Copulatory Fit of Common Bottlenose Dolphin (Tursiops truncatus) Genitalia

Abstract

Sexual selection is the primary force that drives gross morphological variation of male genitalia in many species. Less information is known about how male and female genitalia fit together during copulation or how sexual selection and mechanical constraints during copulation interact and shape reproductive organs. Marine mammals offer important insights into evolutionary forces that act on genital morphology in both sexes because of adaptations to extreme environmental constraints (e.g., hydrodynamic drag forces acting on the penis during copulation while swimming, the necessity to prevent seawater from entering the uterus, and the challenge of successful intromission while moving in a 3‐D space). Cetaceans (whales, dolphins, and porpoises) have a fibroelastic penis type and their vaginas have muscular folds, both characteristics that are rare among mammals and unique to cetartiodactlys. We used excised male and female reproductive tracts from deceased adult common bottlenose dolphins (Tursiops truncatus) to assess how deep the penis penetrates the vagina, which anatomical landmarks of male genitalia contact morphological features of females, and how genital shape influences copulatory interactions. Frozen‐thawed vaginas remained flaccid and were stained with iodine, while penises were mechanically inflated and formalin‐fixed to maintain rigidity. Female and male reproductive tracts were ct‐scanned independently. The penis was then inflated inside the vagina in a position that approximates real intromission and the reproductive tracts were ct‐scanned together. Resulting images were reconstructed in transverse and sagittal planes. In conjunction with video observations of copulations by free‐swimming dolphins, our results suggest that the tip of the bottlenose dolphin penis navigates around the female's single vaginal fold to achieve successful insemination. Understanding the coevolution of male and female reproductive anatomy under extreme environmental constraints has broad applications to conservation and captive breeding programs. Copulatory fit can provide clues of how genitals interact to influence fertility and improve artificial insemination techniques.