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 isoften challenging. This work aimed to apply the principles of skin mark analysis to a cetacean species to test its ability to identify sexdifferences in markings and build a sex-discrimination model exclusively based on photographic material. The common bottlenosedolphin (Tursiops truncatus) was chosen as the model species, and skin marks and their progression over time were identifiedand measured across the whole-body surface of individuals on photographs acquired between 2016 and 2023 in the Tiber RiverEstuary area. Interactions with fisheries were quantified both as the proportion of encounters in presence of trawlers, and as aprobability of fishing gear injuries. Non-parametric tests were used to assess sex-specific differences. A generalized linear modelwith binary response (sex) was built through a stepwise procedure, using mark-based indices as predictors. Relevant differencesbetween 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 thestatistical tests and had a high accuracy and sensitivity toward both sexes. These results highlight the feasibility and efficacyof this methodological approach to sex-ratio investigations and identify differences in anthropic pressures insisting on males andfemales, with important conservation implications. The exclusive use of photographic material makes this methodology potentiallyapplicable and adaptable to multiple species and environments, aiding the efforts of conservationists worldwide.