IMAGE: These primordial cousins of humans are among very few mammalian species in which male-female partners mate with each other year after year. view more
Credit: David Haring, Duke Lemur Center
DURHAM, NC – Humans are not the only mammals to form long-term bonds with special companions – some bats, wolves, beavers, foxes and other animals do too. But new research suggests that the brain circulation that makes love may not last in some species the same as in others.
The study, featured Feb. 12 in the journal Scientific Reports, comparing monogamous and promiscuous species within a group of closely related lemurs, primate cousins of people from the island of Madagascar.
Red bell lemurs and mongoose lemurs are just a few of the species in the lemur family tree in which male-female partners stick together year after year, working together to raise their brood and their area. to protect.
Once attached, pairs spend many of their waking hours adorning each other or going side by side, often with their tails folded around. each other’s bodies. Males and females of that sex spend a third of their lives with the same mate. The same cannot be said of their closest relatives, who frequently change partners.
To biologists, monogamy is something of a mystery. That’s partly because it’s rare in many animal groups. While about 90% of bird species use some form of loyalty to one partner, only 3% to 5% of mammals do. Most of about 6,500 species of mammals have open relationships, so to speak.
“It’s an uncommon arrangement,” said lead author Nicholas Grebe, a graduate fellow in Duke Christine Drea’s lab at Duke University.
Which begs the question: what makes some species biologically prone to converge for the long haul while others play on the field?
Studies over the past 30 years in rodents identify two hormones released during courtship, oxytocin and vasopressin, suggesting that the key to enduring love may lie in between differences in how they work on the brain.
Some of the first mites came from an influential study of prairie voles, small mouse-like mammals that, unlike most rats, breed for life. When researchers compared the brains of monogamous prairie voles with their promiscuous counterparts, montane voles and water voles, they found that prairie voles had more “docking sites” for these hormones, especially in parts of the brain’s reward system.
Since these “cuddle chemicals” were found to strengthen male-female bonds in water voles, researchers have long wondered if they might work the same way in humans.
That’s why the Duke-led team turned to lemurs. Despite being our most distant private relatives, lemurs are genetically matched more closely to humans than water voles.
The researchers used an imaging technique called autoradiography to map binding sites for oxytocin and vasopressin in the brains of 12 lemurs that had died of natural causes at the Duke Lemur Center.
The animals represented seven species: monogamous red bell and mongoose lemurs along with five promiscuous species in the same species.
“They are certainly the only comparative natural test to look for monogamy biological names in primates,” Grebe said.
Comparing the results of brain imaging in lemurs with previous results in water voles and monkeys revealed some obvious differences in the density and circulation of hormone receptors. In other words, oxytocin and vasopressin appear to act on different parts of the brain in lemurs – which means they may also have different effects, depending on where their target cell is.
But within lemurs, the researchers were surprised that there were few consistent differences between monogamous species and promiscuous species.
“We don’t see evidence of a connective tissue cycle” similar to those found in a rodent’s brain, Grebe said.
As a next step, the team looks at how lemur pairs behave towards each other if oxytocin actions are blocked, by feeding them an antagonist that prevents oxytocin from binding to the oxytocin. its receptors in the brain.
So what can lemurs teach us about love? The authors state that their findings are cautious against making simple conclusions based on rodent experiments about how human social behavior evolved.
Oxytocin may be the “potion of devotion” for water voles, but it may be the actions and interactions of multiple brain chemicals, along with ecological factors, that create long-term bonds in lemurs and other primates, humans among them, Grebe said.
“There may be a number of different ways through which instantaneous monogamy takes place inside the brain, and it depends on the animals we are watching,” Grebe said. “There’s more going on than we originally expected.”
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Other authors were: Duke Annika Sharma, Utah State University Sara Freeman, California National Priority Research Center Michelle Palumbo, North Carolina State University Heather Patisaul, and Karen Bales at the University of California, Davis.
This work was supported by grants from the National Science Foundation (SBE-1808803), the National Institute of Mental Health (NIMH R21MH115680), the Josiah Charles Trent Memorial Foundation Implementation Fund, the Charles Lafitte Foundation for Research, and Duke University.
CITATION: “Neural Correlates of Mating System Diversity: Oxytocin and Vasopressin Release in Monogamous and Non-Monogamous Eulemur,” Nicholas Grebe, Annika Sharma, Sara Freeman, Michelle Palumbo, Heather Patisaul, Karen Bales, and Christine Drea. Scientific Reports, February 12, 2021. DOI: 10.1038 / s41598-021-83342-6