The Evolution of Monogamy

Monogamous and polygamous mating behaviors are the result of sexual selection, a form of natural selection. Some organisms have evolved over time to exhibit monogamous behavior because a longer relationship between one female and one male leads to successful reproduction and long-term survival of young more often than other mating behaviors. Species that commit to monogamous relationships can more effectively raise young after birth. If the offspring of the two organisms needs extensive care or food and one parent (usually the mother) is the only one providing care, the offspring may die. Penguins, for example, form monogamous relationships and share in parental care when breeding. Without both parents, it is unlikely that their offspring would be able to hatch or survive to maturity and reproduce. In polygamous relationships, the young may not be as reliant on parental care. If this is the case, it is a better investment of time and energy to mate with multiple organisms of the opposite sex (usually one male with multiple females). It is a matter of investing more or less time in caring for young or mating more often. Males in monogamous relationships must also invest more time in ensuring that the young produced it their own. They often guard females in an attempt to be the only male with which the female mates, and they may also remove sperm from the female’s reproductive tract or increase the likelihood of their fertilizing the egg by introducing large amounts of their own sperm. All of these factors may contribute to a reason for engaging in monogamous behavior.

In the articles by Lukas and Clutton-Brock and Opie et all, there is a strong emphasis on ancestral behavior. Both articles detail phylogenetic comparative methods used to determine the origin of social monogamy and the evolution of traits associated with this behavior. They both introduce three main hypothesis for a transition to social monogamy: high rates of infanticide, need for paternal care, and females occupying discrete ranges making protection of multiple females difficult for males. Lukas and Clutton-Brock detail a test of their own conducted using more than 2500 mammals. After classifying the social systems of all these organisms (into solitary, socially monogamous, or group living), and tracking transitions to social monogamy through a “mammalian super tree,” they came to the conclusion that parent care and protection against male infanticide both developed independently of or proceeding the evolution of monogamous mating behavior. The authors indicated that, “…detailed field studies have found no evidence of any form of male contribution to care in 94 of 229 (41%) socially monogamous species.” They supported their argument against protection from infanticide by stating that, “Male infanticide is typically found in species where the duration of lactation exceeds the duration of gestation: This is the case in few socially monogamous species (20 of 75 species, 27%), compared with females who are solitary…” Finally, the authors came to the conclusion that monogamous relationships evolve where competition for nutritious resources is low, making population-density of females low as well. Lukas and Clutton-Brock state that because many monogamous species may have evolved from solitary ones where population density was higher and there was more competition among females, ranges for female living ceased to overlap to the same extent and was a cause of monogamous relationships. In order to protect their mates and ensure offspring carrying their genes, males had to protect their mates but could not protect multiple mates at the same time.

In the article by Opie et all, the authors use a similar approach but their results differ. They compared models of mating systems and various traits to determine whether or not traits evolved independently. They state that “biparental care is not ubiquitous in socially monogamous mammals,” and that discrete female ranges followed the transition to social monogamy. They reached the conclusion that male infanticide is an explanation for the appearance of monogamy among species. Opie et all argue that “Of the traits [they] tested, high male infanticide alone consistently preceded the appearance of social monogamy across primates.” The authors then ask why so few species exhibit this behavior. They respond saying that although there are very high rates of infanticide in polygamous species like gorillas and langurs, there is strong selective pressure against a transition into a monogamous lifestyle. Sometimes these environmental challenges are a reason not to transition, but good environment conditions are not a cause to transition either. Of the two sources, Opie et all seem to make a more convincing scientific argument. Their use of many clearly stated methods and data from other experiments is more significant than in the article by Lukas and Clutton-Brock. In Opie et all‘s methodology section, they detail where their data is coming from and provide a brief synopsis of each piece, whereas the other article has only a reference page and mention of data collections that are not further elaborated upon but still used to support evidence later on in the text. They both reference a “Bayesian” approach, but only one elaborates on this and mentions a source. In the article by Lukas and Clutton-Brock, there is also mention of “field studies” used in many key sections of their argument with no source or details about said study.

When studying evolution and ecology, it is challenging to determine whether or not one trait preceded another evolutionarily. It was surprising to learn that Lukas and Clutton-Brock suggested that paternal care and infanticide protection my have evolved independently or after the development of monogamous behavior. Opie et all also suggested that paternal care and discrete female range succeeded the development of monogamous mating behavior. It is also interesting to see how these scientists went about determining the cause of monogamy across species. Comparing evolutionary patterns would not have been my first thought when evaluating the causes. It seems relatively likely that biparental care evolved after monogamy, but I am still unsure about whether protection from infanticide or discrete female range is the cause. There is a possibility that neither of these is the reason, but it does seem most likely that one of them is primarily responsible for the transition.

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