Vocal compensation to noise in long-finned pilot whales (Globicephala melas)

Anthropogenic noise is considered a marine pollutant by the European Union and can impact marine animals in various ways. Noise that overlaps in time and frequency with animal communication signals can interfere with communication and thus impact ecological functions mediated by acoustic signals, depending on how animals compensate for increasing noise. Our study investigated Lombard responses in long-finned pilot whales using sound and movement recording tags in an area with high anthropogenic pressure, the Strait of Gibraltar. We tested if pilot whales increase their vocal amplitude in response to noise, and whether vocal compensation is signal or context specific. We analyzed 1336 calls from 18 individuals in small social groups of simultaneously tagged animals, allowing us to parse which individual emitted which call and how call output level depended on masking noise level. We found that pilot whales increase their call output level with 0.50±0.04 dB per 1 dB increase in noise level to partially compensate for the effects of masking noise. Call type significantly influenced the Lombard response magnitude, with vocal compensation ranging from 0.11 to 0.87 dB per 1 dB increase in noise level, whereas animals exhibited similar Lombard responses for different dive contexts (non-foraging, deep dive descent and ascent). Our results indicate that pilot whales only compensate partially for increased noise levels, and that Lombard response magnitude depends on signal type. This emphasizes the need for understanding the ecological consequences of communication disruptions to ensure sustainable management of human activities.