Humans aren’t the only mammals that form long-term bonds with a single, special mate — some lemurs and other animals do, too. Researchers are mapping the hormone receptors that underlie these primates’ ability to pair up for the long haul. Their findings suggest the brain circuitry that makes love last in some species may not be the same in others.

The study, appearing Feb. 12 in the journal Scientific Reports, compares monogamous and promiscuous species within a closely related group of lemurs, distant primate cousins of humans from the island Madagascar.

Red-bellied lemurs and mongoose lemurs are among the few species in the lemur family tree in which male-female partners stick together year after year, working together to raise their young and defend their territory.

Once bonded, pairs spend much of their waking hours grooming each other or huddled side by side, often with their tails wrapped around each other’s bodies. Males and females of these species spend a third of a lifetime with the same mate. The same cannot be said of their closest relatives, who change partners often.

To biologists, monogamy is somewhat a mystery. That’s in part because in many animal groups it’s rare. While around 90% of bird species practice some form of fidelity to one partner, only 3% to 5% of mammals do. The vast majority of the roughly 6,500 known species of mammals have open relationships, so to speak.

«It’s an uncommon arrangement,» said lead author Nicholas Grebe, a postdoctoral associate in professor Christine Drea’s lab at Duke University.

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Materials provided by Duke University. Original written by Robin A. Smith. Note: Content may be edited for style and length.