The basis of lasting love is more complex than expected

Throughout the animal kingdom, there are many species that form monogamous bonds with their mates just like humans. In a new study from Duke University, experts have discovered that the brain circuitry that supports lasting love in some species is not the same in others.

For the investigation, the researchers compared monogamous and promiscuous species within a closely related group of primates known as lemurs.

Red-bellied lemurs and mongoose lemurs are two of the only types of this group in which male-female pairs stick together to raise their young and defend their territory.

Once two lemurs have bonded, they spend about one-third of their lifetime together. The partners spend hours grooming each other or huddled side by side. 

Overall, monogamy is rare among mammals and remains somewhat of a mystery to biologists. While about 90 percent of bird species are usually faithful to one partner, only three to five percent of mammals practice fidelity. “It’s an uncommon arrangement,” said study lead author Nicholas Grebe.

The researchers set out to investigate what makes some species biologically inclined to monogamy. Previous studies have found evidence that two hormones released during mating, oxytocin and vasopressin, have an influence on lasting love according to how they affect the brain.

When experts compared the brains of monogamous prairie voles with two other vole species that behave promiscuously, they found that prairie voles had more “docking sites” for the hormones, especially in parts of the brain’s reward system.

If these “cuddle chemicals” enhance lasting love in voles, the researchers theorized they may have the same influence on humans. The study was focused on lemurs because they are a closer genetic match to humans than voles.

The researchers used an imaging technique called autoradiography on 12 lemurs that had died of natural causes at the Duke Lemur Center. They used the images to map binding sites for oxytocin and vasopressin.

The team compared the brain imaging results in lemurs with previous results in voles and monkeys. This analysis revealed some noticeable differences in the density and distribution of hormone receptors. 

Essentially, oxytocin and vasopressin were found to act on different parts of the brain in lemurs, which means they may also have different effects depending on their target cell’s location.

According to Grebe, 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 lasting bonds in lemurs and other primates like humans.

“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,” said Grebe. “There’s more going on than we originally thought.”

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer