Baboon rumps signal quality of motherhood.

By comparing female baboons' rumps, a male can spot those potential mates best suited for motherhood, say researchers in England.

In females of about 10 percent of primate species, bare areas of their rears swell as they near ovulation. In olive baboons, Papio cynocephalus anubis, water retention in posterior tissues can add 12 percent to a female's weight.

For indicating motherhood potential, bigger means better, report Leah G. Domb, now in Bristol, and Mark Pagel of the University of Reading. They found that wild females with the biggest bulges give birth at earlier ages and see more of their young survive than do females with smaller attributes.

Males seem to get the idea. In Tanzania's Gombe National Park, they threaten each other and get into fights more often over big-bulge females than over small-bulge ones, the researchers say in the March 8 NATURE.

"It's a little like a peacock's tail, except in a female," Domb says. Scientists have linked paternal suitability to various flamboyant signals of males. Except for a study of feather spots on female owls (SN: 5/13/00, p. 310), Domb knows of no other evidence for such signals in females.

To explore the role of female baboons' rumps in reproduction, Domb pieced together the reproductive histories of the Gombe females. She drew on more than 30 years of detailed records from other biologists and spent a year in Tanzania observing wild males scrapping over females with various allurements.

Domb used videotape of wild baboons to measure rump bulges. For calibration, she filmed a field assistant rushing with a meter stick to the spot where the baboon had been.

The irregularly shaped rump swellings ranged from 14.7 to 24.0 centimeters long. Length--but not depth or width--correlates with reproductive success, the researchers report. Females typically bear a baby about every 2 years, but within a range of 0.15 to 0.83 births per year, females with larger rump swelling averaged higher than the smaller-rump group did.

In a commentary accompanying the report, Robin Dunbar of the University of Liverpool listed at least five notions that theorists have proposed to explain the swellings. The new data "are the first set of very conclusive evidence," he says. Still unknown is whether the reproductive payoffs of such splendid displays come with biological costs, such as increased vulnerability to predators.

Charles L. Nunn, a biologist at the University of Virginia in Charlottesville who has studied primate swellings, deems the new findings "an exciting result." But he says he remains puzzled that only the length of the swellings correlates with females' reproductive success.

Another biologist, Alan Dixson of the San Diego Zoo, also admits to being "foxed" by the importance of length, although he says the reported reproductive role of the swellings "does make sense."

He's considered primate swellings in other species and muses that other characteristics of them might also tip off suitors. For example, high-ranking female mandrills cycle through the swollen-rump stage in perhaps 10 days, but subordinate females take several weeks. Could speed of swelling rather than size indicate maternal potential in mandrills? "I think it would be great if we could now have people go out and measure other primates," he says.