The Bridle Mark System on Bottlenose Dolphins (Tursiops truncatus): Pigmented Facial Features Supplement Photo-Identification

Research on free-ranging animal species, which extends from the breadth of stock identification and population dynamics to fine-scaled analysis of behavior, depends on identifying individuals. Recognizing wild bottlenose dolphins (Tursiops truncatus) primarily depends on photographs of distinct patterns of notches which eventually develop on the trailing edge of most dorsal fins. Although photo-identification has advanced to include computer-aided fin identification software, there can still be error related to dolphins whose identifying patterns are subtle, obscured, or absent ("clean fins"). Facial pigment patterns on dolphin species were described as bridle marks over a hundred years ago. The purpose of this descriptive study was to evaluate bridle mark presence, distinctiveness, and permanence as a supplement to dorsal fin photo-identification. A 17-year population census of 860 bottlenose dolphins in Palm Beach, Florida, USA, includes 458 dolphins with photographic records showing bridle marks. A subset of 30 dolphins, based on an encounter history spanning at least 7 years, was coded using a 17-point system. The results confirmed bottlenose dolphin bridle mark distinctiveness and permanence on all 30 dolphins. In conclusion, bridle marks can supplement photo-identification as a double-mark system to improve accuracy. This tool is helpful for recognizing calves with clean dorsal fins post weaning and adults without dorsal fin markings.