In the animal world, monogamy has some clear perks. Living in pairs can give animals some stability and certainty in the constant struggle to reproduce and protect their young—which may be why it has evolved independently in various species. Now, an analysis of gene activity within the brains of frogs, rodents, fish, and birds suggests there may be a pattern common to monogamous creatures. Despite very different brain structures and evolutionary histories, these animals all seem to have developed monogamy by turning on and off some of the same sets of genes.
“It is quite surprising,” says Harvard University evolutionary biologist Hopi Hoekstra, who was not involved in the new work. “It suggests that there’s a sort of genomic strategy to becoming monogamous that evolution has repeatedly tapped into.”
Evolutionary biologists have proposed various benefits to so-called social monogamy, where mates pair up for at least a breeding season to care for their young and defend their territory. When potential mates are scarce or widely dispersed, for example, forming a single-pair bond can ensure they get to keep reproducing.
Neuroscientist Hans Hofmann and evolutionary biologist Rebecca Young at the University of Texas in Austin wanted to explore how the regulation of genes in the brain might have changed when a nonmonogamous species evolved to become monogamous. For example, the complex set of genes that underlie the ability to tolerate the presence of another member of one’s species presumably exists in nonmonogamous animals, but might be activated in different patterns to allow prolonged partnerships in monogamous ones.
“We wanted to be bold—and maybe a little bit crazy” in the new experiment, Hofmann says. Instead of doing a relatively straightforward genetic comparison between closely related species on either side of the monogamy divide, he and colleagues wanted to hunt down a gene activity signature associated with monogamy in males across a wide variety of species—frogs, mice, voles, birds, and fish. So in each of these groups, they selected two species, one monogamous and one nonmonogamous.
Rounding up the brains of those animals took an international team and years of effort. Hostile regional authorities and a complicated permitting system confronted the team in Romania as they tried to capture two types of a native songbird. Hofmann donned scuba gear and plunged into Africa’s Lake Tanganyika to chase finger-length cichlid fish into nets. Delicately debraining them while aboard a rocking boat, he says, was a struggle.
Back the lab, the researchers then grouped roughly comparable genes across all 10 species based on similarities in their sequences. For each of these cross-species gene groups, they measured activity based on how much the cells in the brain transcribed the DNA’s proteinmaking instructions into strands of RNA.
Among the monogamous animals, a pattern emerged. The researchers found certain sets of genes were more likely to be “turned up” or “turned down” in those creatures than in the nonmonogamous species. And they ruled out other reasons why these monogamous animals might have similar gene expression patterns, including similar environments or close evolutionary relationships.
Among the genes with increased activity in monogamous species were those involved in neural development, signaling between cells, learning, and memory, the researchers report online today in the Proceedings of the National Academy of Sciences. They speculate that genes that make the brain more adaptable—and better able to remember—might also help animals recognize their mates and find their presence rewarding.
It makes sense that genes involved in brain development and function would underlie a complex behavior like monogamy, says behavioral neuroscientist Claudio Mello of Oregon Health & Science University in Portland. But because the researchers didn’t dissect out specific brain regions and analyze their RNA production independently, they can’t describe the finely tuned patterns of gene expression in areas that are key to reproductive behavior. “It seems to me unlikely that by themselves these genes will be able to ‘explain’ this behavior,” he says.
“The fact that they got any common genes at all is interesting,” adds Lisa Stubbs, a developmental geneticist at the University of Illinois in Urbana. “It is a superb data set and an expert analysis,” she says, “[but] the authors have not actually uncovered many important biological insights into monogamy.”
The study did turn up a curious outlier. Some of the genes with decreased expression in most of the monogamous species showed increased expression in one of them—the poison dart frog Ranitomeya imitator. Young notes that in this species’s evolutionary history, fathers cared for the young before cooperative parenting evolved. As a result, these frogs may have had a different evolutionary starting point than other animals in the study, later tapping into different genes to become monogamous.
Hoekstra, who has studied the genetics of monogamy in mice, sees “a lot of exciting next steps.” There are likely mutations in other regions of DNA that regulate the expression of the genes this study identified. But it will take more work to show a causal relationship between any particular genetic sequence and monogamous behavior.
People also often opt for monogamy, albeit for a complicated set of social and cultural reasons. So, do we share the gene activity signature common to monogamous birds, fish, and frogs? “We don’t know that,” says Hofmann, but “we certainly would speculate that the kind of gene expression patterns … might [show up] in humans as well.”