Around Valentine's Day, oxytocin (aka the "love hormone") references seem to proliferate. It's not surprising that the so-called "cuddle molecule" or "potion of devotion" is associated with the fourteenth of February, Cupid's arrows, and some people's desire for a monogamous lifelong partner. Oxytocin-related research also tends to make headlines in mid-February; this year is no exception.
A new study (Grebe et al., 2021) published on February 12 in Scientific Reports puts oxytocin in the spotlight, but there's a twist. Nicholas Grebe and his colleagues at Duke University who study lemurs—our close primate cousins from the Island of Madagascar, whose 105 species evolved in isolation there for millions of years—have discovered that having lots of oxytocin receptors in the brain doesn't automatically make every mammal monogamous. According to a press release, this "new research suggests the brain circuitry that makes love last in some species may not be the same in others."
Grebe is a postdoctoral researcher in Duke's Department of Evolutionary Anthropology and a Drea Lab member. The Duke Lemur Center, founded in 1966, houses over 200 lemurs across 14 species belonging to the Eulemur primate genus.
Red-bellied lemurs and mongoose lemurs (Eulemur rubriventer and E. mongoz) are notorious for their monogamous behavior. Male-female partners from this branch of the lemur family tree tend to stick together as monogamous couples for years; once bonded, red-bellied or mongoose lemur pairs spend about a third of their lifespan with the same mate. However, as the diagram in Figure One (below) illustrates, other lemur species belonging to the Eulemur genus change partners frequently and are known for their "polyamory" and non-monogamous behavior.
The latest lemur study by Grebe and his Duke colleagues compared the neural correlates of monogamous and non-monogamous behavior in different lemur species. Grebe et al. investigated a total of seven lemur species: two that were known for being monogamous (red-bellied and mongoose lemurs) and five lemur species (E. rufus, E. rufifrons, E. colaris, E. favifrons, E. macaco) known for their lack of monogamy.
After conducting a side-by-side comparison, the researchers were surprised to find that there weren't consistent differences between the distribution of oxytocin and vasopressin receptors in the neural circuitry of monogamous and non-monogamous lemurs.
"We find little evidence of a 'pair-bonding circuit' in Eulemur akin to those proposed in previous rodent or primate research," the authors write in the paper's abstract. "Mapping neuropeptide receptors in these nontraditional species questions existing assumptions and informs proposed evolutionary explanations about the biological bases of monogamy."
Previous research suggests that monogamy is relatively rare among mammals. Although roughly 90 percent of bird species exhibit fidelity towards one specific partner, only about 3-5 percent of mammals practice monogamy. "The vast majority of the roughly 6,500 known species of mammals have open relationships, so to speak," the Duke authors note.
Contrary to popular belief, a growing body of evidence suggests that oxytocin is not a universal "love hormone" and actually has a "dark side."
For example, last year a study (Anpilov et al., 2020) found that optogenetic stimulation of oxytocin neurons can elevate both prosocial and agonistic behaviors in group-living mice. (See "Oxytocin's Paradox: The 'Love Hormone' Can Fuel Aggression.")
That being said, decades ago, in the 1990s, groundbreaking research found that prairie voles' tendency to form monogamous male‐female pair bonds was linked to oxytocin.
Oxytocin's rise to household-name status can be traced back to a landmark study (Williams et al., 1994) that investigated how centrally administering oxytocin to female prairie voles (Microtus ochrogaster) facilitated the formation of a partner preference to a male prairie vole that was present during the infusion. Jessie Williams and coauthors concluded: "[Our] results suggest that oxytocin may be one factor contributing to the development of partner preferences in this monogamous rodent."
A follow-up prairie vole study (Cho et al., 1999) "compared the effects of centrally administered oxytocin (OT) and arginine vasopressin (AVP) on partner preference formation and social contact in male and female prairie voles" and found that "after one hour of cohabitation and pretreatment with either AVP or OT, both males and females exhibited increased social contact and significant preference for the familiar partner."
This research in small rodents led to some misguided speculation that if oxytocin drives monogamy in prairie voles, it must do the same in humans and other species known for monogamous coupling. However, as mentioned, the latest research from Duke's Drea Lab suggests that oxytocin is complicated and seems to work differently in different species.
"Oxytocin may be the 'potion of devotion' for voles, but it may be the combined actions and interactions of multiple brain chemicals, along with ecological factors, that create long-lasting bonds in lemurs and other primates, including humans," Grebe concludes. "There are probably a number of different ways through which monogamy is instantiated within the brain, and it depends on what animals we're looking at; there's more going on than we originally thought."
Nicholas M. Grebe, Annika Sharma, Sara M. Freeman, Michelle C. Palumbo, Heather B. Patisaul, Karen L. Bales & Christine M. Drea. "Neural Correlates of Mating System Diversity: Oxytocin and Vasopressin Receptor Distributions in Monogamous and Non-Monogamous Eulemur." Scientific Reports (First published: February 12, 2021) DOI: 10.1038/s41598-021-83342-6