Studying shape in sexual signals: the case of primate sexual swellings

Abstract

Despite extensive research on animal signals, their shape has been largely overlooked compared to other components such as size or colour. This may represent a substantial gap in our understanding of animal communication, since shape perception is believed to influence various processes in behavioural ecology, from prey–predator interactions to mate recognition. The technical challenge of measuring shape may explain this bias. This study introduces a morphometric method for the analysis of shape in animal signals and applies it to the study of patterns of shape variation in a classical sexual signal: the sexual swellings of female primates. Using elliptic Fourier descriptors (EFDs), we derived quantitative estimates of the two-dimensional shapes of sexual swellings in two primate populations: wild chacma baboons (Papio ursinus) from Namibia and captive mandrills (Mandrillus sphinx) from Gabon. Despite intra-specific variability, the two species exhibited consistently different swelling shapes. Within species, our analysis further showed more variation in swelling shape between females than across consecutive oestrous cycles of the same female. Using human judges, we confirmed that individual shape differences were visually detectable within both species. Finally, the relationships between individual traits and swelling shape were investigated, revealing age-associated variation in swelling shape in both species. Our study illustrates the high potentialities of EFDs to analyse patterns of shape variation at various scales: not only between species but also between and within individuals.