Social Monogamy: When Game Ain’t Playing the Mating Game

What do you typically think of when you hear ‘monogamy’? If you’re thinking wedding bells, rings, soul mates for life, etc., you might be thinking of the human ideals of a marital monogamous relationship.  Monogamy in human relationships is often considered socially acceptable, and the potential financial and emotional security might be desired…but let’s put aside the fluffy wedding thoughts for a moment and get to what a monogamous relationship means at its core: having one partner at any one set period of time.

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Now let’s add a second layer to human monogamy: social monogamy.  This is where two individuals live together and share resources or children.  So, if you’re living with a partner in an apartment and sharing a salary to support and raise a family, chances are that you’re in a socially monogamous relationship until you break up…even if you're actually spending some nights with someone else. Yes, that someone else.

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OK, OK, now consider social monogamy in the non-human, animal context.  In a journal article by Lukas and Clutton-Brock, social monogamy is defined as: 

“a single breeding female and a single breeding male shar[ing] a common range or territory and associat[ing] with each other for more than one breeding season.” 

Wow. That’s a mouthful. But basically, imagine a long-term living situation between a female and male for the multiple times that offspring could be born.  So instead of living under an apartment room, buying groceries for food, and staying with your partner for multiple pregnancies…imagine mammals living out in the wild, hunting/searching/foraging for food, and mating for multiple breeding seasons.   Animals that are more likely to be socially monogamous are primates and birds.  However, social monogamy is uncommon in some orders, such as Artiodactyla (hoofed animals like pigs and camels), and completely absent in orders like Cetacea (think whales).  When social monogamy is absent, animals will have a new mate every breeding season.

Animals in the Artiodactyla order [3]

The Cetacea order typically consists of whales [4]

Social monogamy in animals does not necessarily mean that males are going to stay home and care for their kids however. (Ain’t nobody got time for that!) In fact, 41% of socially monogamous species do not have males caring for their young. For instance, in dik-diks, males do not guard, carry, feed, or teach their young—these are roles that females take on.  

Dik-diks do not exhibit characteristics of paternal care [5]

However, males that do care for their young often find that their family sizes grow a little bigger.  Their female counterparts can have more litters per year, and the males have a better chance of ensuring that their genes are passed on through more continuous breeding cycles.  However, paternal care is most likely a consequence—a secondary adaptation—rather than an actual cause of social monogamy.  Lukas and Clutton-Brock suggest that certain models show that paternal care began to appear in branches of the phylogenetic tree where social monogamy was already present. 

Lukas and Clutton-Brock's model of how social monogamy transitions from a population's ancestors [6]

Now the big question: under what conditions is social monogamy present? Lukas and Clutton-Brock believe that social monogamy developed from ancestrally solitary females who lived at low population densities in a territory.  Competition for these females would be high, and while these females are solitary, roaming males who happen upon these females may find that it is more effective to stay with one mate and protect their offspring rather than guard multiple female mates.  Although social monogamy is correlated with low population density of females, territory size between socially monogamous and solitary females does not differ that much, possibly due to overlapping home ranges between the two kinds.  When Lukas and Clutton-Brock compared Primates and Carnivora orders with Rodentia and Artiodactyla (an herbivorous order) they concluded that social monogamy may also have developed from relying on a low-abundance food with high nutrient quality.  For instance, in 91% of socially monogamous primates, fruit is the most important part of the diet, as opposed to 28% of solitary primates.  Foods that are not as nutritionally valuable as fruit (such as gum, bark, and fungi) are also present in more solitary species of animals.  Overall, we can see that females and food source play a large role as to whether mammals are socially monogamous.   

That’s the story of social monogamy in mammals, but there’s another evolutionary wrench thrown in.  Humans are closely related to apes, but it is known today that all African apes are polygynous.  This suggests that our common ancestor was most likely polygynous, but we somehow became a monogamous species.  Human monogamy could have developed because of the additional need for paternal care (which would contradict the idea that paternal care is always a secondary adaptation).  Dietary changes that decrease the number of females in a population may also have prevented males from protecting more than one female partner.  That, however, is a story for another day.  In the meantime, enjoy being part of a socially monogamous species…or watch the video below and ask yourself whether mammals could really be monogamous at all times.

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Journal Article:

Lukas, D., and T. H. Clutton-Brock. “The Evolution of Social Monogamy in Mammals.” Science 341, no. 6145 (August 2, 2013): 526–30. doi:10.1126/science.1238677.

http://www.sciencemag.org/content/341/6145/526.full.pdf

Media credits:

[1]: Photo by Ryan G. Smith. https://flic.kr/p/aCZUne

[2]: Public domain image. http://commons.wikimedia.org/wiki/File%3AFamily_drinking_juice_(1).jpg

[3]: "Artiodactyla" by Lorenz Oken - Natural History Prints. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Artiodactyla.jpg#mediaviewer/File:Artiodactyla.jpg 

[4]: Public domain image. http://commons.wikimedia.org/wiki/File:Humpback_whale_noaa.jpg

[5]: Public domain image. http://commons.wikimedia.org/wiki/File%3AKirk's_Dik-dik_in_Samburu.jpg

[6]: Photo from Lukas & Clutton-Brock's paper: http://www.sciencemag.org/content/341/6145/526.full.pdf

[7]: Video by MinuteEarth. http://www.youtube.com/watch?v=bxQdLhOQf5c