When Valentine’s Day rolls around in mid-February, many of us can’t help thinking about finding—or staying with—that one, special person; or, as often described, the “love of our lives.” And although there’s nothing wrong with buying chocolates and flowers for ourselves, dreaming about that unique, cherished partner is hard to escape on February 14.
But humans aren’t the only animals that like to form long-term bonds with a single, special mate. There are several animals that are far more intense about lengthy romances than we are: albatrosses, bald eagles, barn owls, beavers, black vultures, coyotes, elephants, gibbons, otters, pigs, prairie voles, sandhill cranes, swans, shingleback lizards, termites, turtle doves and wolves—all of them endeavor to stay with a single partner throughout their lives.
You can also list lemurs in that above-mentioned group. And now, researchers are mapping the hormone receptors that underlie these small primates’ ability to pair up for the long haul.
But how do lemurs find that unrivaled “one” for them in the first place? Rather than turn to the Internet to find a Mr. or Ms. Right, they just use their noses. For lemurs, an individual’s distinctive body odor reflects genetic differences in the immune system, and they can detect these differences by smell. That ability could help their offspring fight more pathogens.
Sniffing out love
In the wild, lemurs advertise their presence by scent marking, a form of olfactory communication used by animals to transmit signals to other animals. Lemurs rub scent glands against trees to broadcast information about their kin, sex and whether they are ready to mate.
In a study published in the science journal BMC Ecology and Evolution on August 22, 2019, researchers found that from just one whiff, lemurs can tell which prospective partners have immune genes different from their own. This ability could make the immune systems of their offspring more diverse and stronger.
For the BMC Ecology and Evolution study, scientists collected scent secretions from approximately 60 lemurs at the Cincinnati Zoo and Botanical Garden, the Duke Lemur Center and the Indianapolis Zoo. The team used a technique called gas chromatography–mass spectrometry to tease out the hundreds of compounds that make up each animal’s signature scent. They also conducted a DNA analysis, looking for differences within a cluster of genes known as MHC that help trigger the body’s defenses against foreign invaders, such as bacteria and viruses. Their tests revealed that the chemical cocktail lemurs emit varies depending on which MHC types they carry.
To see if potential mates can smell the difference, the researchers then presented the lemurs with pairs of wooden rods smeared with the bodily secretions of two, unfamiliar lemurs and observed their responses. Within seconds, the animals were drawn to the smells wafting from the rods, engaging in a frenzy of licking, sniffing or rubbing their own scents on top. In 300 trials, the team found that the females paid more attention to the scents of males whose immune genes differed from their own. Since different genetic versions respond to different sets of foreign substances, sniffing out genetically dissimilar mates produces offspring capable of fending off a broader range of diseases.
In the future, the researchers plan to use maternity and paternity DNA test results from wild lemurs living in a reserve in Madagascar to see if such lemur couples differ more in their MHC types than would be expected by chance.
This is the first time that the ability to sniff out partners based on their immune genes has been shown in distant primate kin. Growing evidence suggests that primates rely on olfactory cues way more than we thought they did. And, say the researchers, it’s possible that all primates can do this.
Locking on to what makes lemur love last
To biologists, monogamy is somewhat of a mystery. That’s partly because in many animal groups, it’s rare. While around 90 percent of bird species practice some form of fidelity to one partner, only 3 percent to 5 percent of mammals do. The vast majority of the roughly 6,500 known species of mammals have “open relationships,” so to speak.
Which raises a question: what makes some species biologically inclined to pair up for a lifetime while others play the field?
Some of the first clues for finding that answer came from influential research on prairie voles, small, mouse-like mammals that—unlike most rodents—mate for life. When researchers compared the brains of monogamous prairie voles with their promiscuous counterparts—meadow voles and montane voles—they found that prairie voles had more “docking sites” for oxytocin and vasopressin hormones, particularly in parts of the brain’s reward system.
Since these “cuddle chemicals” were found to enhance male-female bonds in voles, researchers have long wondered if they might work the same way in humans. That’s why they turned to lemurs. Despite being our most distant primate relatives, lemurs are a closer genetic match to humans than voles are.
For a study that was published on February 12, 2021, in the journal Scientific Reports, researchers used autoradiography, an imaging technique, 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 mongoose and red-bellied lemurs, along with five promiscuous species in the same genus. Male-female partners of mongoose lemurs and red-bellied lemurs stick together year after year, working side by side to raise their young and defend their territories. Once bonded, pairs spend much of their waking hours grooming each other or sitting with their tails wrapped around each other’s bodies.
Comparing the brain images of lemurs with previous results in voles and monkeys revealed some noticeable differences in the density and distribution of hormone receptors. In other words, oxytocin and vasopressin appear to act on different parts of the brain in lemurs, which means that they may also have different effects, depending on the target cell’s location. But within lemurs, the researchers were surprised to find few consistent differences between monogamous species and promiscuous ones. They didn’t see evidence of a pair-bond circuit akin to that found in rodent brains.
So, what can lemurs teach us about love? Oxytocin may be the “potion of devotion” for voles, say the researchers, but it may be the combined actions and interactions of multiple brain chemicals—along with ecological factors—that create long-lasting bonds in lemurs and other primates, including us.
Moving down multiple paths to love
There are probably a number of different ways through which monogamy is incorporated within the brain, and it most likely depends on what animals we’re looking at. The brain circuitry that makes love last in some species may not be the same in others. Love, I say, is complicated.
Apparently, there are many roads that lead to long-lasting love. And comparing our 3 to 5 percent fidelity rate to the birds’ 90 percent, we must be relative newbies at embarking on that kind of trip.
So, this Valentine’s Day, don’t be too hard on yourself. Other animals may just be far more experienced at long-lasting love than we are.
Here’s to finding your true places and natural habitats,