Whale-Timed Harmonies: Temporal Coordination in Humpback Duet Calls

Humpback whales (Megaptera novaeangliae) migrate annually to their feeding grounds in the Gulf of Alaska. Some of them perform bubble-net feeding there, a cooperative foraging strategy requiring precisely coordinated behaviors (Hanser, 2009). A small group of whales work together to form a net of bubbles around their prey, and produce long sequences of loud, low-frequency ‘food’ calls, which likely function to disorient the fish, making for easier prey capture (Sharpe, 1984). We observed that sometimes multiple whales would produce food calls simultaneously, harmonizing their vocalizations over at least part of a call. These ‘duets’ may function to enhance food call effectiveness by amplifying the impact of vocalizations (Selbmann et al., 2023). Simultaneous vocalizations may also help coordinate the group by acting as a ‘go’ signal (Muntz & Patterson, 2004). These hypotheses are not mutually exclusive. We used Audacity sound analysis software to investigate the role of vocal duets by analyzing acoustic recordings of whales performing bubble-net feeding. We classified duets into three types based on timing of duets and the number of calls involved: Group 1 had duets with multiple calls in the beginning or middle, Group 2 had duets with multiple calls at the end, and Group 3 had a duet on the last call only. We tested our hypotheses by comparing the success of food call series from each group with the success of randomly selected ‘control’ series. These were food call series of similar length, recorded on the same day, but without duets. If duets just amplify the effect of food calls, we expected Group 1 to result in successful feeding events more often than other groups. If duets amplify call effectiveness and help coordinate group behavior, we expected Group 2 to result in successful events more often, and if duets just serve as a ‘go’ signal, we expected Group 3 to result in successful events more often.