Amanda Onion writes in that monogamy may be good for a marriage, but in some animals, it’s bad for the species. Researchers in Ghana have discovered that animals living on reserves, with access to fewer mates than they would have in the wild, have a higher risk of extinction.

“In avoiding extinction, it pays to be promiscuous,” says biologist Justin Brashares. He analyzed the population levels of large mammals in six reserves in Ghana, where rangers have kept careful records for more than 30 years. In that time, 78 species became locally extinct, mostly due to hunting and loss of habitat. But it was monogamy that dealt the final blow: the males had only a small group of females available to mate with.

A small, monogamous antelope called the duiker died out in captivity 10 years after they were placed in one of the reserves. Colobus monkeys, which have two or three mates, also didn’t survive. But African buffalo, with harems of around 15 females, are still thriving. Polygamous green monkeys and baboons are doing fine as well.

This may be why monogamy is rare in the animal world. Among 4,000 species of mammals, fewer than 3% are truly monogamous. Are we part of that group? Recent surveys show that 40% of men and 30% of women in the U.S. admit to having had at least one extramarital affair, and more than 80% of human societies throughout the world practice polygamy. Psychologist David Barash says, “People have all the fingerprints of a mildly polygamous species.”

Birds were once considered monogamous, but now we know they regularly sneak out of the nest for extramarital sex. “We started using biochemical techniques to do paternity testing in birds and the results were flabbergasting,” says biologist David Westneat. “Species that were socially monogamous sometimes have 10 to 55% of their offspring sired by someone else.”

So why does monogamy exist at all? Often monogamous animals?like humans?have children that need a lot of parenting, and only monogamy produces the necessary cooperation. And it may not be entirely to blame for extinction. Brashares says, “There are a lot of factors associated with monogamy that could lead to a higher death rate. I’m not sure you can blame it all on monogamy.” Meanwhile, do you know any blue-eyed parents with a brown-eyed child?

Those of us who believe in monogamy need to learn how to read the right signs.

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