When an animal vocalization shows directionality, or directivity, it radiates more strongly in some directions than others. Directionality in acoustic signals has been shown across many taxa, from insects to higher vertebrates (e.g., Miller 2002) including humans (Dunn and Farnsworth 1939). Some species possess calls with differing levels of directionality, and use them in context-dependent fashion (e.g., Larsen and Dabelsteen 1990, Patricelli et al. 2007, Bernal et al. 2009, Yorzinski and Patricelli 2010). Directionality in acoustic signals is particularly well-suited for animals that use echolocation, as it increases the signal-to-noise ratio of returning echoes, and therefore increases the ef?ciency and accuracy of the system (Au 1993). Strong directionality of echolocation signals has indeed been shown in bats (e.g., Simmons 1969, Schnitzler and Grinnell 1977, Surlykke et al. 2009) and odontocetes (Au et al. 1987, 1995; Au 1993; M?hl et al. 2000). Among mysticetes, Clark et al. (1986) investigated the directionality of two types of calls (upcalls and downcalls) from bowhead whales (Balaena mysticetus) during the spring migration off Barrow, Alaska. They found indications that calls were slightly stronger ahead of the animals. Au et al. (2006) used a vertical array to describe the acoustic properties of singing humpback whales (Megaptera novaeangliae) in Hawaii, and reported directivity in certain types of song units, but the trend was not quanti?ed.
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