Monogamy is a rare thing among mammals, but a few of the old romantics include specific species of bats, wolves, beavers, and lemurs. The latter is of special interest to science as they are a closely related primate species to humans – the distant cousins of humans that once occupied the island Madagascar – and they can be directly compared to other members of the lemur family who aren’t monogamous.
With this in mind, a new study published in the journal Scientific Reports decided to investigate the brain activity of monogamous and promiscuous lemur species. In the monogamous group, we have red-bellied and mongoose lemurs. Couples in this group stay together year on year, working as a team to raise young and defend their territory. Bonded lemurs are easy to spot, spending a lot of time doing each other’s fur and huddling together with their tails wrapped around one another. Some of their genus pals, however, are not so sentimental, chopping and changing partners as and when they please.
Voles proved a pivotal group of animals in the understanding of monogamy, as studying the brains of the mate-for-life varieties revealed that they had more receptors for oxytocin and vasopressin – aka the cuddle chemicals. These hormones are released during mating, and the fact that the monogamous voles’ more promiscuous relatives didn’t have as many receptors showed they were integral to lasting love in these mammals.
The team on this new research wanted to see if the male-female bonds of humans might be influenced in the same way as voles, and so turned to lemurs, who are a closer genetic match to us. Using autoradiography, they mapped the binding sites for oxytocin and vasopressin in the brains of 12 lemurs – some monogamous, some not – that had died of natural causes at the Duke Lemur Center.
When they compared their findings to those of the vole study, they picked up on some significant differences. Oxytocin and vasopressin acted on different parts of the brain in lemurs, which may or may not alter their influence on the body. As well as being different from the voles, the monogamous lemurs’ brains didn’t seem to be all that different from their promiscuous relatives either.
"We don't see evidence of a pair-bond circuit," said lead author of the study Nicholas Grebe, postdoctoral associate at Duke University, in a statement. "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. There's more going on than we originally thought."
The team next hopes to investigate how blocking the cuddle chemicals changes lemur couple’s behavior towards each other, if at all, by temporarily preventing the hormones from binding to receptors in the brain using medication. For now, the exact recipe for the "potion of devotion" remains unclear.
