Roots of Monogamy Feed Scientific Spat: Researchers Clash on How Fidelity Evolved in Mammalian Species

Article excerpt

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Why some mammalian species choose to spend their lives with the same mates has long baffled scientists--and will probably continue to do so as two new massive studies present contradictory results.

One group of researchers says monogamy evolved in primates to counter the threat of males killing babies to boost their siring success. The other team concludes that mammals, including primates, become monogamous when females live far away from one another.

The differences in the conclusions have raised eyebrows. "They do seem to be saying the opposite thing," says Anthony Di Fiore, an evolutionary anthropologist at the University of Texas at Austin. "It's interesting because they use very, very similar methods," Di Fiore says.

The two groups also disagree on whether the research has implications for why humans evolved fidelity to mates (see Back Story, Page 6).

Both teams investigated the evolution of social monogamy, which researchers define as males and females living in breeding pairs. It does not necessarily mean that each animal is always faithful and never mates outside the pair.

Social monogamy is normal for birds but rare in mammals. That's because birds of both sexes can participate in parenting duties such as incubating eggs and feeding chicks, but male mammals can't help gestate or breastfeed a baby. During the long period when a mother mammal is occupied with parenting, an opportunistic father can take off to sire more offspring with other females.

Less than 10 percent of mammal species, such as wolves and beavers, live in pairs in which the male sticks by his mate. This living arrangement is more common among primate species, about a quarter of which live in pairs.

To determine what factors drove the evolution of mammalian monogamy, Dieter Lukas and Tim Clutton-Brock from the University of Cambridge in England collected information about more than 2,500 species--nearly half of all mammals. The researchers used published reports to classify each species as monogamous or not, and then noted whether that species practices infanticide and whether the females live in discrete territories. Using this dataset, the researchers reconstructed the likely evolutionary history of mammalian monogamy.

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The team concludes in the Aug. 2 Science that monogamy evolved independently 61 times, almost always when females lived far from one another.

In those situations, Lukas says, males have difficulty mating with multiple females. By sticking with one female and guarding her against amorous advances from other males, he might produce more offspring than if he attempted to spread himself around.

The other group, led by Kit Opie of University College London, performed a similar evolutionary reconstruction but focused on 230 primate species. …