Based on what we’ve learned in class thus far it appears that there are two major reasons for monogamy in certain mammalian species. The first major reason, or the benefit to females, is that those mammalian females that exhibit the behavior only have a limited number of reproductive opportunities, small and a long gestational period, meaning that they must invest heavily in the care and protection of their offspring once they are born, and having another individual of the species present to aid them in protection and food food gathering, etc. greatly helps insure the survival and viability of the offspring. The second reason, and the benefit to males, is that by providing parental care to their offspring with their mate they can ensure the propagation of their genes better than if they were to simply mate with many females. This is because the investment and care required of many mammalian offspring make the quality over quantity approach more viable to the males in a species, because without two parents the viability of the offspring would be drastically lowered.
However, new research points to different reasons for monogamy in mammals, particularly primates, with two studies arguing for different causes of monogamy. In one study, by D. Lukas et al, it was determined that the cause of monogamy in primates was due to solitary females competing for resources/males, causing the females to spread away from each other leading to a low female density. This low female density over a large area made is so that males could not access more than one female, causing monogamy to be a more viable strategy for males to ensure the propagation of their genes. The article cites numerous studies on primate social structure, with culminating data showing that: “At the highest population densities, there is only a 6% probability that a species will be socially monogamous, whereas the probability rises to 44% at the lowest population densities. (Lukas et al.)”, so the trends in density and monogamy do support the claims laid down by the authors. The second study, done by Opie et al. claims that male infanticide is the cause of social monogamy in primates. In polygamous primate societies it is common for a male to kill the offspring of a female so that they may mate with the female and have her raise their offspring, this happens because females delay ovulation when having given birth so as to avoid having to raise too many children, so killing a child makes the female able to bear children sooner for the new male. The article argues that infanticide is the hypothesis for monogamy that has a causal relationship with a shift from polygamy to monogamy: “male infancicide precedes the switch to social monogamy… is a very stable one (Opie et al.)”, stating that any other hypotheses, including that of Lukas et al. have only a causal or resultant relationship with social monogamy at most.
The Lukas et al. study seems to be a more scientifically credible study than Opie et al. There are a number of reasons for this, and one of the most convincing is either studies’ refutation of the others’ hypothesis. In Lukas et al. when refuting the hypothesis supported by Opie et al. the article mentions that the conditions for infanticide (duration of lactation>gestation) have little-no association with social monogamy “This is the case in few socially monogamous species… that exceed gestation (lrt P>0.40, table S2).(Lukas et al.)” meaning that there is a very low chance of causation between infanticide and social monogamy, because the conditions required for infanticide are more prevalent in species that are polygamous, meaning that the traits did likely not evolve together, the article also uses a lot of data to support these arguments. The Opie et al. article, while refuting the hypothesis of discrete female ranges had only this to say: “Within a few lineages, discreet female ranges arose independently… not a causal factor in its appearance (Fig. 1B).(Opie et al.)” this entire argument has no data being cited, and the only support for it is the reference to figure 1B, which too, has no citations or sources for it’s data. The Opie et al. article also seems to have lower rates of data analysis than the Lukas et al. article, despite being more focused in it’s view (primates vs mammals, respectively) the Opie et al. article only really has data analysis, and not just reference at a few points, whereas Lukas et al. commonly both refers to outside sources, and provides data analysis, not only on the subject of it’s hypothesis (Lukas et al., low female density hypothesis) “Despite the association between social monogamy and low population density… in species where females are solitary. (Lukas et al.)”, but also when dealing with competing theories, where data analysis and reference is found when discussing male infanticide (see above citation), and Male care “Females in socially monogamous species with biparental care… without median=45 months; F=2.10,P=0.17,phy: lambda=0.53, t=-1.1, P=0.31).(Lukas et al.)”. In terms of methodology, the Opie et al. article has a much more detailed and reputable analysis of it’s methodology “Analyses were conducted on a Bayesian posterior… six known fossil calibration points.(Opie et al.)” than that of Lukas et al., which is sparsely even mentioned and far less detailed. However, though the Opie et al. article is far more robust in it’s methodology, it ends up not counting for much because very little of that methodology and data is cited/used in arguments for infanticide causing social monogamy as well as against competing hypotheses.
The articles did change my understanding of monogamy in primates, but not to a huge degree. I already understood that monogamy, even in primates, was a minority affair, and the main thing that these articles taught me about the subject is that there are a few competing hypotheses. It was interesting to learn the various hypotheses, I already knew the male care hypothesis to an extent from our class discussion, but both the infanticide and female density hypotheses were new and interesting to me. The female density hypothesis was interesting, and I did find it more credible, but the infanticide hypothesis really piqued my interest. The hypothesis was interesting to me not only because I had not considered infanticide as something that might drive a shift towards monogamy in a population, but also because of the conditions surrounding primate infanticide in the first place. I had no idea that female primates could delay their ovulation after birth, and that killing their infants can stop that delay, that was honestly the most fascinating thing I’d learned from the article, as I had no idea that that was possible for primates, and the existence of that possibility does explain a lot of things, such as why mammals like primates aren’t perpetually pregnant. It’s definitely something I plan to ask Ms. Ortiz about, because Google searches did not turn up many results.