In the previous posts, a broad overview of the evolution of monogamy was described. Sexual behavior and parental care were compared in several different classes of animals, and different theories for the evolution of monogamy were discussed. These were ultimate explanations for the appearance of monogamy, according to Tinbergen’s four questions. Proximate explanations on the other hand, have to do with how these phenomena occur in an individual. Examples include biological mechanisms that facilitate behavior, learning, and development.
In this post, the function of the neuropeptides oxytocin and vasopressin will be pondered. These chemicals are different from neurotransmitters in that they can also act as hormones (travel through the bloodstream). However, they are in much greater concentration in the brain compared to in the blood.
Oxytocin has quite the pop-science title for it – often being labeled the love molecule. Most people who believe in free-will would agree that hormones and neurotransmitters do not necessarily make us act in certain ways, but rather increase or decrease the likelihood of certain behaviors given specific contexts. Ergo, oxytocin will not make you fall in love. It also seems as though the “love hormone” may have a less discussed dark side to it, possibility facilitating acts of aggression.
Primarily, oxytocin is known for its effects on uterine contractions and the milk let down reflex in mothers. Later discoveries began to reveal its affects on social behavior and pair-bonding, and it got coined the love molecule. How accurate (or inaccurate) is this? Let’s consider some examples:
In one study on marmosets, researchers had three groups where the controls were compared to individuals treated with an oxytocin antagonist or agonist, which would decrease and increase oxytocin levels respectively (Smith, Ågmo, Birnie, & French, 2010). Results indicated that increased oxytocin levels were associated with increased huddling behavior with one’s partner. The opposite effect was also observed, meaning that the oxytocin antagonist increased distance between the animals and decreased huddling behavior. Food sharing behavior was also affected by oxytocin levels (Smith et al., 2010). This study was done using intranasal oxytocin administration, which has been subject to skepticism due to the fact that the amount of oxytocin reaching the brain is questionable due to the size of the molecule and the blood-brain barrier. However, recent studies have suggested that oxytocin and other molecules of that size may use an alternative pathway to reach the brain (Quintana, Smerud, Andreassen, & Djupesland, 2018).
In another study done on zebra finches, researchers showed that oxytocin was at least partially responsible for their monogamous behavior (Klatt James D. & Goodson James L., 2013). Birds treated with an oxytocin antagonist were less likely to remain in a stable monogamous pair-bond. However, sex differences were prevalent in this study. Oxytocin seemed to consistently have more significant effects on females compared to male subjects (Klatt James D. & Goodson James L., 2013). In another study done on finches, an abundance of sex-mediated effects were also found when manipulating vasopressin and oxytocin. For example, a knockdown (gene manipulation) decreasing vasopressin levels increased aggression in males but decreased aggression in females (Kelly & Goodson, 2014). The knockdown of vasopressin decreased the amount of time spent near the group in both sexes, while the knockdown of oxytocin only showed this effect for females (Kelly & Goodson, 2014). Less oxytocin was also associated with less pair-bonding in females, but not males (Kelly & Goodson, 2014). Overall, it seems that the scientific community has come to a relative consensus when referring to sex differences in oxytocin and vasopressin effects.
Oxytocin and vasopressin are involved in more than just positive affectivity, and can actually promote acts of violence out of protection and jealousy. Both molecules seem to modulate feelings of anxiety, and increased anxiety has been correlated to less pup defense behavior in rodents (Bosch Oliver J., 2013). Increased levels of oxytocin and vasopressin related to motherhood have also been linked to increased pup defense behavior (Bosch Oliver J., 2013). In male prairie voles, increased vasopressin from the anterior hypothalamus is associated with aggression towards females other than the males pair-bonded partner (Gobrogge, Liu, Young, & Wang, 2009).
The neuropeptides oxytocin and vasopressin seem to regulate a plethora of social behaviors. Although pop-science has made oxytocin quite famous for its role in feelings of “love,” it is clearly much more complex than that. These molecules have been evolutionarily preserved from some of the oldest living organisms, and serve a greater purpose – which is arguably to increase the likelihood of survival for the offspring in many species.
Bosch Oliver J. (2013). Maternal aggression in rodents: brain oxytocin and vasopressin mediate pup defence. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1631), 20130085. https://doi.org/10.1098/rstb.2013.0085
Gobrogge, K. L., Liu, Y., Young, L. J., & Wang, Z. (2009). Anterior hypothalamic vasopressin regulates pair-bonding and drug-induced aggression in a monogamous rodent. Proceedings of the National Academy of Sciences, 106(45), 19144–19149. https://doi.org/10.1073/pnas.0908620106
Kelly, A. M., & Goodson, J. L. (2014). Hypothalamic oxytocin and vasopressin neurons exert sex-specific effects on pair bonding, gregariousness, and aggression in finches. Proceedings of the National Academy of Sciences, 111(16), 6069–6074. https://doi.org/10.1073/pnas.1322554111
Klatt James D., & Goodson James L. (2013). Oxytocin-like receptors mediate pair bonding in a socially monogamous songbird. Proceedings of the Royal Society B: Biological Sciences, 280(1750), 20122396. https://doi.org/10.1098/rspb.2012.2396
Quintana, D. S., Smerud, K. T., Andreassen, O. A., & Djupesland, P. G. (2018). Evidence for intranasal oxytocin delivery to the brain: recent advances and future perspectives. Therapeutic Delivery, 9(7), 515–525. https://doi.org/10.4155/tde-2018-0002
Smith, A. S., Ågmo, A., Birnie, A. K., & French, J. A. (2010). Manipulation of the oxytocin system alters social behavior and attraction in pair-bonding primates, Callithrix penicillata. Hormones and Behavior, 57(2), 255–262. https://doi.org/10.1016/j.yhbeh.2009.12.004