Communication Sounds of Wild Narrow-Ridged Finless Porpoises (Neophocaena asiaeorientalis) in Ise Bay, Japan

Captive condition sometimes evokes behaviors that are not observed in animals in the wild. A confirmation of the behaviors in the wild is needed to exclude the possibility of captivity-specific behaviors. Narrow-ridged finless porpoises are reported to produce two sound types for communication: "packet sound", which is a series of pulses with repeated pulse packets (a sound series consisting of a few pulses with irregular, short inter-pulse intervals), and "burst pulses", which are a pulse train with significantly shorter inter-pulse intervals compared to packet sound. Burst pulses were reported from wild narrow-ridged finless porpoises of Seto Inland Sea–Hibiki Nada population in Japan, while the packet sound was only recorded in captive conditions. Here, we report the packet sound as well as burst pulses from wild narrow-ridged finless porpoises of Ise–Mikawa Bays population in Japan. We found six packet sounds and six burst pulses out of 5 h recording of a self-contained underwater sound recorder, at a frequency of 0.020 times/min for each sound type. It is suggested that packet sound is produced by not only captive but wild narrow-ridged finless porpoises. Future studies should focus on the behavioral state when producing communication sounds.