This entry was written by Dean Rub as part of a project done in BIAN 6133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
Among all the taxa, the primate linage has demonstrated a diversity of mating systems that’s have shaped the evolution of many primate families amongst the linage. This diversity in mating systems touched upon in this entry include; monogamous, polygamous, polygyny, promiscuous mating and same sex mating. Moreover, Charles Darwin never lived out to see how his ideology of sexual selection grow and diversify, to the point of understanding on how mating systems have evolved over time. In context of this paper will be applying the sexual selection theory and comparing different mating systems across the primate linage, to map out how primate mating systems have evolved and influenced many human mating systems, that are expressed today across human civilization.
In this entry, I will provide a brief overview of these studies and their results, whilst comparing similarities amongst primate species to draw possible explanations on how primate mating systems evolved and influenced human mating systems today.
Main Text
Primate Mating Systems
When looking at the evolution of sexual selection in humans, a good approach is to look deeper into our own linage. Studying primates enables humans to better understand how our ancestors may have utilized certain strategies, that are used today within some of our closest relatives. Muller et al (2007) reviewed male coercion and costs for female chimpanzees (Pan troglodytes). They concluded that females that had shown physical symptoms of sexual swellings, correlated with males being more aggressive towards them, the higher frequency of aggression, the higher reproductive success for the male. They also noted within the review that male coercion would be seen more relevant in low ranking males, who would apply this strategy to overcome female resistance. Moreover, high ranking males tend to use aggression to constrain promiscuity. This is especially seen in preovulatory periods (Muller et al, 2007). Aggressive coercive behaviors are still expressed today within humans but tend to be a lot less frequent than elaborated above, this enabling the notion that these aggressive behaviors have been thinned out through the evolution of humanity.
Bonobo chimpanzees (Pan paniscus) are amongst the most sexually active polygamous nonhuman primate species. As humans have an array of mating systems, this enables the notion that bonobos are great comparison for studying the evolution of sexual selection amongst the primate linage. Research from Hare et al (2012) investigated the evolution of bonobo psychology regarding sexual selection, through the use of the self-domestication hypothesis. What they found was that not only do P. paniscus have the highest rates of heterosexual copulation amongst the great apes, but they also show a significant occurrence of sexual acts towards individuals of the same sex (Hare et al, 2012). These behaviors expressed are seen within humans, thus creating the hypothesis that bonobos sexual behaviors may have an evolutionary link with regards to mating systems within the frame works of sexual selection.
Pair-bonding and monogamy is seen predominantly in the Hylobatidae branch within the primate linage. Borries et al (2011) aimed to investigate the level of monogamy amongst white handed gibbons (Hybolates Lar) and to see if infanticide was related to polyandrous gibbons. With their results they concluded that when an adult male left a mate to increase their reproductive fitness, or an adult male had died, male immigration had occurred, and all the infants from all immigrated groups over the study period had been lost. This research indicated that polyandry plays a part within Hylobatidae social groups (Borries et al, 2011). Although monogamy is the dominant mating system within Hylobatidae, their mating system is strikingly similar to western human mating systems, which could be an evolutionary link to how humans have altered the way we select.
Human Mating Systems
The evolution of sexual selection within humans, comes within the two pillars of sexual selection, Intersexual selection and intrasexual selection. Puts (2016) mapped out sexual selection amongst humans. Sexual selection amongst men comes in the form of intrasexual selection, this is due to males being more muscular (roughly 15-20%) than women, more violent, tendency to utilize weapons against each other, and express sex specific traits like beards and deeper tones. In contrast, female sexual selection comes in a more placid intersexual approach, although they do utilize the following expressions to attract mates. Higher-toned voices, gracile facial features, enhanced youthfulness and reduced body hair. Anatomically women have lean body mass then men but have 40% more adipose tissue. These deposits of fat tend to be around buttocks, hips and breast regions, which promotes fecundity (Puts, 2016). Although sexual dimorphism is apparent, it is on a similar level as our closest relatives P. troglodytes, and P. paniscus (Hare et al, 2012). This is suggesting the closer within our primate linage, the less outward sexual dimorphism becomes.
