Monogamy.

What makes one species pair off, while members of a closely related species play the field?

The answer may lie in their genes.

Researchers at the University of Texas at Austin were interested in how complex characteristics arise during evolution.

"We chose to investigate this question using monogamous mating systems because animals with monogamous mating systems are available in all of the different vertebrate clades."Rebecca Young,a research associate and evolutionary biologist,who led the study.

"And we were able to find species that had independently evolved monogamy in each of these lineages."Young's colleague Hans Hofmann,professor of integrative biology, adds: "So we decided early on that we didn't just want to study a particular group of animals, like mice or fish for example or a particular group of birds, and compare between monogamy or non-monogamy there. But instead take a very broad look across vertebrates, across 450 million years of evolution when these fish and birds and frogs and us shared the last common ancestor." The researchers chose five pairs of species…and looked to see if they could spot a signature pattern of gene activity that was shared only by animals that were monogamous.

And they discovered a set of 24 genes whose activity in the brain is strongly associated with monogamy…including genes involved in neural development,learning and memory,and cognition.

The results appear in the Proceedings of the National Academy of Sciences.

"And again this is surprising because they've evolved monogamy independently. And their, the species have diverged for hundreds of millions of years from one another. So we might expect because of this distance, evolutionary distance, that gene expression in the brain would be quite different. But in fact we find this shared signature that seems to be related to the mating system of the organism. Now, those genes may not be setting up entirely new patterns of behavior. They may just be building on underlying mechanisms that all species share. Take for example, pair bonding. "To form a pair bond,one has to tolerate another individual for a long period of time." Yet, even members of the most intolerant species have to put up with one another…at least for as long as it takes to get the mating done. "Shrews is a great example…so they tolerate each other for about one day a year.

So those mechanisms already exist in very aggressive species.