Bottlenose dolphins (Tursiops truncatus) produce individually distinctive
vocalizations called signature whistles, first described by Melba and David Caldwell
(1965). The Caldwells observed that isolated, captive dolphins produced whistles with
individually distinctive frequency contours, or patterns of frequency changes over time,
and hypothesized that these whistles were used to transmit identity information (Caldwell
and Caldwell 1965; Caldwell et al. 1990). Since the Caldwell’s work with isolated,
captive dolphins, several studies have documented signature whistles in a variety of
contexts, including free-swimming captive dolphins (e.g., Janik and Slater 1998; Tyack
1986), briefly restrained wild dolphins (e.g., Sayigh et al. 1990, 2007, Watwood et al.
2005), and free-ranging wild dolphins (e.g., Watwood 2003; Watwood et al. 2004, 2005;
Buckstaff 2004; Cook et al. 2004). Janik and Slater (1998) demonstrated that signature
whistles are used to maintain group cohesion, thus supporting the Caldwells’ hypothesis.
Janik et al. (2006) verified experimentally that bottlenose dolphins respond to signature
whistles produced by familiar conspecifics even after voice featured have been removed,
reinforcing the notion that the contour of a signature whistle carries identity information.