Do Some Birds Cheat to Avoid Inbreeding?

Hillary Mayell
for National Geographic News
October 9, 2002

An international team of biologists is offering more evidence that when it comes to mate selection, genes count.

In a study of three species of monogamous shorebirds, the researchers found that "illegitimate" chicks were found overwhelmingly in the nests of partners with a high degree of genetic similarity.

"We discovered that female sandpipers 'cheat' and seek extra-pair matings if they are closely related to their mate—a behavioral adaptation that would minimize the deleterious effects of inbreeding," said Brett K. Sandercock, an avian ecologist at Kansas State University, and co-author of the study published in the October 10 issue of the journal Nature.

How are species able to recognize a genetically similar partner?

Some species—rodents and humans, for instance—can tell by smell. One study has shown that human females prefer men whose major histocompatibility complex (MHC) genes are the least similar to their own, which they can tell by body odor. The ability to make this distinction was muted among women taking birth control pills. [See sidebar.]

But among birds, how they recognize genetic incompatibility remains a mystery.

Seeking Evolutionary Success

The ability to test DNA and conclusively identify parents and their young forced a change in scientific thinking as mounting evidence showed that most species are not monogamous.

"One of the first papers to mention the fact that females in many species are actively involved in promiscuity was published in 1988," said Olivia Judson, author of a recently published book on the evolutionary biology of sex.

For years, scientists believed that the war between the sexes was based on opposing strategies to achieve reproductive success: monogamy for her, multiple partners for him.

From an evolutionary standpoint, success is having your young survive to adulthood and then reproduce themselves.

Females can give birth to only a limited number of offspring, so to enhance their chances for survival it would be to her benefit to seek a good provider—whether he's offering protection, food, or territory—and to keep him to herself. Conversely, males would reap evolutionary success by mating with as many females as possible.

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