Sexual swellings in wild baboons (Papio cynocephalus anubis) at Gombe National Park, Tanzania

The females of many Old World primate species produce prominent and conspicuous swellings of the perineal skin around the time of ovulation. The function of these swellings has long been debated, with no consensus on their function to date. This study tested the hypothesis that sexual swellings evolved as sexually selected traits, honestly advertising female quality, with the predictions that sexual swelling size influences mate interest, and is associated with some aspect(s) of female quality. Results are presented from thirteen months of observation of twenty-nine parous, females in five troops of olive baboons in Tanzania. Methodology was developed to quantify male behavioral interest. Swelling sizes, particularly their length and width, could be reliably assessed using photographic methods developed in this study. Reproductive histories on the study subjects were provided by Drs. Jane Goodall, Craig Packer and D. Anthony Collins from their long-term study of the baboons at Gombe.;This study showed that length of a female's swelling was correlated with three measures of female fitness. A strong negative association was found between the length of a female's sexual swellings and her age at sexual maturity. Females with longer swellings also produced both more offspring and more surviving offspring per year than females with smaller swellings. Females from higher-ranked families had longer swellings in most troops, the one exception being an unstable troop that fissioned at the end of the study. In addition, this study demonstrated that troops with longer swellings also showed faster growth and reproductive rates. Finally, male baboons were found to be more interested in females with longer swellings, fighting more aggressively to consort them, and spending more time grooming them.;From these results it is concluded that sexual swellings in wild baboons reliably advertise female reproductive quality. These results demonstrate an unusual case of a sexually selected trait in females, and show how male mate choice based on the size of the sexual swelling increases the chances that males mate females more likely to produce surviving offspring. A review and discussion of honesty in signalling is provided, along with a discussion of sexual signaling in human females.