Aversive behavioural responses of killer whales to sounds of long-finned pilot whales.

Interactions are common among marine species which use sound as the primary form of communication, yet the role of acoustic signals in mediating these interactions remains poorly understood. Long-finned pilot whales (Globicephala melas) are attracted to killer whale (Orcinus orca) sounds, leading to antagonistic interactions. To test whether these interactions are acoustically mediated in both directions, playback experiments (n = 15) using pilot whale and control sound stimuli were conducted on eight killer whales equipped with multi-sensor tags. To assess behavioural responses to the playbacks, we applied hidden Markov models (HMMs) to movement and acoustic data, and fitted univariate regression models to a horizontal movement reaction score, calling rate, and group behaviour variables. The tagged whales exhibited an avoidance response to pilot whale sounds, evidenced by fast, directed movement away from the sound source and increased cohesion and alignment of group members. Calling rate often increased initially, followed by a pronounced decrease. These findings demonstrate that killer whales, the oceans' apex predators, respond to acoustic signals of pilot whales and likely perceive their presence as a threat, similar to naval sonar. This study provides important insights into the complexity of cetacean behaviour and acoustic mechanisms shaping multi-species community dynamics.