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Analysis of the natural markings of Risso's dolphins (Grampus griseus) in the central Mediterranean Sea

Risso's dolphins are known for the persistency of their natural markings, possibly due to the loss of pigment during the healing process of skin wounds. Nonetheless, the actual longevity and reliability of each mark type has never been assessed. In this paper, we used photographs to investigate the etiology of skin marks in the species, analyze their distribution and temporal variability, and discuss implications for photo identification. Nineteen mark types were described on the dorsal fin of Risso's dolphin, including 2 new to the literature: the snake-like mark and the protruding fat. Longevity of skin marks ranged from 6 years for the protruding fat to several decades for scrapes and dots. Persistent and reliable marks were notch, tooth-rake, and thick single and parallel scrapes. Mark change was sufficiently low that all mark types could be used for photo identification, provided that backlit or underexposed images were discarded as photographs taken under suboptimal light conditions proved to be unreliable. Finally, mark distribution and variability were unequal between age classes; juveniles were less marked and showed a higher rate of mark change than older individuals so that, even if they possessed enough notches to be classified as reliably marked, they could be confidently matched over a time interval of up to 3 years.