The One and Only

September 17, 2019

Written by: Rebecca Somach

 

If you have a Facebook, Instagram or other social media account, you may experience an uptick in the number of wedding posts you will be seeing in the next few months. According to a report of 18,000 couples surveyed by WeddingWire.com¹, September and October are the most popular months to get married. Between all the flowers, cake, and decorations, a wedding focuses on the relationship between a couple and the strong feelings of connection that exist between them. The passion and love that brings people together sounds great in movies and books, but what makes a marriage last is the commitment that couple shares with one another.

However, if we take out the sappy notions of love and comparisons of a person to a summer’s day, we all would probably agree that having a single couple stay together forever doesn’t make sense from a biological perspective. If the ultimate goal of every creature is to pass on their genes by having as many children as possible, then marriage or monogamy is actually the worst strategy of them all. Despite that, many people will stay committed to the same partner for most of their lives. Is there biology behind this seemingly unreasonable behavior? Can we understand how the brain causes this pair-bonding?

Humans are not the only animals that form monogamous bonds. Small mammals called prairie voles (Figure 1) are also monogamous and both parents share a role in taking care of their young. If one vole dies, the other will often not take another partner. However, not all voles behave this way. The closely-related meadow and montane voles are not monogamous and don’t take life partners. Scientists can compare these similar animals and see what differences might exist in their brains that would make them behave differently.

What scientists have found is that pair bonding might rely on the release of different neuropeptides. Neuropeptides are small proteins that are released by neurons in order to communicate information to other neurons. These are different than other communication molecules called neurotransmitters because neuropeptides are larger, slower to act, and their effects last longer².

Researchers discovered that the most prominent neuropeptides related to forming pair bonds in prairie voles are oxytocin, arginine vasopressin (AVP), and dopamine³, which they found using a “Partner Preference Test”^4,5. Scientists place two male voles on a leash, while one female vole can move around freely. One of the males is the female’s partner and the other is a stranger. The scientists measure which male vole the female spends more time with. Normally, it can take 24 hours for the female to form a bond with a male vole, but in one study, scientists found that when they injected oxytocin into the females, they would spend more time with their partner in just 6 hours⁴.

Where do these neuropeptides act in the brain? The scientists found that pair formation uses the reward circuitry of the brain. One region is called the nucleus accumbens. When scientists blocked the oxytocin and dopamine receptors in this area of female voles they didn’t form partner pairs, but when these receptors were activated the females did form partner pairs⁵. There is also some evidence that oxytocin and dopamine interact in a planning region called the prefrontal cortex to help form pair bonds⁶. These studies demonstrate that the formation of pairs in prairie voles happens because the prairie voles feel rewarded when they spend time with their partner which reinforces the behavior.

These neuropeptides are found in other species of voles as well, so why don’t they also form pair bonds? The answer may have to do with the places where these neuropeptides act. The brains of monogamous voles have more receptors for oxytocin and AVP in the nucleus accumbens, thalamus, and amygdala⁷. These areas are associated with reward, but also with sensory processing. In this way, the animals that have fewer receptors may still experience some type of positive feeling from being around other voles, but don’t associate that reward with any specific partner the way the monogamous voles do. This idea was further tested by genetically altering non-monogamous voles. If scientists added more AVP receptors to the ventral forebrain of non-monogamous meadow voles, then the meadow voles began to show a preference for a single partner⁸. So it isn’t that these voles don’t have the same neuropeptides, but that they act in different places in monogomous voles.

While voles are quite cute when they cuddle with their partners, it isn’t quite the same as watching a human couple get married. Do these neuropeptides have any relevance to humans? As it turns out, oxytocin and vasopressin do have effects in humans as well. Oxytocin is released in response to stressful situations as well as when infants breastfeed. And in some studies higher oxytocin levels in humans were associated with a greater perception of love and gratitude from a partner⁹. What’s more, fMRI studies show activation reward systems when humans were shown images of their romantic partners¹⁰,similar to the results of the rodent studies.

While scientists can’t directly measure the presence of oxytocin and vasopressin in the human brain, there is some evidence that these neuropeptides may be involved in the rewarding feeling humans feel when they see their partners. That might mean that the way we form partnerships could be similar to the monogamous rodents that have already been studied. Maybe at the next upcoming wedding your toast to the couple can include a wish that they are loyal to each other, just like the prairie voles.

 

Image References:

Cover Image via Pixabay, https://pixabay.com/photos/couple-bride-love-wedding-bench-260899/

Figure 1 via Pexels, https://www.pexels.com/photo/closeup-photo-of-tan-rat-1010267

 

References:

1. Wedding Wire Newlywed Report: https://go.weddingwire.com/newlywed-report
2. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Two Major Categories of Neurotransmitters. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10960/
3. Young L., Wang Z., The neurobiology of pair bonding. Nature Neuroscience. 2004;7,1048-1054
4. Williams JR., Carter CS., Insel T., Partner preference in female prairie voles is facilitated by mating or the central infusion of oxytocin. Ann NY Acad Sci. 1992
5. Liu Y., Wang Z.X, Nucleus accumbens oxytocin and dopamine interact to regulate pair bond formation in female prairie voles Neuroscience 2003
6. Lieberwirth C, Wang Z. The neurobiology of pair bond formation, bond disruption, and social buffering. Curr Opin Neurobiol. 2016;40:8–13. doi:10.1016/j.conb.2016.05.006
7. Insel, T.R. & Shapiro, L.E. Oxytocin receptor distribution reflects social organization in monogamous and polygamous voles. Proc. Natl. Acad. Sci. USA 89, 5981–5985 (1992).
8. Lim, M.M. et al. Enhanced partner preference in promiscuous species by manipulating the expression of a single gene. Nature 429, 754–757 (2004).
9. Algoe SB, Kurtz LE, Grewen K. Oxytocin and Social Bonds: The Role of Oxytocin in Perceptions of Romantic Partners’ Bonding Behavior. Psychol Sci. 2017;28(12):1763–1772. doi:10.1177/0956797617716922
10. Bianca P. Acevedo, Arthur Aron, Helen E. Fisher, Lucy L. Brown, Neural correlates of long-term intense romantic love, Social Cognitive and Affective Neuroscience, Volume 7, Issue 2, February 2012, Pages 145–159, https://doi.org/10.1093/scan/nsq092