Janik, V.M., Sayigh, L.S. and Wells, R.S. (2006). Signature whistle shape conveys identity information to bottlenose dolphins. PNAS, 103(21): 8293-8297.

In this paper, the authors investigated how bottlenose dolphins transmit identity information to other dolphins. It has previously been shown that approximately 50% of all whistles emitted by wild bottlenose dolphins are individually distinctive whistles, termed “signature whistles.” However, it is not known exactly what features of these whistles the dolphins actually use to transmit identity. To investigate this question, the authors conducted playbacks of manipulated signature whistles to temporarily captured wild dolphins in Sarasota, Florida. Versions of previously recorded whistles from this population of dolphins were synthesized using a computer so that they contained the same frequency modulation shape but without any of the acoustic voice cues. These synthetic signature whistles were played to each dolphin captured, and the authors recorded head turns towards the speaker. The authors found that dolphins turned toward the speaker more often if the synthetic signature whistle was from a close relative than from a known unrelated individual. However, cross-correlations of spectrograms of whistles of the test dolphin and the stimuli revealed that the dolphin was not discriminating whistles based on similarity of the dolphin’s own whistle.

These results suggest that bottlenose dolphins use signature whistles to transmit individual identity and not kin discrimination based on similarity of related individuals’ whistles. The response of the dolphins to the synthetic whistles suggests that the important information in signature whistles is frequency modulation, not voice cues from the signaler. Signature whistles are most likely important for maintaining contact between individuals in underwater conditions with few landmarks and constantly changing water pressure, which may alter voice information.

The paper pertains to our lecture and chapter in the textbook on social integration. Part of this lecture was on recognition, or as our textbook defines it, the discrimination and identification of a target individual or group among similar nontarget individuals or group. Recognition mechanisms may evolve when some individuals care for or give aid to others. In order for those aid-giving individuals to gain some type of fitness benefit, they must recognize the offspring, relative or partner they are giving care to.

We learned about several recognition mechanisms. The mechanism used depends on the identification level necessary: species, populations, kin groups, families, age classes, status classes, or individuals. The simplest type of recognition mechanism is spatial location, in which location is a good cue for recognition, because target individuals or groups are usually found in a particular place. However, this would not work for bottlenose dolphins because they are constantly changing locations.

Another recognition mechanism is familiarity. This mechanism requires the learning of specific characteristics of individuals or groups that have been encountered before. As the number of individuals or groups increases, there is strong selection for signature signals. Bottlenose dolphins tend to travel in groups and live in a fission-fusion society, so the number of individuals that need to be discriminated may be high. Individually distinctive recognition signals can be visual (contrasting patterns), olfactory (chemical mixes) or auditory (frequency or temporal patterning). Thus a learned familiarization of individually distinctive vocalizations could provide an effective method of individual recognition in the bottlenose dolphin.