What do prairie voles, water pipits, and humans have in common? Individuals of each species forge monogamous pairings. 

While they might have very little else in common, at least two of these animals share a bunch of genes that encourage this form of romantic bonding. That is according to a study recently published in the Proceedings of the National Academy of Sciences.

Researchers from the University of Texas (UT) at Austin came to this conclusion after analyzing the male brains of 10 different species, five that are monogamous and five that are not. Each animal defined as monogamous was compared to a close cousin who enjoys a more freewheeling lifestyle, and so loved-up prairie voles were compared to meadow voles, water pipits to dunnocks, etc. 

"Our study spans 450 million years of evolution, which is how long ago all these species shared a common ancestor," first author Rebecca Young, research associate in UT Austin's Department of Integrative Biology, said in a statement.

Interestingly, the definition of monogamy adopted by the researchers might not quite fit with what we have all come to expect with the concept – namely, your respective other will remain faithful. Extra-pair liaisons were permitted provided the species met the following criteria:

1. 1. Pairings last for a mating season or more.

2. 2. The male and female share at least some parental responsibilities.

3. 3. The male and female band together to defend their joint offspring from threats.

4. Young and her colleagues identified a cluster of 24 candidate genes (including those involved in neural development, learning and memory, and signaling between cells) that appear to be the most strongly tied to monogamous mating systems – regardless of whether the species in question was a bird, a mammal, or a fish. However, there may be up to hundreds more that are more loosely connected to the behavior.

5. This suggests that because of whatever selective pressures that make monogamy a more attractive lifestyle choice, the same evolutionary processes occur again and again across the animal kingdom. Possibly with one exception. Unlike the other monogamous species that showed decreased expression of these genes, the mimic poison frog displayed increased expression.

"Most people wouldn't expect that across 450 million years, transitions to such complex behaviors would happen the same way every time," Young added.

While there are certainly perks to monogamy (stability, certainty, and co-parenting), there are also downsides, senior author Hans Hoffman told The Guardian. Think: having to tolerate another animal for a long period of time – one that might steal your food, make you ill, or even try to hurt you. Meanwhile, offspring take up valuable resources and make their parents more of a target to predators. 

"What evolution came up with is brilliant," said Hoffman.

"When we enter into a pair bond, or have offspring we must take care of, we find it rewarding. The reward system gets hijacked. It says, ‘Hey, I love this shit.’"

The researchers cannot confirm whether these same genes are responsible for human monogamy, which may be driven by biological or sociological factors (or both). That is for future studies to find out. But Hoffman did tell Science that "we certainly would speculate that the kind of gene expression patterns… might [show up] in humans as well."