Sex-specific variation in conspecific-acquired marking prevalence among dusky dolphins (Lagenorhynchus obscurus)

Despite logistical challenges that limit direct observations of behavior for some species, physical scars can provide indirect evidence of aggression. Dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)) off Kaikoura, New Zealand, derive most of the notches and scars on their dorsal fins from conspecifics. However, aggressive encounters have rarely been observed in this sexually monomorphic species. If injurious interactions are associated with intrasexual competition among males but not females, we predicted males would have more conspecific-acquired markings than females. High-quality dorsal fin photographs of dolphins of known sex (n = 79) were collected during boat-based surveys off Kaikoura. We measured three quantitative variables (notch height ratio, notch depth ratio, and dorsal fin rake direction) and one qualitative variable that scored the overall prevalence of markings (notches and scars). All four measures yielded consistent results. Males had significantly more conspecific-acquired markings than females. We reject the hypothesis that there is no difference between the sexes in marking prevalence. We propose that brief events of direct competition are missed by observers but recorded in accumulated patterns of conspecific-acquired markings that differ between the sexes.