Stable affiliative social relationships among captive narrow-ridged finless porpoises

Abstract

It is challenging to study the social relationships of wild narrow-ridged finless porpoises because they do not have the dorsal fin or clear body coloration patterns usually used by researchers for individual identification to record interactions among individuals. Here, we investigated the affiliative and aggressive behaviors of individually identified captive narrow-ridged finless porpoises (five males and six females) in Toba Aquarium and Minamichita Beachland Aquarium, Japan, using 437 hours (54 days) of visual observations and video recordings to reveal their social relationships and the robustness of the relationships between individuals. We found a bout structure in the back-rubbing behavior and defined the bout as successive behaviors occurring within 2 min. Various social relationships were found, including three long-term stable affiliative pairs that were independent of sex and age. Within the three long-term stable affiliative pairs, one pair maintained the affiliative relationship after a social disturbance, defined as an event, in which the individuals in the same tank were changed, and another pair restored their affiliative relationship following one day of aggressive behavior after the social disturbance. These results suggest that several captive narrow-ridged finless porpoises have robust and stable affiliative social relationships that are not affected by social disturbance.