Buzz
Falling in love is one of life's greatest joys—and humans might not be the only species to experience it. Pair bonding, the act of animals partnering up, is observed throughout the animal kingdom. While it’s debatable whether robins truly experience love, the idea of animals forming bonds is captivating. Biologists have made remarkable progress in recent decades, utilizing technology to better understand the physiological processes behind pair bonding and love.
Getting In-Vole-ved
In their quest to uncover the secrets of pair bonding, scientists chose to study voles, rather than robins. Prairie voles and montane voles are closely related rodents with drastically different mating behaviors: prairie voles form lifelong monogamous bonds after mating, while montane voles remain promiscuous. Research led by Thomas Insel, director of the National Institute of Mental Health (NIMH), revealed that these differences in mating patterns are linked to the hormones oxytocin and vasopressin. Oxytocin plays a crucial role in childbirth and lactation, while vasopressin regulates kidney function and constricts blood vessels. Recent studies have also connected these hormones to various social behaviors, from collaboration to mate selection and even interpreting the emotions of others. Oxytocin, in particular, has calming properties and appears to foster trust between individuals.
The key physiological distinction between these two vole species lies in the distribution of oxytocin and vasopressin receptors within their brains. Prairie voles have a higher concentration of these receptors in the amygdala, a brain region associated with emotional memory, and in areas tied to the dopamine reward system. The release of both hormones during mating fosters the formation of pair bonds. When this hormonal release is blocked, prairie voles abandon their monogamous behaviors. Even more telling, when montane voles are genetically altered to possess prairie vole-like vasopressin receptor distributions, they adopt monogamous tendencies.
Dopamine also plays a significant role. When released, dopamine induces feelings of pleasure, and it’s involved in rewarding behaviors such as mating or enjoying a satisfying meal. Voles that are made more sensitive to dopamine can form partner preferences even without mating—simply encountering a potential partner can trigger this preference. On the other hand, voles with blocked dopamine receptors do not develop such preferences. Interestingly, many of the dopamine receptors involved in pair bonding in voles are the same ones that play a role in cocaine addiction in rodents, providing a possible explanation for why love can feel addictive.
This is what love does to your brain
In humans, Donatella Marazziti, Professor of Psychiatry and Director of the Laboratory of Psychopharmacology at the University of Pisa, discovered that the initial stages of romantic love are associated with lower levels of serotonin and serotonin receptors. These molecules are also reduced in obsessive-compulsive disorder. Given that both conditions (albeit to varying degrees) are linked to anxiety and intrusive thoughts, early love might be considered a temporary, mild form of obsessive behavior. Additionally, early romance triggers increased levels of molecules associated with stress. After twelve to eighteen months, both serotonin and stress-related molecules return to normal levels.
Researchers such as Helen Fisher from the Department of Anthropology at Rutgers University and Semir Zeki from UCL in London have used functional MRI studies to identify the brain areas activated or deactivated by romantic love. Their findings show that individuals in love exhibit heightened activity in several brain regions involved in the dopamine reward system. Parental love activates many of the same regions but not the hypothalamus, suggesting the hypothalamus is responsible for the sexual component of romantic love.
Regions that display reduced activity include the amygdala, along with the frontal and prefrontal cortices. The amygdala is linked to fear and the process of learning from negative experiences. The frontal and prefrontal cortices are involved in executive functions such as analysis, judgment, delayed gratification, and predicting future outcomes. It can be inferred that the reduced activity in these areas might explain why people in love often appear to lack full control over these functions. Fisher eloquently summarizes some of her findings in her 2008 TED talk.
While the full physiology of love remains unclear, research continues to unfold. Studies on the pair bonding of animals like voles have shed light on the hormonal and brain activity patterns that occur in humans in romantic relationships. What physiology reveals about love confirms much of what we already suspected—that love can be a stressful experience, bordering on addiction or obsessive disorder, that it clouds judgment and drives impulsive behavior, and that as time passes, these disruptions subside, eventually giving way to calm and happiness.
