In class, we understood the cause of monogamous relationships in mammals to be based on the viability of the offspring, especially the needs of the young and how certainty the father is that the offspring are his own. The needs of the young influence whether biparental care is needed for the offspring to survive. For example, birds tend to be monogamous because many species need two parents to supply enough food and care. If the offspring do not survive, the genes of the parents do not get passed on to future generations. From an evolutionary point of view, it is beneficial for the parents to raise their young so their genes can perpetuate. However, if the young can survive on their own without aid or simply with the care from their mother, monogamous relationships will not be selected for in a species. A father, who cannot sure of the parentage of the offspring, will produce more viable offspring to future generations by having more mates if the young will survive without him. Remaining in a monogamous relationship can cause the male to waste time, resources, and other mating opportunities by raising children that are possibly not his own. However, if monogamy ensures
These two articles attempted to find the key factor in determining the reason for social monogamy in mammals, although they came to different conclusions. In the first article written by Christopher Opie and his team, they found that monogamy resulted from a reduction in the rate of male infanticide when the father is present. D. Lukas and T. H. Clutton believe that, based on their results, social monogamy arose from ancestors where the females live solitary and farther apart from other females, preventing males from mating with other females with success. Lukas and Clutton disprove Opie’s theory by showing that paternal care developed after social monogamy, not as a “precondition.” Although there is a correlation, it is not the root cause.
Opie and his team used likelihood-based phylogenetic comparative methods in a Bayesian framework to look at three supported hypotheses for the start of social monogamy: paternal care, female ranging patterns, and male infanticide. Then, they used correlated evolution of social monogamy and each of the factors and compared them, and decided whether the traits came before or after social monogamy. Their ancestral trait constructions and model rates showed that paternal care evolves after monogamy, so it cannot be the cause of it. Male infanticide was the only trait that could explain the root cause of social monogamy.
When two parents are able to provide the resources necessary for lactation, the length of lactation will shorten. When the lactation period for females is longer than the development of a fetus, it is difficult for a female to feed two infants of different ages. Because the female will avoid mating during this time, it can be beneficial for another male to kill the infant so the mother can begin reproducing again. If the couple is monogamous, the father will care for the offspring, preventing male infanticide. I find this chain of reactions to be very interesting and very plausible.
The research by D. Lukas and T. H. Clutton-Block seemed to disprove Opie’s theory as the main driving force because male infanticide is only found in species where the length of gestation is shorter than lactation, which only occurs in 27% of socially monogamous species. However, 44% of species that are socially monogamous have females that are solitary. Additionally, their data found using an analysis of phylogenetic independent contrasts seem to show that the social monogamy and male infanticide came about independently. Furthermore, they found a pattern for the conditions needed for multiple mating types.
Lukas and Clutton-Block found that socially monogamous mammals tend to have a female that was solitary, and the mammals in general live at lower densities. This trait occurs when the females live solitary and the males roam. Due to female competition and mutual female intolerance, it is beneficial for males to remain with one female mate.
I think that the article written by Lukas and Clutton-Block is more convincing scientifically, although I believe both are at least partially correct. Their team had a larger sample size than Opie and was able to disprove the theory of male infanticide being the driving force of social monogamy. Lukas and Block also examined more factors for their research, such as sexual dimorphism between females that live in solitary and females that live in socially monogamous societies, diet patterns, density, and body mass. They also have more numerical data and figures to support their data. While I believe that Opie’s group has found a strong correlation, I do not believe that they have enough evidence to encompass all the factors that may cause social monogamy to be selected for in animals.