Valentine’s Day celebrates the triumph of love: the bond that is formed between two people who have decided to spend the rest of their lives together. But humans aren’t the only mammals to form long-term relationships with a single special partner: so do some bats, wolves, beavers, foxes, and other animals, including lemurs. Now, new research published in the journal Scientific Reports suggests that the brain circuits that cause love to last differ by species. The research compares monogamous and promiscuous species within a closely related group of lemurs, the primate ‘distant cousins’ of humans. For example, red-bellied lemurs and mongoose are among the few species in which male-female pairs stick together year after year. They even work together to raise their offspring and defend their territory. And the relationship doesn’t end there: once bonded, they spend much of their time grooming or snuggling next to each other, often with their tails wrapped around their partner’s body. Your relationships last a third of your life. In contrast, other species of lemurs change partners often.

In general, for biologists the monogamy of mammals is a mystery since it is not a common behavior: of the 6,500 known species, only 3 to 5% are monogamous. In contrast, about 90% of bird species practice some form of fidelity to a partner. “This is a rare practice,” says Nicholas Grebe of Duke University and lead author of the study. This is how the question arises of what mechanism is behind that some species are biologically inclined to mate in the long term and others are not. Studies conducted over the past 30 years in rodents point to two hormones released during mating: oxytocin (also known as the “love hormone”) and vasopressin, and the different way in which both act in the brain.
Prairie voles VS lemurs.

Some of the first clues come from extensive research into the monogamy of prairie voles, one species, unlike most other rodents, spending its life with the same pair. When the researchers compared the brains of prairie voles (monogamous) with mountain voles (promiscuous), they found that those of prairie voles had more “places” where hormones – oxytocin and vasopressin – could “couple”, particularly in parts of the brain’s reward system. But is it the same with other species, including humans? This is how the Duke University team chose the lemurs. “Despite being our most distant primate relatives, they have a closer genetic compatibility to humans than prairie voles,” the researchers explain. Using an imaging technique called autoradiography, they mapped the 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-bellied lemurs and mongoose, whose fidelity was already known, along with five other promiscuous species of the same genus. The results revealed notable differences in the density and distribution of hormonal receptors concerning prairie voles. That is, oxytocin and vasopressin act in different parts of the brain in lemurs, which means that they can also have different effects, depending on the location of the target cell. But there was more: When comparing between monogamous and promiscuous species of lemurs, the researchers didn’t find many differences. “We don’t see evidence of a pair-bonding circuit similar to that found in rodent brains,” says Grebe. That is why the team’s next step will be to analyze how the pairs of lemurs behave with each other but temporarily blocking the oxytocin receptors in their brains.

So what can lemurs teach us about love? The authors say their findings caution against drawing simple conclusions based on rodent experiments about how human social behaviors emerged. There are probably several different ways in which monogamy is instantiated within the brain, and it depends on the animals we are looking at. More things are happening than we originally thought, ”says Grebe. That is, oxytocin may be a “love potion” for prairie voles, but lemurs tell us that there may be the combined actions and interactions of multiple brain chemicals, along with ecological factors, that create lasting bonds. in lemurs and other primates, including humans. That is why the magic of love remains a mystery, At least for science.