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.
According to the researchers, females try to delay having more babies while they're still in the throes of tending to a slowly developing infant. Rival males, then, try to kill the baby to induce the mother to conceive again, with them.
To reach this conclusion, the researchers, from University College London, the University of Manchester, Oxford University, and the University of Auckland, gathered data on some 230 primate species.
They plotted the species on a sort of family tree, reflecting the evolutionary relationship between them.
Through statistical analysis of the tree, the team was able to determine a timeline of which different behaviors evolved together over time, and which appeared first.
According to this timeline, infanticide from rival males drove fathers to stick around to protect their young, leading to the switch from multiple mating partners to just one.
But 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 Tim Clutton-Brock.
By classifying some 2,500 mammalian species as either solitary, socially monogamous, or group living, they found monogamy tended to appear in species whose food sources were spread out -- like meat and fruits -- where the animal has to range over wide distances to find enough.
These species -- which included several types of rodents, many kinds of primates and some carnivores, including jackals, wolves, and meerkats -- also tended to have low density of females and low levels of home-range overlap.
But unlike in the PNAS study, the University of Cambridge researchers said they did not include humans in their analysis, and they are skeptical their results apply to homo sapiens.
"Humans are such unusual animals, depending so excessively on culture, which changes so many of the ground rules of evolution," Clutton-Brock added during a telephone press conference.