I beg your pardon? Acoustic behaviour of a wild solitary common dolphin who interacts with harbour porpoises

Kylie is a solitary common dolphin who inhabits a restricted area within the Firth of Clyde (Scotland). She spends most of her time around navigational buoys in the Hunterston/Fairlie channel, where she has been seen interacting with harbour porpoises. Recordings from 2016 and 2017 were used to study her acoustic behaviour when seen alone and with a porpoise. Clicks were classified as potential porpoise or dolphin clicks based on the waveform, power spectrum, and spectrogram, as well as direction of arrival, inter-click interval, amplitude, and centroid frequency variations. Kylie emitted clicks exclusively, which were of variable nature, including low, mid, and high-frequency (HF, centroid frequency >100 kHz) as well as broad or narrowband. Some of Kylie’s HF clicks were similar to porpoise clicks both in the time (e.g. polycyclic) and frequency (e.g. narrowband with most energy between 100 and 150 kHz) domains, which cannot be explained by physical phenomenon alone.