Monogamous relationships are a very rare phenomenon within animal species with only 3% of mammals expressing such relationships. This currently is believed to be because certain species are altricial and therefore require constant nourishment to survive infancy. With this demand for protection and resources, a monogamous relationship in which two parental figures has developed to ensure the propagation of their genes. It is in the best interest of both the male and female to devote their resources in nurturing the child.
In Male Infanticide Leads to Social Monogamy in Primates by Opie et al., it is argued that male infanticide was the root cause of social monogamy in primates. Male infanticide is when a male kills off the offspring of other mates in order to hasten the females mating availability. According to the article, it is the only correlated factor of monogamous relationships that precede its evolution. Both parental care and female ranges are modeled to have developed after the evolution of monogamy as supported by Figure 1. There is also evidence that “social monogamy with high infanticide is an unstable state, whereas social monogamy with low infanticide is a very stable one”. It is therefore suggested that social monogamy developed to counter high infanticide rates and foster a stable environment. However, social monogamy is limited by ecological pressures such as predation pressure and resource distribution impact population density and social grouping and therefore hinder social monogamy from starting.
According to D.Lukas and T.H. Clutton-Brock in The Evolution of Social Monogamy in Mammals, the primary cause of the evolution of social monogamy was the distribution of females, resulting in the inability of males to maintain access to multiple mates. The main support of this hypothesis is the relationship between socially monogamous species and their socially solitude ancestry. Data suggests that socially monogamous species have been derived from ancestry where the females were socially solitary. This is supported by Figure 1 where the probability of a species being socially monogamous rises to 44% at the lowest population densities. Female intolerance to each other and the competition of resources, as seen in the example of high fruit concentration required socially monogamous primates. Male infanticide is regarded as unlikely to be the cause of monogamous relationships in mammals because of the lack of socially monogamous species that expresses lactation period that exceeds that of the gestation period, a typical indicator of where male infanticide is found. The range of females may have forced males to guard individual females as their best method of breeding.
Male Infanticide Leads to Social Monogamy in Primates creates a more convincing scientific argument because of its methodology and conclusions. In the article by Lukas and Clutton-Brock, male infanticide is deemed unlikely because of the lack of species with lactation times longer than gestation time, as is commonly found in species where male infanticide is found. However, it is mentioned by Opie et al. that the shorter lactation times may actually reduce infanticide risk as a mechanism of why social monogamy evolved. Opie et al. also demonstrate more care in using data as they eliminate bias by including species with at least 20 publications, classifying rates of infanticide as to allow for non-documented species, and the use of an independent infanticide risk index. Lukas and Clutton-Brock fail to provide or mention this depth of methodology. However, both articles both do mention how low population density could permit monogamy.
Monogamy in primates have a much more complicated and speculative evolutionary history than I believed. I had not considered male infanticide as the primary reason for monogamy and had believed that limited resources and the taxation of alrticial offspring would force parental care to develop monogamous relationships. What is most surprising to me is the consensus that parental care by is the consequence of monogamous relationships and not the cause for it. The rarity of monogamous relationships also stand out as they are not even found within all primate species but only certain ones where ecological conditions permitted.