By Cara Burke
Monogamy is still an evolutionary and molecular mystery. It is a rare sight for mammals – only around 3 to 5% of them mate for life, which is a very small number in comparison to the 90% of bird species who do the same (Carter & Perkeybile, 2018). Even the term “mate for life” is controversial, as monogamy has been found to not necessarily just involve sexual partnership. Exclusive mating with only one sexual partner is much rarer than social partnership, where a male and female may stay with each other socially, with many of the behaviours associated with this partnership, but would also mate with individuals outside of the partnership (Carter & Perkeybile, 2018).
The term “social partnership” also encompasses other forms of social behaviour, such as child-rearing between the two of them, aggression towards outsiders, particularly after mating, close-knit family cycles, etc. However, these behaviours are very rarely found across all monogamous species (Solomon & Ophir, 2020). It is very hard to categorise monogamy, because not only does it differ greatly between species, but how it presents itself can also differ greatly among individuals of the same species.
One reason for this may be its evolutionary history, about which we still know very little. It has emerged independently in mammals around 60 times . This could be the reason exciting new research has suggested there are different mechanisms for “love” (or at least, what holds individuals together in a monogamous relationship) in different species.
Much of our previous research had been based on prairie voles, which form socially monogamous relationships, which can be directly contrasted with their close relatives who do not. The neuropeptides oxytocin and vasopressin were particularly implicated in the molecular basis of social monogamy (Carter & Perkeybile, 2018). They are hormones released during mating. Insensitivity to gonadal androgens and steroids such as testosterone and estradiol have also been heavily implicated. Voles are insensitive to androgens during development, which may also affect sexual dimorphism, which tends to be less extreme in monogamous species. After mating, both males and females have heightened aggression, which vasopressin can induce. It was found in males that pre-natal castration can disrupt pair bonding, but these effects could be reversed by vasopressin. Oxytocin also has lots of effects, particularly maternal ones and upregulation of alloparental care, which suggests it could be involved in the child-rearing aspects of monogamous relationships (Carter & Perkeybile, 2018). The effects of these four steroids and neuropeptides on behaviours present in particularly monogamous species hints at their role in enabling monogamy.
In order to further investigate their effects, comparisons were made between the oxytocin and vasopressin receptors in prairie voles and closely-related non-monogamous rodents. Oxytocin has only one receptor, G protein-coupled receptor XTR, and vasopressin has three known receptors; AVPR1a, AVPR1b, and AVPR2 (Grebe et al., 2021). A greater amount of receptors were found in prairie vole brains, particularly in the nucleus accumbens, which is a region usually related to reinforcement and reward (Carter & Perkeybile, 2018).
Both the upregulation of these neuroproteins, their downregulation, and investigation into their receptor density in the brain have strongly implicated them in the formation of monogamous relationships. But now, a paper from February 12th has turned this idea on its head somewhat (Grebe et al., 2021).
Red-bellied lemurs and mongoose lemurs form monogamous relationships which can last around one-third of their lifetime, and they spend much of the day cuddled up to their partner (Duke University, 2021). But when the oxytocin and vasopressin receptors were compared between them and five species of the same genus, there was no significant difference in the amount of distribution of receptors (Grebe et al., 2021). Just how upregulating or downregulating these hormones affect the lemur’s monogamous behaviour is ongoing research (Duke University, 2021).
This might be particularly interesting for us humans, as we form socially and genetically monogamous relationships, and are much more closely related to lemurs than to prairie voles. However, the mysteries behind monogamy are still far from being solved, and it is hard to discern any real implications for human feelings of “love” yet. It is clear that the mechanisms behind monogamy differ between species, showing that there is not a “one-size-fits-all” model for love.
Carter, C. S. & Perkeybile, A. M. (2018) The Monogamy Paradox: What Do Love and Sex Have to Do With It? Frontiers in Ecology and Evolution. 6 202. Available from: https://www.frontiersin.org/article/10.3389/fevo.2018.00202.
Solomon, N. G. & Ophir, A. G. (2020) Editorial: What’s Love Got to Do With It: The Evolution of Monogamy. Frontiers in Ecology and Evolution. 8 110. Available from: https://www.frontiersin.org/article/10.3389/fevo.2020.00110.
Grebe, N. M., Sharma, A., Freeman, S. M., Palumbo, M. C., Patisaul, H. B., Bales, K. L. & Drea, C. M. (2021) Neural correlates of mating system diversity: oxytocin and vasopressin receptor distributions in monogamous and non-monogamous Eulemur. Scientific Reports. 11 (1), 3746. Available from: https://doi.org/10.1038/s41598-021-83342-6. Available from: doi: 10.1038/s41598-021-83342-6.
Duke University. (2021) Lemurs show there’s no single formula for lasting love: Brain imaging reveals that not all monogamous mammals are ‘wired for love’ in the same way. ScienceDaily. Available from: http://www.sciencedaily.com/releases/2021/02/210212094110.htm.