males care for their offspring and their mother by protecting them from predators and the threat of other males. This is consistent with the findings of Borries, Savini, & Koeng who indicated that the percentage of infant loss was significantly lower in monogamous species than in polyandrous ones. However, there still needs to be more empirical evidence in order to further test this hypothesis.

Evolutionary consequences

relates to the phenotypic differences between a male and a female of the same species, often referring to the species' body size. Monogamous males tend to be smaller compared to females, which results in sexual competition between the male candidates for a larger female.
Comparing to monogamous species, polygynous species tend to display more sexual dimorphism, or difference in body size; it is believed that sexual dimorphism could be an evolutionary consequence to a monogamous mating system. In other words, monogamous animals do not compete as strongly, hence physical characteristics are not favored as much. Consequently, Weckerly concluded that mating systems do affect the extent of sexual dimorphism, with sexual dimorphism being reduced in long-term pair bonding.

Monogamy and cooperative breeding

is defined as a social system where individuals take care of offspring other than their own. These individuals may include nonbreeding adults or subadults, alloparents, or simply reproductive adults who share the care amongst each other. This care often includes provisioning for food or protection from predators. The association between cooperative breeding and monogamy has been connected to monogamous pairings within mammalian societies.
Lukas and Clutton-Brock extensively discuss this association between monogamy and cooperative breeding. Most females provision and care for their young; however, there are certain species such as mongooses, New World monkeys, and porcupines that care for their young in assistance of non-breeding helpers. This concept triggers the question as to why any individuals would take care of offspring other than their own. As described by the study, this social system could be triggered by the non-breeding helpers' benefit to maximize their fitness by assisting in the rearing of the young.