BioWorld International Correspondent

LONDON - The extent to which a female baboon's bottom swells when she is in estrus can predict her future ability to reproduce, research on wild baboons has shown. Baboons with larger swellings reached sexual maturity earlier and were able to breed more frequently. They also had more offspring, more of which survived.

Male baboons seem to be able to "read" this information. Leah Domb, formerly of Harvard University, and Mark Pagel, of the University of Reading in Reading, UK, who carried out the study, also found that males fought more fiercely with each other for the chance to mate with females with larger swellings. They report their findings in a letter to Nature on March 8, 2001, titled "Sexual swellings advertise female quality in wild baboons."

In common with many Old World monkeys, female baboons develop large red swellings on their bottoms around the time of ovulation. These are sometimes so large as to make it difficult for the animals to sit down. The Victorians found the sight so embarrassing that they often banned females in this condition from public view.

Sexual ornaments of this kind - the peacock's tail, the nightingale's song, the stag's antlers - are well known among male animals. In some cases, such as the peacock, researchers have managed to show that peahens prefer males with larger tails that have more eye spots, and that offspring of peacocks with more elaborate tails are more likely to survive than those of less well endowed males. When Domb and Pagel started their study, however, no one had looked at a mammal or bird to find out the role of female secondary sexual characteristics.

Domb told BioWorld International, "This is a ground-breaking finding because it's new to find this type of trait being displayed in females. Normally, males will fight vigorously over any female that comes into estrus, so there should not be any need for females to produce elaborate signals. This discovery is exciting because we found that females are doing the same thing as males in their display."

Domb observed a population of wild olive baboons (Papio cynocephalus anubis) living in Gombe National Park in Tanzania. She measured the length of females' swellings, using a video technique, and compared that with information from long-term demographic records.

She found that females with larger swellings began to reproduce at an earlier age, that they produced a larger number of offspring each year, and that more of these offspring survived each year. Overall, a greater proportion of the offspring of females with larger swellings survived. All of these observations were statistically significant.

The researchers also showed that males were more interested in females with larger swellings. Males who consorted with females who had larger swellings had to endure significantly more aggression from other males than males consorting with females with smaller swellings. Females with large swellings also received more grooming from their male consorts.

Writing in Nature, Domb and Pagel conclude: "Together, our results show that sexual swellings advertise females' long-term reproductive value, and that males use the swellings to determine their mating effort. Males are more interested in and expend greater effort consorting females whose large swellings advertise a higher reproductive rate, and whose offspring have a higher probability of survival."

Domb noted that her findings are particularly interesting because humans may be among the handful of species that have similar sexually selected traits, in the form of secondary sexual characteristics. Permanent fatty breasts are not required for milk production, she pointed out, nor is the distribution of fat on the hips and buttocks in women related to their ability to bear children. For this reason, she said, some researchers have argued that these characteristics could be signaling reproductive fitness to potential mates.

So is it possible that large breasts and buttocks in human females indicate their reproductive potential? "There is some evidence to support this idea," Domb said, "but it's not conclusive. Some studies have found that women with larger breasts reached menarche [the onset of menstruation] earlier, and it has been shown, as in my study, that individuals that experience menarche earlier can have higher measures of reproductive success, with the possibility that they are more likely to ovulate during each menstrual cycle for the rest of their lives."

Intriguingly, as mentioned in the Nature paper, men of the Kipsigi tribe in Kenya pay higher bride prices for women who reach menarche earlier, even though they are not aware of the ages at which their brides' experienced menarche.

Domb added, "Other research has shown that in artificial insemination programs, women with a lower hip to waist ratio conceived quicker than women with a larger waists and a more male-like figure. Also, women with a lower hip to waist ratio reached menarche earlier, and married women with a lower hip to waist ratio had their first child earlier than women with a higher hip to waist ratio."

If research in baboons is anything to go by, perhaps women with an hour-glass figure like that of Marilyn Monroe are giving an honest indication to men of their potential abilities to produce and rear children.