1. The reasons for monogamous relationships in mammals involve how much care both parents must put into raising their offspring so it (or they) will be grow to be biologically fit, that is, able to mate and pass on its DNA successfully. Since female mammals have a limited number of gametes, they will dedicate a significant amount of energy to bolstering their young’s fitness before and after their offspring are born. The care of the female is ensured because she can be certain that her offspring belongs to her. Monogamy allows the male to also know that the offspring is his, ensuring he will be as invested in its fitness because his offspring can pass on his genes.
When the sole care of the mother is not enough to ensure the future fitness of her offspring, the father must also contribute energy to performing acts of parental care such as feeding or supervising the young in order to maximize its survival and future ability to reproduce successfully. With the care of both parents, there is a higher likelihood of their young being fit and able to pass on both the mother and father’s genetic material. In mammals, the fertilization of a female’s egg cell and a male’s sperm and the crossing over that occurs during meiosis, ensures that both parents’ DNA is present in the offspring. Therefore both mother and father have reason to contribute energy to make sure their offspring will be biologically fit.
2. The Evolution of Social Monogamy in Mammals, by D. Lukas and T. H. Clutton-Brock, supports several suggested reasons among biologists for why monogamy occurs in some mammal species. Lukas and Clutton-Brock state that these reasons exist due to an “ancestral condition” exhibiting a “social system” outlined in the article. One of the main elements of that social system that the authors mention as possibly giving rise to monogamous societies amongst mammals is the isolation females who were likely “intolerant” of one another during breeding and “lived in individual home ranges.” Lukas and Clutton-Brock support this factor by stating that that social monogamy “may represent a mate guarding strategy and may have evolved where males were unable to defend access to more than one female” which would cause a change in mating strategy as well as in this case, a transition to monogamy.
Another mentioned factor causing the evolution of monogamy in mammal populations is population density, which is closely related to female isolation. Figure 1 suggests that monogamous mating occurs more often where there is a low density of the species exhibiting monogamous pairing. The caption of Figure 1 states that “At the highest population densities, there is only a 6% probability that a species will be socially monogamous”, supporting the graph’s indication that a low population density is conducive to monogamous pairing to some extent.
Lukas and Hutton-Brock also identified the presence of male infanticide in mammal societies as possible precursors to the evolution of social monogamy but state “the available evidence suggests that male infanticide is unlikely to be the principal mechanism for the evolution of social monogamy in mammals.”
Primates are specifically given as a supporting example when Lukas and Clutton-Brock mention a need for high quality nutrition as a contributing factor to the evolution of social monogamy among mammals. They state that the “range overlap in females and social monogamy may be a consequence of a reliance on resources of high nutritional quality but low abundance. They go on to say that 91% of socially monogamous primates eat such fruit while only “28% of solitary primate species” have the “high quality-low density” diets of the monogamous species.
In the article, Male Infanticide leads to social monogamy in primates, by Opie et al., the authors make the argument that male infanticide is the strongest factor that caused many species of primate to transition to monogamous mating, negating the several alternative factors examined by Lukas and Clutton-Brock as having less influence in this transition. Opie et al. state that “The ancestral state reconstructions (of the monogamous primates) and model rates suggest that only male infanticide precedes the initial shift to social monogamy” further stating “there is little support for a transition from polygamy to monogamy with low infanticide.” Opie et al. acknowledge factors previously mentioned by other biologists in facilitating this transition, stating “we find decisive support for correlated evolution between social monogamy and paternal care, female ranging patterns and male infanticide” which support the “intuition” of Opie et al. “that these traits represent a suite of social behaviors linked to social monogamy” but that this correlation “does not identify any direction of causality” according to their data. Opie et al. clearly view infanticide as the leading cause of a transition to monogamy amongst primate species.
3. I consider the argument put forth by Male Infanticide leads to social monogamy in primates, by Opie et al. to be more convincing than The Evolution of Social Monogamy in Mammals by Lukas and Clutton-Brock. The authors focus on primates in order to bolster one of their main points, that “These phylogenetic analyses support a key role for infanticide in the social evolution of primates, and potentially humans”, or that male infanticide is an extremely significant factor in the transition to monogamy in primate populations relative to the three other hypotheses debated by biologists, as listed by Opie et al.: “paternal care, female ranging patterns, and male infanticide.” The authors go on to explain their findings when examining each of these factors and clearly state their conclusion that “Paternal care only evolves after a switch to social monogamy…[suggesting] biparental care was not a factor driving the shift to monogamy.” This clear indication supports an equally clear overall purpose of supporting the authors’ hypothesis that male infanticide in primates is the most influential factor in the transition to monogamy. What made the Opie et al. article more convincing than that of Lukas and Clutton-Brock, is that the former acknowledged possible counter arguments.
The experiments of Opie et al. utilized a large amount of data to make their conclusions, leading me to believe their conclusions were supported by actual happenings and not coincidences. In the “Methods” section of the article, the authors indicated that they use the distribution of 10,000 phylogenies for 230 primate species and a large posterior sample, “while accounting for phylogenetic uncertainty.”
While the Lukas and Clutton-Brock article was not unconvincing, it is less convincing than the Opie et al. work because it covered a much wider ground than the other article. It spoke generally about the causes of the transition to monogamy in some mammals. Lukas and Clutton-Brock state, “Social monogamy has evolved in nonhuman mammals where where breeding females are intolerant of each other….” As the Opie et al. article points out, the rates of monogamy vary amongst various orders of mammals by stating “Social monogamy is more common in primates than in other mammalian orders.” Therefore, the transition to monogamy may vary from order to order, species to species etc. This makes me believe that the Lukas and Clutton-Brock article may be too general to be as convincing as Opie et al.s’ article which has evidence for their claims regarding specific orders and species.
4. These articles have somewhat changed my perceptions of monogamy in primates. I knew a little of the mating strategies of primates prior to reading these articles. I now better understand some of the innate behaviors that drive the fascinating behavior of primates and their possible evolutionary causes.
After reading these articles, I was struck by the complexity of a population. Even though I learned about behavioral ecology and the various rules scientists have applied to animal populations, reading them unfold in legitimate scholarly articles was very interesting. I was surprised by the influence behavior could have on the evolution of a species, namely that of male infanticide. I was also surprised (or rather fascinated to see) the influence chemical changes had in this behavior and evolution, like in the Opie et al. article where it states that “females are expected to avoid suckling two infants of different ages simultaneously by delaying the return to oestrus after partition. When oestrus is delayed, it can pay a male, who is not the father, to kill and unweaned infant so that the female can return to oestrus sooner.” Not only is the brutality of the animal kingdom striking, but also the complexity of the interactions that occur within.