Female baboons' big red bottoms are not just showy ploys to attract mates, according to new research completed by Leah Domb through Harvard's bio-anthropology department and recently released in last week's issue of Nature magazine--the swelling during mating time actually serves to predict a female's fertility.

Baboons are one of the few ape species whose reproductive area swells during ovulation. Biologists used to view the swelling of the baboon

behind as just another example of a sexual trait evolved to attract

mates--similar to the decorative tales of male peacocks.

But Domb's research has demonstrated that the baboons' swelling is more than just decoration.

"Our study has shown that females are producing an honest signal of their

reproductive quality," Domb said.

The idea is that the bigger the bottom, the better the mom, according to the study released by Domb and her partner, Mark Pagel of the British University of Reading. The signal is then picked up by the male baboons.

"Male babbons use the size of the sexual swelling to determine motherly success," Domb said in Nature.

The scientists spent 13 months in Tanzania studying olive baboons. Among the baboons studied, females with the largest bottoms reached puberty the earliest and gave birth to more offspring. These children also had a better chance of surviving.

Domb also noted that "males fought more aggressively over females with larger swellings."

Domb siad the reason baboons publicize their fertility probably has to with the high rates of infanticide in the baboon population.

If females can mate with as many baboons as possible, there will be doubt about the paternity of the offspring. And baboon males are less likely to kill a baby they think is their own.

"Female baboons may be trying to mate with as many males as possible to confuse paternity and therefore avoid infanticidal action from nonfathers," Domb said.

Advertisting fertility is quite costly for female baboons. Swelling is so pronounced that a female can gain up to 14 percent of her body weight during ovulation. Swelling also hampers movement and makes her more vulnerable to infection.

With such high costs, the gain must be even higher.

"What needs to be examined next is what females with the largest swellings are achieving: are they mating with more males, the best males, or a better combination of the two across their fecund period?" Domb said.