Discussion

Partial Lombard response

Our results show that pilot whales only partially compensate for increasing noise levels, a pattern consistent with previous findings in both terrestrial and cetacean species. Initially, we quantified the Lombard response using broadband noise levels, which yielded a lower compensation slope. However, when using call-specific frequency bands, the estimated response was notably stronger. This highlights that the degree of vocal compensation is influenced by the spectral overlap between the noise and the call.

Partial compensation means that calls may still be masked in noisy environments, leading to a reduction in communication space. This can have important ecological consequences, potentially impairing foraging efficiency, social coordination, mother-calf bonding, and overall group cohesion.

Call type influences strength of vocal compensation

We showed that each call type exhibits a distinct Lombard response magnitude, a pattern also observed in finches and bottlenose dolphins. While pilot whales produce a wide variety of calls, there is currently no standardized protocol for classifying these call types. As such, it is possible that some calls with specific functional roles were not fully captured in our categorization. Nonetheless, our classification likely reflects relevant acoustic variation, allowing us to detect meaningful differences in vocal compensation.

We hypothesize that the variation in the Lombard response across call types reflects a combination of factors, including masking susceptibility, physiological constraints, and the communicative function of each call type.

Calling under pressure

In our study, we found no significant differences in Lombard response magnitude across the four dive contexts, suggesting that pilot whales adjust their call amplitude similarly regardless of dive phase. However, we did observe a decrease in call output level and a reduction in the number of calls with increasing depth. This supports earlier findings in short-finned pilot whales and beluga whales, and likely reflects physiological constraints related to increasing pressure on the sound production system at depth.

Interestingly, calls produced during the final ascent phase of deep dives exhibited higher output levels than both surface and deep descent calls within the 0–100 m depth range. During deep foraging dives, pilot whales often disperse and must reunite as they ascend. We suggest that these louder ascent-phase calls may serve a reunification function, facilitating group coordination before surfacing, potentially explaining their elevated amplitude.