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Humpback whales (Megaptera novaeangliae) produce an array of social behaviors on their tropical breeding grounds. The most commonly reported behaviors are surface active displays, which include tail, pectoral, or full body slapping events (Kavanagh et al., 2017). Social interactions also comprise a diverse range of subsurface behaviors that include calls (Dunlop et al., 2007; Zoidis et al., 2008; Seger, 2016). The function of most social behaviors within humpback whale groups remains unclear; whales spend most of their time underwater, and their behavioral repertoire is, thus, obscured due to inherent difficulties in documentation in this environment. For example, synchronized movements and tactile signals occur underwater during social group interactions (e.g., between mothers and calves or within competitive groups), and these behaviors may play an important role in their social lives and communication (Darling & Bérubé, 2001; Zoidis et al., 2008, 2014). As such, multiple functions have been proposed, and the significance of social behaviors, including underwater displays, are still debated for humpback whales and most marine mammal species (Dudzinski et al., 2009).
Humpback whales produce vocalizations named social calls (Silber, 1986), and these are considered to be emitted by individuals of all ages and sexes (females/males/calves; Zoidis et al., 2008). Some social calls can also be units as part of the male song display (Rekdahl et al., 2013). Social calls are vocalized as opposed to social sonances, which occur during surface activity and are made by the percussive nature of a body part hitting the water surface (Dunlop et al., 2008; Seger, 2016). Social calls can be short (tenths of a second) or long (multiple seconds) and can be emitted once or arranged into bouts (Dunlop et al., 2007; Zoidis et al., 2008; Rekdahl et al., 2015). For instance, social calls in mother-calf groups are often accompanied by bodily contact between the mother and calf, and are considered an important communication component in maternal care (Zoidis et al., 2008; Zoidis & Lomac-MacNair, 2017). Also, social calls have been recorded during agonistic behavioral interactions in competitive groups (Herman et al., 2008). The importance and function of social calls during different stages of a humpback whale's life are poorly understood, as is the role of these calls within the diverse and complex range of marine mammal social interactions.