Feb. 12 (UPI) -- The hormones that encourage monogamy among rodent pairs don't appear to work the same way in the brains of lemurs, according to a new study.

The research, published Friday in the journal Scientific Reports, suggest there are multiple biochemical recipes of monogamous love.

The vast majority of bird species, approximately 90 percent, are monogamous, but lifelong devotion is rare among mammals -- fewer than 5 percent of mammal species practice monogamy.

For decades, scientists have been trying to figure out why some species commit themselves to a single partner and others play the field. The results from a number of rodent studies suggested a pair of hormones were responsible.

Experiments showed the so-called "cuddle chemicals" oxytocin and vasopressin are released in the brains of monogamous rodents during mating. Scientists found the brains of monogamous prairie voles featured more docking sites, or receptors, for oxytocin and vasopressin than their more promiscuous peers.

For the new study, researchers at Duke University wanted to see if cuddle chemicals worked similar magic on lemurs, which spent more than a third of their lives with a single partner.

When they're not caring for their young or looking for food, lemur pairs can often be found side-by-side, cuddling with their tails wrapped around one another.

The team of scientists used autoradiography, an X-ray imaging technique, to study the distribution of docking sites for cuddle chemicals in the brains of 12 lemurs that had died of natural causes.

The survey featured the seven lemur species: monogamous red-bellied and mongoose lemurs, in addition to five promiscuous species from the same genus.

"They're really the only comparable natural experiment to look for biological signatures of monogamy in primates," lead study author Nicholas Grebe, a postdoctoral researcher at Duke, said in a news release.

Scientists found the density and distribution of receptors for oxytocin and vasopressin on lemur brains were distinct from the patterns found on rodent brains.

The findings suggest the cuddle chemicals work differently in the brains of primates than they do in prairie voles and other rodents.

In rodent brains, the differences between monogamous and promiscuous circuitry were apparent. Scientists found no such distinction among lemur brains.

"We don't see evidence of a pair-bond circuit," said Grebe.

In future experiments, researchers are planning to block oxytocin from binding to receptors in the brains of lemurs and watching how mates behave toward one another.

While it's likely oxytocin and vasopressin still play a role in regulating the romantic decisions of lemurs and other primates, the latest research suggests monogamy in lemurs -- and probably humans, too -- is the product of a unique combination of different brain chemicals and ecological factors.

"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," Grebe said. "There's more going on than we originally thought."

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Lemurs prove there's more than one biochemical recipe for monogamous love - UPI News