(Presented by Sally Crossley and Pam Mohr).

• Playback experiments conducted with wild bottlenose dolphins, Tursiops truncatus, to determine if there is sufficient information in individual distinctive signature whistles for individual recognition.
• Each bottlenose dolphin has its own whistle, which is different in tone, form and duration from any other bottlenose dolphin whistles. (P. Thompson, B. Wilson, 1994).
• Target dolphins were members of a community near Sarasota, Florida.
• Dolphins use acoustic communication, therefore is the most likely channel for individual recognition.
• So far, only the ability to discriminate among whistles of different individuals has been demonstrated experimentally.
• 3 Criteria were used for this study of individual recognition where:

1. Vocal variability is shown
2. Evidence for individual recognition noted in natural behavior
3. Ability to discriminate among individuals confirmed through playback experiments

• Predicted that mothers would respond more strongly to the whistles of their own independent offspring, rather than that of a similar aged dolphin of their offspring.
• Independent offspring would respond more strongly to the whistles of their own mother, rather than of a similar whistle from another familiar female adult.

1. Vocal variability

• Distinctive frequency contours of bottlenose dolphin signature whistles provide information on individual identity.
• One dolphin capable of distinguishing between several different signature whistles, using many different examples from each individual.
• Studies on cognition have shown dolphins are capable of discriminating among different computer-generated whistle like sounds.
• Dolphins can learn to associate arbitrary sounds with arbitrary objects

1. Observational evidence for individual recognition

• Mothers and calves remain in close association for 3-6 years.
• They do frequently separate out of visual range and then reunite.
• Almost immediately after birth, the mother may whistle nearly continuously for several days presumably to provide a strong acoustic imprinting stimulus. This early whistle is relatively uniform but then acquires a ‘signature’ characteristic, which may serve for individual recognition. As calf grows older, the bond with the mother weakens. More time is spent away from her and interactions with other young increases. (Peter Evans, 1987).

1. Playback experiments

• Playback experiments were used to test whether there is sufficient information in signature whistles for individual recognition.
• Primary targets were mother and independent offspring.
• Between close association of the two, signature whistle appears to function as a contact call, and to initiate reunions between the when out of visual contact.
• This experiment was designed to test if mothers and offspring continue to recognize signature whistles of one another.
• From another study we found that ‘the whistle repertoire model, proposes that dolphins posses a larger and more complex whistle repertoire, share whistle types both within and across social groups, and produce a predominant but not individualized whistle type’ (B. McCowan, D. Reiss, C. Gubbins, 1998).

• This experiment was a capture/release project.
• Animals were surrounded using a large net in water approx. 1m deep, then they were processed on the deck for 1 hour.
• They used a paired playback design to control contextual cues in any whistles.
• The experiment took place after the dolphins were released into the water with the nets.
• Each target animal was presented with two familiar stimuli, with one stimulus predicted to elicit a stronger response than the other.
• In tests with mothers as target animals we compared the mothers response to sequences from an independent offspring and a familiar similar aged non-offspring.
• In tests with independent offspring as target animals, we compared the offspring’s responses to whistle sequences from the mother and a familiar adult female.
• In a third set of experiments we targeted females from different bands to test whether adult females preferentially respond to whistles of closely associated females as compared with whistles of familiar but less closely associated females.
• Stimuli sounds were monitored on a Kay Elemetrics Corporation, Model 5500 DSP Sonograph.
• Audio output was recorded on a sonogram on a Marantz PDM-430 stereo cassette recorder, and recorded all tapes at the same input level.
• Within each pair, the order of stimulus was the one predicted to elicit the higher response.
• All twenty individuals were used as target animals in the eleven paired trials. Two individuals were used as target animals in two different experiments, in different years to increase the number of pairs of similar aged animals.

• From 1989-1994, 38 playback experiments were completed, but results were based on only 11 paired trails, due to death, disappearance and equipment failure.
• Responses to playback were assessed by counting number of head turns towards speaker.
• Head turns represent efforts by the dolphin to echolocate on the source of whistles and/or to locate the source visually.
• Because echolocation is directional, the dolphin must be oriented in the direction of the object to visualize it effectively, but a persons body was placed in front of a speaker so the dolphin could not detect directly where the sound was coming from.
• Head turns were counted to determine whether the bottlenose dolphin turned more in response to the predicted stimuli.
• Target mothers and offspring turned their heads towards the speaker more than other pairs.

• Dolphins interact with many other different individuals over periods of many years.
• It seems likely that recognition of individuals is very important.
• The playback experiments demonstrate that bottlenose dolphins are capable of discriminating between whistles of different familiar individuals.
• Overall, what is known about bottlenose dolphins behavior and social structure supports the idea that individuals do have concepts of one another and that they track the history of their individual relationships.
• Loss of stereotypy was particularly true in long term captives housed alone or with one or two other calves (H.E. Winn, 1926).
• However, they speculated that the gas-filled nasal sacs believed to be the source of vocalizations in dolphins may not provide reliable clues for individual recognition, because the shape of the nasal sacs is likely to be altered by pressure changes that take place during dives.
• Target animals showed a significantly stronger head turning response to the predicted stimulus, which was either the mother or an independent offspring.
• Offspring used as targets and stimuli had been independent for periods ranging from 1 month to 14 years and most had formed close association with other individuals.
• Since separation from mothers, independent offspring showed low rates of association with mothers and other female adults used in the experiment.
• It is highly unlikely that discriminations were based on familiar versus unfamiliar basis.
• When taken in the context of what is know about the social structure and behavior of bottlenose dolphins, these playback experiments suggest that signature whistles are used for individual recognition.
• Due to death, disappearance and equipment failure, only 11 paired trials were counted, which suggest that the sample size was too small and perhaps another experiment should be done with a larger sample size.
• A mother investing in a new calf may not be strongly motivated to respond to her independent offspring.
• From that it is hard to differentiate between lack of motivation to attend kin or the inability to recognize them.
• From this experiment however, head turns indicated that dolphins recognized their kin and are motivated to respond to them.
• Target animals do respond to playbacks of kin or associates which implies that individually specific social relationships are important to dolphins.
• Idiosyncratic differences in coloration, dorsal fin shape and natural scars contribute to visual individual recognition (L.M Herman, 1980).
• An experiment involving a pair of bottlenose dolphins has shown the possibility of a dolphin language (R. Burton, et al.).

• Burton, R., Devaney, C., Long, T. The Living Sea.
• Evans, P. (1987) The Natural History of Whales and Dolphins.
• Gubbins, C., McCowan, B., Reiss, D. (1998). Social Familiarity Influences Whistle Acoustic Structure in Adult Female Bottlenose Dolphins (Tursiops truncatus). Aquatic Mammals.
• Herman, L.M. (1980) Cetacean Behavior.
• Thompson, P. (1994) Bottlenose Dolphins.
• Winn, H.E. (1926) Behavior of Marine Mammals.

[Course Homepage] [Syllabus] [Lecture Schedule] [Lab Schedule] [Student Presentations] [Marine Mammal Links]