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Why are some mammals monogamous? Two definitive studies not so definitive.

One says some mammals are monogamous to prevent infanticide. The other says they are monogamous because females are spread too widely for males to have multiple mates.

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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. Gibbons are one of the monogamous species among the world’s mammals.

Heng Sinith/AP/File

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Wolves do it, beavers do it, even some termites pair off as monogamous couples to procreate.

Now, two teams working independently have offered competing explanations for why some mammal species, including those among primates, stick with one mate.

One team analyzed patterns in the evolution of reproductive behavior in more than 2,500 mammal species, including primates, and found that monogamy most likely arose as a male response to breeding females' penchant for guarding their turf.

Females needed access to relatively scarce, high-qualify food, to help ensure their reproductive success, the team holds. That scarcity led them to defend their territory from other females. Because females were distributed over a wide area, it would have been too difficult for a male to gather and defend a harem. So, he would settle down as the sole partner for a female, who would share her territory with him for more than a year.

"In mammals, social monogamy is a consequence of resource defense," said Dieter Lukas, a zoologist at Cambridge University in Britain, during a press briefing Monday. "Female behavior is influenced by the distribution of food. And male behavior is influenced by the distribution of females."

That holds true for primates, as well as other mammals, he said.

Dr. Lukas conducted the study with Cambridge University colleague and evolutionary biologist Tim Clutton-Brock. A formal description of the work and results are set for publication Friday in the journal Science.

A second team, led by Kit Opie, an anthropologist from University College London, acknowledges a role for territorial-defense among breeding females in the evolution of social monogamy in its study group – 230 species of primates.

But, the researchers argue, that piece of the puzzle fell into place later. The main driver behind social monogamy, they say, was a need to have a male on hand to reduce the risk of infanticide by another male trying to mate with a female while she is still nursing an earlier offspring.

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