From class discussions and textbook readings, it is currently believed that monogamous relationships in mammals exist for the purpose of maximizing the offspring’s survival by pooling together the resources of the mother and father. Opie, Atkinson, Dunbar, and Shultz argue that the “evolution of both female ranges and paternal care followed the shift to social monogamy rather than preceded it” and that it is much more likely that high male infanticide dictates the emergence of social monogamy by encouraging males to stay and protect their offspring from infanticidal males in their population. Lukas and Clutton-Brock take a completely different stance on the emergence of social monogamy and argue that “male care is probably a consequence rather than a cause of the evolution of social monogamy” and that arguments that due site protection of offspring against infantifical males by fathers ignore the fact that male infanticide is not very present in socially monogamous species. In fact, only 20 of 75 socially monogamous species have lactation periods that are longer than gestation periods (Lukas, Clutton-Brock 527), giving infanticidal males a larger window of opportunity to perform infanticide.
Lukas and Clutton-Brock provide a much more thorough and detailed road map for the evolutionary emergence of social monogamy than do Opie et al. In table 1 of “Male infanticide leads to social monogamy in primates”, discrete female ranges have a high impact on the emergence of social monogamy in species but the impact or value of these traits is discussed little in the paper. Solitary and limited females create more competition amongst males for a females making it much more advantageous for a male to breed with one female for every breeding season in order to ensure that they will be able to perpetuate their genes. Opie et al do not derive their data from species observation while Lukas and Clutton-Brock derived their conclusion from observations on 2545 mammalian species. The volume of data considered lends more validity to the arguments of the later and sheds skepticism on the purely statistical models of the former. Lukas and Clutton-Brock incorporate the important characteristic of population density when analyzing their data. Considering that social monogamy is based on the likelihood of individuals reacting with each other within a given space, Opie et al’s conclusion that the probability of social monogamy decreases with higher population density is valid. As individuals have a greater probability of interacting with members of the opposite sex from their species, they are less likely to settle on one individual and will attempt to breed with as many individuals as possible in order to maximize the offspring they product. In regions with low population density, individual species would be happy to find whatever mates are available in the region and would be less hesitant to move on after one breeding season.
These articles have thoroughly confused and enlightened me about the evolution of monogamy in primates. Nonetheless, the article did introduce me to concepts not discussed in our discussion or textbook such a male infanticide. I was aware of the action in some species but not of its importance in evolutionary history. Most surprising to me was the correlation between lactation period and infanticide rates, a longer lactation period not only allows a mother more time to nurse and encourage a greater fitness for her offspring, it also allows an infanticidal male to completely eliminate any chance of survival for the offspring. In this manner, a longer lactation period is a double edged sword that can endanger an vulnerable infant or greatly increase the fitness of a protected infant.