When Mammals Became Socially Monogamous: How Single-Partner Pairings Evolved

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Wolves usually travel in packs that consists of a male, female, and their offspring. Image by Johannes Jansson.

If you are like me, you are probably wondering, “Mammals… monogamous?”

Most of us learned in school that, in order to have a reproductive advantage, animals, specifically males, have to mate frequently- to generate as many offspring as possible. If this is the case, why then, is there such a thing as social monogamy and how did it evolve?

Social Monogamy Among Animals

There are two main explanations for social monogamy: 1) it is a byproduct of some form of paternal care and 2) it is a strategy that males use to guard their mates.

Lukas and Clutton-Brock show, in their study “The Evolution of Social Monogamy in Mammals,” that social monogamy in nonhuman mammals evolved from a social system in which the animals are solitary, where females are “intolerant of each other,” and where “males are not able to defend their access to multiple females.

Through phylogenetic reconstruction, or reconstruction of the evolutionary history of species, researchers found that in order for an animal to become socially monogamous, its ancestor, in most cases, must have lived in solitary conditions.  In these ancestors, the females lived solitarily in their own home ranges while the males roamed independently in territories that overlapped those of the females.

So, in addition to having solitary ancestors, what else had to have happened for a species to develop a single-partner system?

Social Monogamy and Paternal Care

One theory to explain the evolution of social monogamy is that it was a result of some form of paternal care.

When looking at the difference between the male population who cared for their offspring and those that did not, paternal care plays a role in the fitness of both sexes. In biparental care, females produced more offspring and had more breeding cycles than those where only one parent provided care.

Field studies have found that 41%  of observed mammal males do not contribute to the care of their young, and that regular paternal care was seen in 59% of males. So, although social monogamy and paternal care are related, this suggests that paternal care is only a consequence of social monogamy and not the cause of it.

Social Monogamy and the Defense of Females

The other explanation of social monogamy in nonhuman mammals is that it evolved from a situation where males were unable to defend access to more than one female at once.

In an interview with Decoded Science, Dr. Lukas of the University of Cambridge’s Large Animal Research Group, explains why males might not have and maintain access to more than one mate:

It might be more difficult for a male to defend more than one mate when females are widely dispersed because the chance of detecting a roaming female within a large area might be lower and because males might face increased risks of injury or predation while traveling long distances for extended periods of time, especially if they cannot familiarize themselves with the area.”

Monogamy Linked To Lower Population Density, Larger Bodies

Lukas and Clutton-Brock agree with other studies that socially monogamous species come from ancestors where females are solitary but unlike other studies, their data shows that socially monogamous mammals live at lower densities than solitary species. This could be because socially monogamous species have higher body mass than solitary species.

Dr. Lukas explains that as individuals get larger, they “require more food and, in consequence, the number of individuals that can be found within a certain area declines as these individuals get larger.”

Since these animals in are usually larger, the females require resources that are high in nutrition, that also happen to be low in abundance. In order for them to survive, there must be competition for these resources. This is where intolerance comes into play. According to Dr. Lukas, females in socially monogamous species “appear less likely to tolerate the presence of other females in their home ranges and the females react with physical aggression to the intrusion of other females into their ranges.”

Beavers are generally monogamous and, usually, will only take on a new partner after losing one. Image by Ted Guzzi.

Monogamous Relationship Evolution

Lukas and Clutton-Brock’s results shows that social monogamy in non-human mammals evolved from ancestors with particular states (where there is feeding competition between females, where females are intolerant of each other, and where population density is low).

Because of these conditions, guarding multiple mates may be difficult,  so the best strategy is to guard only an individual. The benefits of paternal care came after the evolution of social monogamy.

Resources:

Lukas, D. and Clutton-Brock, T.H. The Evolution of Social Monogamy in Mammals(2013). Science. Accessed on August 1, 2013.

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