The females of many Old World primate species produce prominent and conspicuous swellings of the perineal skin around the time of ovulation. These sexual swellings have been proposed to increase competition among males for females1 or to increase the likelihood of a female getting fertilized, by signalling either a female's general reproductive status1,2,3,4,5, or the timing of her ovulation6. Here we show that sexual swellings in wild baboons reliably advertise a female's reproductive value over her lifetime, in accordance with a theoretical model of honest signalling7. Females with larger swellings attained sexual maturity earlier, produced both more offspring and more surviving offspring per year than females with smaller swellings, and had a higher overall proportion of their offspring survive. Male baboons use the size of the sexual swelling to determine their mating effort, fighting more aggressively to consort females with larger swellings, and spending more time grooming these females. Our results document an unusual case of a sexually selected ornament in females, and show how males, by mating selectively on the basis of the size of the sexual swelling, increase their probability of mating with females more likely to produce surviving offspring.
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We thank C. Packer and D. A. Collins for providing access to the reproductive histories of the females in this study; the Government of Tanzania for permission to carry out this study; K. Hawkes, C. Packer, R. Wrangham and D. A. Collins for comments on this work; and A. Sindimwo, M. Bwenda, I. Rukamata, M. Omari, S. Hamisi, R. Mkono, J. Bushingwa, F. Juma. and D. A. Collins for assistance in data collection at Gombe National Park. L.G.D.'s field work was supported by National Science Foundation and Wenner-Gren Foundation for Anthropological Research. Data collection at Gombe was supported by the Jane Goodall Institute. M.P. is supported by the Leverhulme Trust and the Natural Environment Research Council.
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BMC Evolutionary Biology (2012)