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Size of sexual swellings reflects ovarian function in chimpanzees (Pan troglodytes)

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Abstract

While many theories exist on the evolution of the sexual swelling signal in female primates, the significance of the very large and variable swellings found in some catarrhine primates is still debated. As suggested by Pagel [(1994) Anim Behav 47:1333–1341], these exaggerated and costly signals have the potential to provide information about differential female reproductive quality to males. While Domb [(2000) Department of Anthropology, Harvard University, Cambridge] and Domb and Pagel [(2001) Nature 410:204–206] propose that swellings serve to reliably indicate between-female fitness differences, we hypothesize that they may indicate current reproductive cycle quality, within or between females. This study examines the between-cycle relationship between ovarian function, gauged via fecal steroid analysis, and absolute size of sexual swellings, yielded through photogrammetric methods. Fourteen cycles from eight captive, group-living chimpanzees (Pan troglodytes) exhibited considerable variability both in ovarian steroid levels and maximum sexual swelling size. Nearly all measures of a female's maximal swelling in a cycle correlated with early follicular and early luteal estradiol averages and the duration of luteal progesterone elevation. Swelling area was also correlated with late follicular estradiol. Given past evidence that such measures of ovarian function predict conceptive success, these data strongly suggest that swellings could convey information to competing males about the fecundability of a potential mate.

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Acknowledgements

Thanks to Jim Moore for his generosity in loaning M.A.E. the Nikon and Photoscale-2 photographic equipment used in this study. We also thank Katja Liebal for assistance in sample collection and Brian Hare, Signe Preuschoft, and Michael Seres, for introduction to the chimpanzees and for allowing collections during their studies. Elizabeth Russell provided valuable assistance in the Laboratory of Reproductive Ecology and Environmental Toxicology at Emory University. The authors are indebted to Cheryl Knott, Richard Wrangham, and Jeremy Dahl for helpful comments during planning stages, to C.K., R.W., Peter Ellison, David Pilbeam, Melissa Gerald, Martin Muller, Charles Nunn, and several anonymous reviewers for comments on the manuscript, and to Randall Collura and Ofer Bar-Yosef for use of their scanning equipment and expertise. This study was made possible by a NSF Graduate Research Fellowship, Mellon Foundation training grant (to M.A.E.), NIH Grant RR-00165 (to Yerkes), and Thomas Gordon and the animal care staff at the Yerkes Regional Primate Research Center field station. The Yerkes Primate Center and this research project are approved by the American Association for the Accreditation of Laboratory Animal Care.

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Emery, M.A., Whitten, P.L. Size of sexual swellings reflects ovarian function in chimpanzees (Pan troglodytes). Behav Ecol Sociobiol 54, 340–351 (2003). https://doi.org/10.1007/s00265-003-0648-6

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