The fallacy of ‘signature whistles’ in bottlenose dolphins: a comparative perspective of ‘signature information’ in animal vocalizations

In sharp contrast with descriptions of contact calls in all other species, the contact or cohesion calls used by bottlenose dolphins, Tursiops truncatus, in contexts of social isolation have been historically described as individually distinctive and categorically different whistle types, termed ‘signature whistles’. These whistle types have been proposed to function as labels or names of conspecifics. Other studies have reported an absence of signature whistles and have demonstrated that dolphins, like other species, produce a predominant shared whistle type that probably contains individual variability in the acoustic parameters of this shared whistle type. To further understand the discrepancies between different studies on dolphin whistle communication and the vast differences reported between the isolation calls of dolphins and other species, we conducted a study replicating the approach and methodologies used in the studies that originally and subsequently characterized signature whistles. In contrast to these studies, we present clear evidence that, in contexts of isolation, dolphins use a predominant and shared whistle type rather than individually distinctive signature whistles. This general class of shared whistles was the predominant call of 10 of the 12 individuals, the same shared whistle type previously reported as predominant for individuals within both socially interactive and separation contexts. Results on the further classification of this predominant shared whistle type indicated that 14 subtle variations within this one whistle type could be partially attributed to individual identity. Thus, individual variability in the production of a shared contact call, as reported for other taxa, probably accounts for individual recognition in dolphins. No evidence was found for categorically different signature whistle types in isolated bottlenose dolphins using both qualitative and quantitative techniques.