What makes a creature commit to one sexual partner? Evolutionary biologists have trawled through 450 million years of genetic history and found one genetic formula, across several different species, that appears to have turned non-monogamous animals monogamous.

The definition of monogamy in the animal kingdom is slightly more flexible than in the human dating terrain: The authors of the paper, published on January 7 in the journal PNAS (paywall), consider an animal monogamous if it stays with a mate for at least one mating season, and the two jointly raise their offspring. An occasional sexual rendezvous with another partner does not preclude an animal from being deemed monogamous, according to these biologists.

The researchers identified the monogamous gene expression by studying five pairs of species, where one species was monogamous, and the other non-monogamous. These 10 total animals encompass a wide variety: the researchers studies two different species of mice, two species of voles, as well as two species of bird, two frogs, and two cichlid fish. And, in each pair, one species become monogamous in evolutionary history, while the other, closely related species, stayed non-monogamous.

In all five cases, despite the wide variety of animals studied, scientists found the same changes in gene expression as a species became monogamous. Essentially, they found that the monogamous species shared certain genes with increased activity, and some genes with decreased activity. They theorize that the evolution of monogamy is manifest through these particular genes, across the species.

“Most people wouldn’t expect that across 450 million years, transitions to such complex behaviors would happen the same way every time,” Rebecca Young, research associate in UT Austin’s Department of Integrative Biology and first author of the study, told Psych.org. Of course, there’s a huge leap between monogamy in animals and how the behavior is expressed and understood among humans. But the research certainly suggests that there’s a genetic element to this social behavior. Animals’ monogamous instincts aren’t simply a reflection of personal choice, it seems, but a reflection of their genes.