A cute, monogamous dik dik: how did it get that way?
A couple of new papers have just appeared that attempt to identify the evolutionary roots of social monogamy. One of these papers—by Christopher Opie, Quentin Atkinson, Robin Dunbar, and Susanne Shultz—focuses on phylogenetic reconstructions of social monogamy among primates. The other paper—by Dieter Lukas and Tim Clutton-Brock—addresses mammals more broadly. Media coverage of these papers has recognized and highlighted that they reach different conclusions. One research group says that male infanticide is the root of social monogamy in primates, whereas the other says that female resource use best accounts for evolutionary shifts to social monogamy across mammals. Of course, many readers want to know what these analyses might say about shifts toward slight polygyny (or if you prefer mostly monogamy) among recent human ancestors. It’s all so confusing, if lead researchers can’t seem to find similar evolutionary grounds behind social monogamy.
Let’s focus on something that authors of both papers agree on, and a subject that also answers the title question for this blog post. Both groups suggest social monogamy precedes paternal care. As Opie et al. note, “Paternal care only evolves after a switch to social monogamy and not in polygynous mating systems.” (p. 2) Similarly, Luka and Clutton-Brock note, “Although paternal care and social monogamy are associated, an analysis of transitions suggests that male care is probably a consequence rather than a cause of the evolution of social monogamy.” (p. 527) Indeed, in the latter analysis, it’s only in a slight majority (59%) of socially monogamous species that paternal care (such as provisioning or carrying) is also found. Moreover, paternal care is found in only three species of non-monogamous mammals, including Goeldi’s monkey and a lemur. By this line of thought, social monogamy is effectively a necessary but insufficient condition for paternal care.
Another finding on which both papers agree is that paternal care increases reproductive rates. Opie and colleagues note, “In primates, paternal care is associated with a shortening of interbirth intervals and an increase in reproductive rates similar to that seen in birds and other mammals.” (p. 2) Lukas and Clutton-Brock recognize, “Females in socially monogamous species with biparental care produce more litters per year…”(p. 527) Male care, such as carrying or provisioning, can thus aid female reproductive output. Another benefit of male care championed by Opie and colleagues is that it reduces the length of time a female is nursing relative to gestation length; accordingly, a female’s offspring spend relatively less time in a potentially highly vulnerable state nursing, when a would-be infanticidal male may seek to kill her babies in order to father his own with her. Whether one buys the contention that infanticide drives social monogamy, another punch line of both papers is that paternal care enhances female reproductive rates.
For discussions concerning the evolution of human paternal care, both of the key points on which these papers agree are relevant: reconstructions of paternal care among our hominin ancestors do well to see it as a consequence rather than cause of long-term sociosexual relationships, and a primary benefit to females of paternal care is in increased reproductive output. These views are at odds with some evolutionary scenarios concerning human paternal care—views that see the origins of hominin paternal care as synchronous with long-term bonds when a hominin female traded sexual fidelity for provisioned paternal resource (the provisioning likely arose later), or scenarios in which male carrying or protection originated to increase offspring survival (survival effects are less important than increased reproductive output).
Neither of these papers settles the evolutionary origins of long-term (slightly polygynous/mostly monogamous) sociosexual bonds or paternal care among our ancestors, but they do help orient our thinking toward the best ways to try doing so—ways that draw upon evolutionary and comparative thinking. As one last parting shot, there’s another detail in Lukas and Clutton-Brock’s paper that could catch on: in their analysis of 2545 species of mammals, 9% were classified as socially monogamous. The longstanding and oft-cited point that 3% of mammalian species are socially monogamous traces to a 1977 paper by Devra Kleiman, even though much has obviously been learned since then and that percentage seemed to low-ball the estimate. There is a relatively higher percentage of primates and carnivores that are socially monogamous compared to most other groups of mammals, even if we are still perplexed by the evolutionary paths our ancestors took the last few millions of years to get that (mostly monogamous/slightly polygynous) way.
Kleiman, D. G. (1977). Monogamy in mammals. The Quarterly Review of Biology, 52, 39-69.
Lukas, D., & Clutton-Brock, T. H. (2013). The evolution of social monogamy in mammals. Science, 341, 526-530.
Opie, C., Atkinson, Q. D., Dunbar, R. I. M., & Schultz, S. (2013). Male infanticide leads to social monogamy in primates. Proceedings of the National Academy of Sciences, early edition.