The scientists studied the DNA of five pairs of closely related species – four mammals, two birds, two frogs and two fish — comparing monogamous and non-monogamous species. These five pairs represented the five times in vertebrate history that monogamy independently evolved.
'Our study spans 450 million years of evolution, which is how long ago all these species shared a common ancestor,' said Dr Rebecca Young, an author of the study and research associate in University of Texas at Austin's Department of Integrative Biology.
The scientists in this study defined social monogamy as a pair bond for at least one mating season, the sharing of some parental care duties and the joint defense of offspring against predators. This behaviour is not always favoured by evolution.
'Offspring are parasites. They eat you, they take your resources, they make your life more dangerous because it is easier for a predator to find you,' lead author Professor Hans Hofmann at the University of Texas at Austin told the Guardian.
With some difficulty, the international team scoured sites from African lakes to Romanian forests to obtain tissue samples from the brains of three male individuals in all ten species. The researchers performed RNA sequencing, and found that the same 24 changes in gene expression occurred in each monogamous species. The results were unexpected.
'Most people wouldn't expect that across 450 million years, transitions to such complex behaviors would happen the same way every time,' Dr Young said.
There was an exception. One poison dart frog species, Ranitomeya imitator, bucked the genetic trend. This may be linked to the fact that in this species monogamy evolved after males started caring for offspring.
'What evolution came up with is brilliant,' Professor Hofmann told the Guardian. '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 speculate that the same gene pattern may occur in humans. However, the team has not uncovered a causal link or biological mechanism between these genes and monogamy. Future studies could potentially use genome editing in animals to see if it could increase or reduce monogamous behaviours.