Social structure and site fidelity of an oceanic delphinid: the rough-toothed dolphin (Steno bredanensis) off the southwest Pacific coast of Mexico

Rough-toothed dolphins (Steno bredanensis) typically inhabit deep oceanic waters from tropical to temperate waters worldwide. However, their ecology and behavior remain poorly understood. We investigated site fidelity and social structure of rough-toothed dolphins using photo-identification data collected between 2014 and 2022 in the continental shelf waters of the southwestern coast of Mexico. A total of 133 groups of rough-toothed dolphins were recorded. Mean group size was 4.9 individuals (SD = 4.5), and dolphins were encountered in waters of a mean depth of 110 m (SD = 188). Of 228 individuals identified, 55% were observed once, 39% were seen between two and four times, and 6% were seen five times or more. Site fidelity analysis suggests three levels of site fidelity: transients, occasional visitors, and regular visitors, which are likely to affect the social structure. Through network analysis, nonrandom social patterns were identified (S = 1.25 ± 0.26) and dolphins exhibited both preferences and avoidances of social partners. Cluster analysis suggested significant divisions within the population (Q = 0.43 ± 0.05). Despite limitations related to sample size and design, this preliminary study suggests that rough-toothed dolphins are best characterized by a fission-fusion society, which is similar to most coastal delphinid populations.