Biologists have long thought that baboon males prefer females with bigger backsides as the mark of a good mother but new research suggests it isn't so simple.

A Duke University study of wild baboons in southern Kenya found the size of the female's behind doesn't have as much of an influence as scientists thought it did.

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Baboons breed throughout the year, and mating occurs during times when a female's behind is swollen, a sign she may be ovulating. For 10 to 20 days each month, the tissue in a female baboon's hindquarters swells up, reaching peak size when a female is most fertile and then shrinking back to normal.

Scientists used a camera technique to monitor the baboons' swelling and mating habits. Measurements from 34 female baboons showed a variation of rump swelling ranging from four to 6.5 inches.

They found this difference in size didn't have an influence on the baboons' mothering abilities or ability to get a mate. Instead of being drawn to females with larger backsides, male baboons tended to prefer females that had cycled more times since their last pregnancy.

The researchers also recorded male courtship behavior during the time when females were swollen. They found that big-bottomed females were no more likely to attract mates than their smaller-bottomed counterparts.

"It's almost as if the males are counting. Our study suggests that, at least in part, males follow a rule along the lines of 'later is better' rather than 'bigger is better,'" said Duke University researcher Courtney Fitzpatrick.

Researchers are also planning to find out if females that have had more postpartum menstrual cycles mate with more males, and whether or not this has an influence on their offspring's chance of survival.

The findings appeared in a recent edition of the journal Animal Behavior.

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