Monogamy in Mammals

In two recently published articles concerning monogamy in certain mammal species, The Evolution of Social Monogamy in Mammals by D. Lukas and T. H. Clutton-Brock and Male Infanticide Leads To Social Monogamy in Primates by Christopher Opie et al., the researchers introduce three proposed factors in the evolution of social monogamy in primates and proceed to argue that one is the driving force behind this behavior, while the other two are results that developed because of certain advantages provided by monogamous relationships. Lukas and Clutton-Brock argue in favor of paternal involvement resulting in reduced male infanticide and therefore increased fitness as the cause of social monogamy in species that exhibit it; Opie et al., on the other hand, argue that social monogamy is the result of females of given species exhibiting solitary behavior to such an extreme that it is no longer advantageous or even possible for a male to spend his time roaming between different females’ territories.

An interesting factor when considering these articles is that both sets of authors acknowledge that the other’s scenario does in fact correlate with socially monogamous behavior, but each proposes that it is their scenario that is ultimately causal of this behavior, and not just related to it. Because of this, the article that, in my opinion, makes the most convincing argument is the one that is best able to address the arguments in favor of opposing positions. Both articles provide a large quantity of statistical analysis (that I didn’t always understand but which nevertheless seemed quite impressive); both articles provide a clear explanation of their methods and analyses; but, in my opinion, Lukas and Clutton-Brock are much more successful in their ability to take the data a step further and discuss it in relation to other theories regarding the evolution of social monogamy in mammals.

To me, Opie et al.’s article read like something along the lines of, “These are three reasons that have been proposed as to why some mammal species are socially monogamous, and you should believe that male infanticide is the real one because of all of our fancy log likelihoods and q- and z-values.” Lukas and Clutton-Brock, however, provide their data and then explicitly address why they believe that neither paternal care nor male infanticide provide as compelling arguments for the evolution of social monogamy in mammal species. I believe that, without discounting other theories for the cause of social monogamy in mammals, Opie et al.’s findings could just as well be a heavy correlation for a trend that occurs around the same time that social monogamy develops for the same reasons that it does. Essentially, Opie et al. convinced me that their theory was good; Lukas and Clutton-Brock convinced me that their theory was the best.

For example, Lukas and Clutton-Brock address the male infanticide proposal by saying that it is typically found in “few socially monogamous species (20 of 75 species, 27%) compared with species where females are solitary” and that “this difference does not appear to be a consequence of a direct association between social monogamy and male infanticide, because an analysis of [various statistical models] suggest an independent evolution of the two traits”. Opie et al., on the other hand, do little to explicitly address other possible explanations for social monogamy in mammals, only pointing out that “the demonstration of correlation between traits does not identify any direction of causality” and suggesting that other potential causes are distinguished from their own because they “make monogamy more profitable” instead of actually causing it, which is not a very strong argument and could just as easily be used in favor of the very suggestions that they are trying to contradict. They even go so far as to admit that their own explanation “raises the question of why more primate species are not monogamous”, an issue that does not arise with Lukas and Clutton-Brock’s proposal.

Honestly, the thing that was most surprising to me was a relatively uncontroversial piece of information – the fact that very few mammal species (9% of the species studied by Lukas and Clutton-Brock) exhibit monogamous behavior. Upon reflection, I can easily think of a variety of examples of non-monogamous mammal species that I have known are non-monogamous for as long as I can remember (horses, elephants, lions, wolves….the list goes on and on) but I don’t think I was really consciously aware of the fact that most mammals are non-monogamous until I read these articles and was forced to think about it. This is probably because the significance of monogamous relationships is impressed on human society, but it was weird to find myself making that assumption without even realizing it – and also making that assumption while being aware of a lot of counter-examples and still not really thinking about my preconceptions.

I also found the whole concept of male infanticide really interesting, as I had never heard of it before. It had never occurred to me that for males without a mate, killing a female’s offspring in order to induce oestrus and allow her to mate with them would be advantageous. It seems counterintuitive for the survival of a species as a whole, though I suppose it does make sense if only passing on one’s own genes is taken into consideration.

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