When looking at mating systems, comparing the evolutionary link between primates and humans is vital to understand how humans came to mate today. Wilson et al (2017) conducted an overview of humans as a model of sexual selection. They proposed that since we have multiple definitions of relationships, these include monogamy, polygyny, polyandry, polygynandry and same sex relationships. Some of these relationships can be obtained through status or acceptability in certain societies. They also stated polygyny in some cultures can be acquired by men of high status marrying multiple wives. Within these societies a mean of 12.4% had more than one wife, with men of high status obtaining multiple wife’s correlated with men of lower status having a shortage of wives. Polygynandry is more of a life stage for majority then a style of life, They also noted that most people who identified as polygynandry eventually wanted to obtain a monogamous marriage, but postponed reproduction to put time into education and to further oneself Wilson et al (2017). The human relationship systems are indeed diverse, all of these mating system preferences are all linked throughout our primate linage, moreover, an effective form of study is to clarify if the sociality of primates and the development of cultures may be the evolutionary link that structures human and primate mating systems.
Comparison of Humans and Primates
Now with comparison of humans to the non-human primate species studied in this entry, we found that humans exhibit all mating systems that are seen across the sampled primate species. Sexual dimorphism was a component that occurred in polygamous mating systems. This had been witnessed in P. troglodytes and P. paniscus due to certain levels of competition. Monogamous and pair bonded mating systems had only been expressed amongst H. lar, however there had been small study samples that expressed small level of polygamy, which in-turn enable the expression of the infanticide behaviors amongst those small populations.
Table1) Demonstrates the sampled primate species and what mating systems they utilize
| Monogamous/ Pair-bonded | Polygyny | Polygamous | Promiscuous Mating | Same Sex Mating | |
| Human | x | x | x | x | x |
| P. trolodytes | x | x | x | ||
| P. paniscus | x | x | x | x | |
| H. lar | x| |
Conclusion
The evolution of primate sexual selection has shaped the structure of human mating systems and shown significant similarities. Resemblance in sexual dimorphism across the amongst humans’, chimpanzees and bonobos have similar small levels of dimorphism. Mating systems also show quite similar structures when it comes to the sociality of mating. Humans tend to be predominantly pair-bonded however express multiply mating system in relation to cultures. The expression of pair-bonding has been seen within the Hylobatidae branch. Although there is some polyandrous within that branch, monogamy is predominantly expressed. Societies that mainly express the polygamous mating systems are cultures spread all across the eastern world, and some growing subcultures within the west. P. troglodytes and P. paniscus are our closest living relatives and predominantly share polygamous lifestyle. This questions the link between some of our living relatives, and puts forth the notion, is this link cultural across the evolution of sociality within primates or is the link of mating systems placed on a biological level. As the definition of Charles Darwins sexual selection hasn’t changed, his field of sexual selection has evolved and included an array of topics that now fit under the banner of sexual selection like mating systems. With this understanding advocates further research within this field, to better understand our evolution of human mating systems with connections to our fellow primates.
Literature Cited
Brindle, M., & Opie, C. (2016). Postcopulatory sexual selection influences baculum evolution in primates and carnivores. Proceedings of the Royal Society B: Biological Sciences, 283(1844)
Borries, C., Savini, T., & Koenig, A. (2010). Social monogamy and the threat of infanticide in larger mammals. Behavioral Ecology and Sociobiology,65(4), 685-693.
Hare, B., Wobber, V., & Wrangham, R. (2012). The self-domestication hypothesis: Evolution of bonobo psychology is due to selection against aggression. Animal Behaviour,83(3), 573-585.
Puts, D. (2016). Human Sexual Selection. Current Opinion in Psychology, 7, 28-32
Muller, M. N., Kahlenberg, S. M., Thompson, M. E., & Wrangham, R. W. (2007). Male coercion and the costs of promiscuous mating for female chimpanzees. Proceedings of the Royal Society B: Biological Sciences,274(1612).
Wilson, M. L., Miller, C. M., & Crouse, K. N. (2017). Humans as a model species for sexual selection research. Proceedings of the Royal Society B: Biological Sciences, 284(1866)