Why Only One?

Monogamy, or the practice of having one mate at a time, has been a subject of interest likely for all of human history. The question of when and why monogamy evolved are debated, but not as much so as whether or not human beings are truly meant to be monogamous. In this series of posts, the evolution and neuroendocrinology of non-human animals and humans will be discussed in order to shed light on some of the questions surrounding monogamy, and hopefully give any individual reading enough knowledge so that they could form their own educated opinion regarding human monogamy subsequent to reading.

Firstly, it is important to always remember that humans evolved from a common ancestor of chimpanzees and bonobos, so at least part of our instinctual behavior is derived from them. Secondly, it is crucial to understand that it is extremely unlikely that monogamy would exist in any species that it didn’t benefit. Therefore, monogamy likely exists only in certain species because it either helps the individual of said species pass on their genes, or helps their offspring survive. Thirdly, nature does not have the morality that our societies possess. In some monogamous animals, there can be “enforced monogamy” in which the males guard their chosen mate and often become violent if their monogamous bond is violated (Reichard & Boesch, 2003). Males often injure each other over access to a mate, and the female can even be subject to collateral damage during all the hullabaloo. There is also a high rate of infanticide in many animals. There are exceptions, but normally this is where a nearby male will kill the offspring of another male, such as in lions. This kind of brutal behavior was simply selected for throughout evolution because it often ensured that the dominant male would pass on their genes before they got old and bested by a new alpha.

With all that being said, it seems that the monogamy originated either from a high rate of mate-guarding under certain conditions, a need for biparental care under certain conditions, or perhaps a combination of both.

In many animals, males are said to be the less picky ones when it comes to mating, and females are said to be the more picky ones. This is because for many animals, parental care and the cost of caring for the eggs or the internally developing offspring are the responsibilities of the mother. However, in a case where access to a fertile female is more rare, monogamy actually might increase fitness (which can be measured by number of surviving offspring) compared to another mating strategy (Schacht & Bell, 2016). Ergo, in many thorough examinations of the evolution of monogamy, authors place the high level of mate guarding by males prior to high levels of biparental care on the timeline (Burley & Johnson, 2002; Lukas & Clutton-Brock, 2013; Schacht & Bell, 2016). In other words, it doesn’t seem like the necessity for two parents was so great that it drove the evolution of monogamy. Biparental care however, seems to be more of an effect of monogamy rather than a cause (at least in mammals) (Lukas & Clutton-Brock, 2013).

In summary, under certain conditions, fertile females become rare, so males increase mate-guarding behavior. In these contexts where there is a high prevalence of mate-guarding behavior showed by males, the males are more certain of their paternity status when the offspring are developing / born. Since the goal of the individual is to pass on their genes, and resources are very limited in nature, it is unlikely for any given male to show parental care to offspring that aren’t his (or even mercy in many cases – hence the infanticide). Since the females are rare, and the males will guard the ones that they choose, the females can selectively choose mates that will display higher levels of parental care, thus practically birthing the concept of the nuclear family.


Burley, N. T., & Johnson, K. (2002). The evolution of avian parental care. Philosophical Transactions of the Royal Society B: Biological Sciences, 357(1419), 241–250. https://doi.org/10.1098/rstb.2001.0923

Lukas, D., & Clutton-Brock, T. H. (2013). The Evolution of Social Monogamy in Mammals. Science, 341(6145), 526–530. https://doi.org/10.1126/science.1238677

Reichard, U. H., & Boesch, C. (2003). Monogamy: Mating Strategies and Partnerships in Birds, Humans and Other Mammals. Cambridge University Press.

Schacht, R., & Bell, A. V. (2016). The evolution of monogamy in response to partner scarcity. Scientific Reports, 6, 32472. https://doi.org/10.1038/srep32472 3

One thought on “Why Only One?

  1. Thanks for your post and the introduction to your topic, Taylor! When you cited some authors as holding the opinion that “Biparental care however, seems to be more of an effect of monogamy rather than a cause”, I wondered what kind of experimental evidence (if any) you would want to see to support this idea!

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