Female sexual swellings in the Asian colobine Simias concolor

During a preliminary study of pig‐tailed langurs ( Simias concolor ) in the Pagai Islands, Indonesia, it was discovered that females exhibited conspicuous swelling of the urogenital triangle. The pig‐tailed langur is the first Asiatic colobine found to have prominent sexual swellings and the only colobine with sexual swellings that lives in one‐male groups. Because all anthropoids with conspicuous sex skin typically live in groups having female‐biased adult sex ratios, it is possible that females might compete amongst themselves for the male in one‐male groups, or the best males in multimale groups. Sexual swellings may therefore have resulted from sexual selection for signals attractive to males in female‐female competition, as suggested earlier by Bercovitch (California Anthropologist 8:9–12, 1978). Prolonged observation of recognizable individuals will be required to test this hypothesis in the pig‐tailed langur and other species.