Scientists are coming closer to understanding the evolutionary reason behind monogamy, with two new studies out on Monday exploring different advantages of the practice that pairs mates for the long haul.
A leading theory had been that men stuck around to help raise children - especially ones, as among humans, who take a long time, and a great deal of energy to rear to adulthood.
But both studies, one in the Proceedings of the National Academy of Sciences and the other in Science, determined that dads become involved parents later, after they'd already begun being monogamous.
"Paternal care evolves after monogamy is present and seems to be a consequence, rather than a cause, of the evolution of monogamy," said University of Cambridge zoologist Dieter Lukas.
"Once it does evolve," he noted, "it provides a clear benefit to the female."
However, the two teams differed in their conclusions about what brought the males and females to stay together in the first place.
In the PNAS study, the British and New Zealander researchers found the practice of monogamy helped fathers protect their vulnerable young from being murdered by rival males.
However, a second study by University of Cambridge researchers in Science used a different method to come their conclusion that monogamy appeared as a result of competition.
"Where females are widely dispersed, the best strategy for a male is to stick with one female, defend her, and make sure that he sires all her offspring," said cambridge's Tim Clutton-Brock.
"Humans are such unusual animals, depending so excessively on culture, which changes so many of the ground rules of evolution," Clutton-Brock added.