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FILE - This Aug. 29, 2008 file photo shows a yellow-cheeked crested gibbon sitting in a cage at Cambodia's Phnom Tamau Zoo in Takeo province, south of Phnom Penh, Cambodia. Two different studies looking at why monogamy evolved in some species, pointed at separate reasons. Gibbons are one of the monogamous species among the world s mammals and chimpanzees are at the other end of the spectrum and wildly promiscuous, said anthropology researcher Kit Opie of University College London, an author of one of the studies.

The answers aren't even the same.



So it's not about romance, said researcher Dieter Lukas of the University of Cambridge, lead author of the mammals study. "It's just really the best he can do."


But both teams discounted a long-standing explanation for monogamy, that it provides two parents rather than one for rearing offspring. That's just a side benefit, they said.





Researchers said they hesitated to apply their conclusions to humans, and they acknowledged that their results aren't exactly the stuff of Valentine's Day.

Less than 9 percent of mammal species pair up socially.




Why? Because primates breast-feed their offspring for a long time, even for years, and competing males kill off infants if the dad doesn't stick around to fight them off.




So they stick around and monogamy occurs.

Frans de Waal of Emory University, who wasn't part of either team, said he thought the Opie infanticide paper offered quantifiable support for that theory, but he wasn't sold completely.


Both teams did agree that they wouldn't quite put humans in the monogamous category.





Proceedings of the National Academy of Sciences:

The journal Science:


Seth Borenstein can be followed at