Cracking Under Pressure: Vocal Stress and Warm-Up in Humpback Whale Food Calls

Humpback whales (Megaptera novaeangliae) are baleen whales that travel to the northern Pacific Ocean in summer to forage on herring. Some of these whales engage in a cooperative foraging strategy known as bubble-net feeding, in which group members release a net of bubbles which surrounds a school of fish, and produce loud, low-frequency vocalizations, known as food calls, which function to further aggregate prey into denser schools (Hanser, 2009). These calls can be classified into two primary categories: tonal and modulated. The fundamental frequency, or lowest frequency component, of tonal food calls remains relatively stable, but it fluctuates rhythmically in modulated food calls. We observed that sometimes the whale’s voice seems to ‘crack’ during a food call, exhibiting a brief but sudden shift in fundamental frequency. We proposed that the exertion associated with food call production induces stress on the vocal tract, leading to voice cracks (Fitch et al., 2002). Previous research has shown no differences in the occurrence of vocal cracks between tonal and modulated food calls, suggesting both are equally stressful. We hypothesized that longer call series would contain more vocal cracks due to stress generated by repeated production of difficult calls. We further hypothesized that whales may need to ‘warm up’ their vocal tract before producing food calls to avoid vocal cracks. We tested these hypotheses by using Audacity sound software to analyze acoustic recordings of humpback whales performing bubble-net feeding. We matched food call series with cracks to randomly selected call series containing similar call types but without voice cracks to examine the effect of series length and the presence of calls preceding the series on the occurrence of voice cracks. If ‘warming up’ helps prevent vocal cracks, we expect that food calls series without vocal cracks will be preceded by calls containing significant frequency modulation, much like those produced by professional singers prior to performing.