Throughout human history, pining lovers have sought so-called aphrodisiacs to win over affection. From oysters and dark chocolate to rhinoceros horns, these reported love potions sometimes have questionable ties to truth. However, scientists are beginning to unravel the genuine biology of ‘love’ by studying the neuroscience of monogamy and bonding. The monogamous love lives of titi monkeys (Plecturocebus cupreus) have been a fruitful focus. These non-human primates share an equal role in raising their young and mate for life, often romantically entwining their tails in their slumber. Only about 3% of mammals – including humans – exhibit this kind of social monogamy and we know that these strong attachments are cemented by powerful hormones, such as oxytocin and vasopressin. In fact, oxytocin is being explored as a potential treatment for disorders such as autism. Given the potential for the hormone to transform lives and the strong bonds that titi monkeys form, Rocío Arias-del Razo and colleagues from University of California Davis, USA, examined the long-term behavioural and neural effects of oxytocin treatment on the loyal animals.
Initially, the researchers dripped oxytocin dissolved in salt water into the nostrils of young titi monkeys daily for 6 months. Each monkey lived with their parents and siblings for another full year after the oxytocin treatment ceased, before they were rehoused with a stranger who would become their mate. Of the original 15 treated, only one monkey did not take kindly to his arranged ‘marriage’, showing some initial signs of aggression, but even this ornery fellow quickly settled into his new role.
To examine how oxytocin treatment affects animal behaviour, the researchers observed the couples each day during their first 4 months together. They found that monkeys treated with oxytocin exhibited more tail twining (a more ‘intimate’ form of affection) at the expense of proximity and contact with their mate. The hormone seemed to affect how the monkeys interacted with their mates, rather than the total time spent interacting.
Besides these daily observations, the researchers also tested the behavioural responses of the monkey couples, 4 months after they were introduced. First, they presented each monkey with either their mate or a complete stranger in an adjacent cage. Both male and female monkeys spent more time touching their partner’s cage than that of the stranger, but the males that had been treated with oxytocin as youngsters showed a stronger preference for their partner. The second test examined the monkeys’ reactions to strangers. As titi monkeys lack self-recognition, they respond to their own reflection as if they were a stranger of the same sex, while they react to their partner's reflection as a stranger of the opposite sex. Testing the monkeys’ responses to these reflections, the team found that the oxytocin-treated male monkeys were friendlier towards the strangers in the mirror, making more curious approaches, contacts, movements and lip smacks.
Finally, the researchers PET scanned the monkeys’ brains, both before and after forming a relationship with their partner, to examine the long-term effects of oxytocin in the brain. The scientists discovered brain activity patterns that confounded their expectations. Although they had anticipated that the long-term oxytocin treatment would increase activity in the regions of the youngsters’ brains that are associated with attachment and social skills, the activity levels remained unchanged. However, the team did find that the females that had been treated with oxytocin had higher brain activity than the males.
Oxytocin seems to sit at the epicenter of social monogamy, potentially regulating social engagement and the formation of long-term relationships as adults. Further research on the hormonal ties that bind could also have implications for how to treat autism and may even help us to be more successful in love.
