Modeling Skin Mark Temporal Variations to Estimate the Sex of Delphinids with Low Sexual Dimorphism: Implications for Management and Conservation.

Sex ratio is an important population metric in animal conservation, but its study in marine species with low sexual dimorphism is often challenging. This work aimed to apply the principles of skin mark analysis to a cetacean species to test its ability to identify sex differences in markings and build a sex-discrimination model exclusively based on photographic material. The common bottlenose dolphin (Tursiops truncatus) was chosen as the model species, and skin marks and their progression over time were identified and measured across the whole-body surface of individuals on photographs acquired between 2016 and 2023 in the Tiber River Estuary area. Interactions with fisheries were quantified both as the proportion of encounters in presence of trawlers, and as a probability of fishing gear injuries. Non-parametric tests were used to assess sex-specific differences. A generalized linear model with binary response (sex) was built through a stepwise procedure, using mark-based indices as predictors. Relevant differences between sexes emerged in markings, with males presenting more social, aggression-related, and fishing-related marks overall, and a stronger tendency of mark accumulation over time. The estimated model reflected the differences highlighted by the statistical tests and had a high accuracy and sensitivity toward both sexes. These results highlight the feasibility and efficacy of this methodological approach to sex-ratio investigations and identify differences in anthropic pressures insisting on males and females, with important conservation implications. The exclusive use of photographic material makes this methodology potentially applicable and adaptable to multiple species and environments, aiding the efforts of conservationists worldwide.