Partial Lombard response
When including only noise level as a fixed effect, the model estimated an average Lombard response of 0.50 dB increase in call amplitude per 1 dB increase in ambient noise, indicating a partial compensation for rising background noise.
Call type influence
The Lombard response varied significantly across the four call types identified in our study, with each type exhibiting a different degree of vocal compensation to increasing noise levels. Specifically, the slope of the relationship between ambient noise and call output level (dB/dB) was:
- Two-component calls: 0.17
- Short pulsed calls: 0.20
- Low-frequency calls: 0.36
- High-frequency calls: 0.86
These findings suggest that high-frequency calls are most strongly adjusted in noisy environments, while two-component calls show minimal compensation.
Dive context influence
Dive context had no significant effect on the magnitude of the Lombard response, indicating that pilot whales adjusted their vocal output similarly across surface, shallow, and deep dive phases.
What about depth?
With increasing depth, both call output level and the number of emitted calls decreased. Interestingly, ascent calls were louder near the surface than typical surface calls, despite being produced at similar depths. In contrast, descent call output remained relatively stable across depths.
