Determining the sex of bottlenose dolphins from Doubtful Sound using dorsal fin photographs

Sexing cetaceans usually requires time-consuming observation, or genetic sexing via biopsy sampling or skin swabbing. We developed a method to determine the sex of bottlenose dolphins (Tursiops sp.) in Doubtful Sound, Fiordland, using laser-metric dorsal fin photographs. From dorsal fin photographs of 43 bottlenose dolphins of known sex (25 females, 18 males) we analyzed the shape, proportion of fin area covered in scarring and epidermal lesions, and the number of fin nicks. Males had significantly higher rates of scarring (P < 0.001) and dorsal fin nicks (P < 0.01) than females, whereas the severity of epidermal lesions was higher in females (P < 0.05). A logistic regression applied to all measured variables, and measurements of dorsal fin size, indicated that the proportion of dorsal fin scarring (P < 0.001), number of fin nicks (P < 0.01), and dorsal fin surface area (P < 0.01) were significant variables and together correctly predicted the sex of 93% (40/43) of the dolphins. The classification function may not be applicable to other populations due to geographic variation in bottlenose dolphin morphology and social structure. The method is quick and noninvasive to apply, and further increases the value of dorsal fin photo-identification pictures.