High-resolution X-ray computed tomography scanning of primate copulatory plugs

In this study, high-resolution computed tomography X-ray scanning was used to scan ring-tailed lemur (Lemur catta) copulatory plugs. This method produced accurate measures of plug volume and surface area, but was not useful for visualizing plug internal structure. Copulatory plug size was of interest because it may relate to male fertilization success. Copulatory plugs form from coagulated ejaculate, and are routinely displaced in this species by the penis of a subsequent mate during copulation (Parga [2003] Int. J. Primatol. 24:889-899). Because one potential function of these plugs may be to preclude or delay other males' successful insemination of females, we tested the hypothesis that larger plugs are more difficult for subsequent males to displace. Plugs were collected opportunistically upon displacement during data collection on L. catta mating behavior on St. Catherines Island, Georgia (USA) during two subsequent breeding seasons. Copulatory plugs exhibited a wide range of volumes: 1,758-5,013.6 mm3 (n = 9). Intraindividual differences in plug volume were sometimes greater than interindividual differences. Contrary to predictions, larger plugs were not more time-consuming for males to displace via penile intromission during copulation. Nor were plugs with longer vaginal residence times notably smaller than plugs with shorter residence times, as might be expected if plugs disintegrate while releasing sperm (Asdell [1946] Patterns of Mammalian Reproduction; Ithaca: Comstock). We found a significant inverse correlation between number of copulatory mounts leading to ejaculation and copulatory plug volume. This may indicate that if males are sufficiently sexually aroused to reach ejaculation in fewer mounts, they tend to produce ejaculates of greater volume.