Dorsal fin mark changes for assigning sexes to individual bottlenose dolphins, Tursiops truncatus

Abstract

. Sexing cetaceans typically requires extended periods of observation or expensive molecular methods. An alternative approach using photo‐identification may provide a cost‐effective, noninvasive method for assigning a sex to free‐ranging individuals. We investigated two methods for predicting the sex of common bottlenose dolphins (Tursiops truncatus) from Namibia, using the rate of change in dorsal fin (1) trailing edge marks or (2) surface features (scarring and lesions) and binary logistic regression models. Our results suggest animals with frequent and extensive changes to dorsal edge marks, or a high number/proportion of the dorsal fin covered in surface features are likely male, while those with little to no changes in dorsal edge marks or low numbers/proportion of surface features are likely female. Both sex prediction models performed reasonably well (accuracy: edge marks: 100%, surface features: 83%), with a high degree of consensus between models in sex predictions (78.6%). Our models are straightforward to implement and robust for the study population, providing useful insights into individuals' sex and population demographics. This reliable, inexpensive, and minimally invasive framework for assigning sex could be widely applicable, if population‐specific information of known sex individuals is included, due to the variable social systems and behaviors exhibited by Tursiops spp.

Season, age, sex, and location impact the density of tooth rake mark and dorsal fin notch of Indo‐Pacific humpback dolphins (Sousa chinensis) in the northern South China Sea

“…Variables included fin shape, scarring and nicks, and epidermal lesions with scarring being one of the most important predictors of sex. Several recent studies on other species validated the reliability of using tooth rake marks as an indicator of sex (e.g., Cuvier's beaked whales, Ziphius cavirostris; Coomber et al, 2016; Indo-Pacific humpback dolphins, S. chinensis; Wright et al, 2017; Australian humpback dolphins, S. sahulensis; Brown et al, 2016; common bottlenose dolphins, T. truncatus; James et al, 2022). A previous study of sexual dimorphism in common bottlenose dolphins revealed that the number of scars but not the surface covered by scars was higher in males than in females (Tolley et al, 1995), showing the need to test different parameters to find the one(s) best predicting the sex of dolphins for each species or even each population.…”