From an evolutionary standpoint, monogamy doesn't seem to make much sense – especially for males. And yet, it's practiced by a significant number of mammalian species, including humans. Now, in a fantastic example of science in progress, two newly published studies with divergent conclusions seek to explain why.
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"Monogamy is a problem," said University of Cambridge zoologist Dieter Lukas at a teleconference yesterday afternoon. "Why should the male keep to one female?"
It doesn't take a degree in biology to understand what Lukas is getting at. Females, with their extended gestational periods and finite physical resources, can only give birth to healthy offspring every so often. No such biological limitation exists for males, who can, in theory, seek out other females and sow their seed as far and wide as they please.
And yet, social monogamy, where a male and female pair off to mate and rear offspring, is regularly observed throughout the animal kingdom. It's seen more often in birds than in primates, and more often in primates than most other mammals, but it's popped up frequently enough to attract the attention and curiosity of evolutionary biologists. In an investigation recounted in the latest issue of Science, Lukas and study co-author Tim Clutton-Brock summarize the two leading hypotheses about the evolution of social monogamy in mammals:
One suggests that it is a consequence of selection for some form of paternal care, such as contributions to carrying or provisioning young or their protection from infanticide by competing males. Alternatively, social monogamy may represent a mate guarding strategy and may have evolved where males were unable to defend access to more than one female
Many studies have found evidence of correlation between monogamy, mate-guarding and paternal care, but the current state of affairs is pretty much your archetypal chicken-and-egg problem. Which drove the evolution of monogamy: mate-guarding, or paternal care?
Lukas and Clutton-Brock, for their part, come down on the side of the mate-guarding hypothesis. After tracing the evolution of social- and mating-strategies in 2,545 non-human mammalian species, the pair concludes that mammals transitioned from solitary lifestyles to social monogamy 61 times in the 170-million-years of evolution separating them from a shared ancestor. When the researchers searched for a common denominator among these mammals, they concluded that in species where breeding females are intolerant of one another, they tend to give each other such a wide berth that males become unable to prevent other males from breeding with them. If a male wants to ensure a female gives birth to its offspring, it must stand by her and her alone, or else risk her being impregnated by another male while he's away. As for the offspring-protection hypothesis, the researchers conclude that "paternal care is a consequence rather than a cause of social monogamy."
Which is interesting. Because in a different study, published yesterday in Proceedings of the National Academy of Sciences, researchers led by University College London anthropologist Christopher Opie conclude something quite different.
Like Lukas, Opie searched for evidence of both the mate-guarding and parental-care hypotheses, albeit with a focus on primates, specifically. Opie's team looked at 230 species in total, comparing data on things like mating behavior, paternal contribution to the care of offspring, and rates of infanticide by competing males and running it through a computer simulation that teases out correlations between these traits and the evolution of monogamy. After running the program through millions of iterations, the researchers had found correlative evidence in support of both hypotheses – but of all the behaviors, infanticide by rival males was the only one to reliably precede a shift to monogamous mating, suggesting a causal link. The findings suggest that the threat of male infanticide is what drove the evolution of monogamy in primates, and that monogamy, in turn, facilitated the emergence of mate-guarding and paternal care.
It bears mentioning that Opie and his colleagues examined the tendency for males to protect offspring from competing males separately from their contribution to infant care. Lukas and Clutton-Brock, in contrast, lumped the two together, defining paternal care as "contributions to carrying or provisioning young or their protection from infanticide by competing males."
In other words: when Opie concludes that monogamy may have facilitated the emergence of paternal care, he's only in half-agreement with Lukas's conclusion, quoted above, that "paternal care is a consequence rather than a cause of social monogamy." For Opie, protection against infanticide begets monogamy, and monogamy begets paternal contribution as well as mate-guarding. For Lukas, mate guarding begets monogamy and monogamy begets a host of behaviors lumped under the umbrella of "paternal care," including both male contribution and male infanticide.
Why the discrepancy? One explanation could be that monogamy in primates simply tends to evolve via a different route than it does in most other mammals (remember Opie's team looked at primates, exclusively). Close to a third of primate species are socially monogamous, compared to about 5% of mammals as a whole; it's not unreasonable to assume they've developed monogamous social systems by alternate evolutionary routes. That being said, Lukas and Cluton-Brock claim that when they repeated their analysis looking only at primates, they found no effect of male care or infanticide on the evolution of monogamy.
Therefore, Clutton Brock says another possible explanation could boil down to "some difference in classification in the way the species are classified between [Opie's team] and ourselves, but as yet we’ve not been able to see the classification of particular species by Opie... so we don’t know whether that’s the answer."
Another explanation could be that the two teams relied on different methods of analysis. Lukas and Clutton-Brock used on a novel technique that allows for the evolutionary comparison of species based on genetic data, whereas Opie and his colleagues depended primarily on Bayesian probability, a longstanding method of rigorous statistical analysis.
The debate over how monogamy evolved, in other words, is far from settled. If anything, these two studies – both comprehensive in scope, and creative in their approach – have only served to reopen a longstanding evolutionary discussion. What we have here is an excellent example of science in progress. We look forward to seeing where the discourse moves from here.