Spontaneous Vocal Mimicry and Production by Bottlenose Dolphins (Tursiops truncatus): Evidence for Vocal Learning

Spontaneous Vocal Mimicry and Production by Bottlenose Dolphins (Tursiops truncatus): Evidence for Vocal Learning

Introduction

In captivity, the bottlenose dolphin has

been proven to possess a remarkable ability

for behavioral and vocal mimicry.

Several studies show dolphins spontaneously

mimicking whistles of their own species as well

as other biological and artificial sounds.

This suggests that dolphins develop their vocal repertoire through learning.

The mechanistic details by which vocal mimicry influences learning and communication in bottlenose dolphins is unclear.

This study was designed in order to gain a better understanding of the functional and developmental aspects of vocal learning in dolphins.

Subjects

• Two female bottlenose dolphins (aged 9 and 20 years old) and their male offspring.
• The subjects were all captive Atlantic bottlenose dolphins.
• The two male offspring were born at the research facility and at the time of this study were two-years old.

This study used a free-choice keyboard system.

Most studies train animals with food reinforcement, this type of reinforcement gives information mainly on the animals’ shaped responses. This study allowed the animals to physically interact with a self-reinforcing system, a methodology that provides more information about the subjects' behavioral and cognitive abilities.

An underwater keyboard was presented to the dolphins and their interactions with the keyboard were observed. When the dolphins selected visual forms on the keyboard, an underwater whistle was generated, and then an object or activity was presented to the dolphin.

The keyboard provided the dolphins with a systematic chain of events:

*the dolphin selected one of the visual forms

* followed by an underwater computer generated whistle

*the dolphin was presented with an object or an activity

Results

Behavioral Concordance: Facsimile

Productions and Combination

Whistles

• Researchers were able

to determine the contexts in which the dolphins produced whistle facsimiles by analyzing behavioral narratives recorded during Year 2.

Inappropriate: contexts in which the dolphin physically interacted with an inappropriate object or activity.

Figure 9

Figure 10

• NOTE

• The dolphins could never

have heard the two computer

whistles in succession. There

was a 0.3 second delay between

computer-generated whistles.

• These combination whistles

appear to be novel signals

produced by the dolphins.

Overall, dolphin’s whistles were similar to the computer’s whistles’

• duration
• relative frequency
• frequency modulation

But their whistles didn’t match

• fundamental frequencies
• time (expanded or compressed parameters)
• spectral parameters (but sounded the same)

• vocal productions (n=210) were 1,005% higher than vocal mimics (n=19)greater structural fidelity to computer sounds in Year 2 than in Year 1matched relative time and frequency parameters better, but some facsimiles were still expanded or compressedhigher rate of “ball” facsimiles (n=104 for mimicry, n=90 for production) than “ring” facsimiles (n=45 for mimicry, n=34 for production).

Figure 6: Spectrograms of the computer-generated ball whistle and dolphin productions of ball facsimiles

Figure 7:

Vocal mimicry and facsimile production during Years 1 and 2

Figure 5:

Ring spectrograms

Facsimiles showed close fidelity to the model sound’s frequency modulation. But the facsimiles were actually a slightly higher or lower frequency.

Figure 4:

Rub spectrograms

Figure 8:

Spectrograms of facsimile

productions of the ring

whistle during Year 2

“Vocal mimicry of the computer-generated ball and rub whistles was first recorded during the 10th keyboard session in which only the ball and rub keys were presented. The ball whistle was mimicked after 19 exposures to the computer model” (Reiss et al., 1993, p. 304).

Combination Whistles:

Figure 3:

Ball spectrograms

“Although all 4 dolphins had the same opportunity for interaction, only the 2 young males used the keyboard consistently” (Reiss et al., 1993, p. 304).

B

Figure 1

Two or more discrete whistles

that were apparently combined

to form one continuous

whistle

Discussion

Dolphins quickly and spontaneously mimicked the frequency modulation, the duration, and often the harmonic structure of the model sounds, suggesting that there are salient acoustic features which may play an important role in the dolphins' discrimination and use of whistles in communication

By: Diana Reiss and Brenda McCowan

Unlike other studies, including one in which over 1,000 trials were required to train initial dolphin vocal mimicry, the dolphins in this study learned to mimic and produce facsimiles relatively quickly.

Botlenose dolphins

Instead of examining to what extent they could train and control dolphin behavior, the researchers set up a situation in which the dolphins could explore the contingencies of key-board use.

Finally, it is important to remember that the two juvenile dolphins were the only ones who demonstrated significant learning/mimicking, and they were also the only ones to continually use the keyboard. The researchers mention that previous studies have used older dolphins.

• Researchers found that dolphins began emitting these combination whistles.

Reiss and McCowan write, "The contexts in which facsimiles were produced suggest the dolphins developed associations between the visual forms, the whistle facsimiles, and the objects and activity."

Reiss and McCowan suggest that their study is successfully inconsistent with previous studies of this sort in several ways:

1. Their methodology more effectively enabled opportunities for the dolphins to demonstrate learning.

2. The freedom of the dolphin to choose what noises to mimic fostered mimicking.

3. Their dolphins didn't lose the fidelity of the sound production acquired.

4. They found spontaneous and continued use of facsimiles in behaviorally appropriate contexts.

The tendency of the dolphins to initially mimic the last element and then the first element of the model sounds suggests that the processing of new stimuli may involve ordeing effects and selective attention similar to that for humans

Table 1

They also concede that there is insufficient evidence to support any claim that associations were developed between the visual element and the object or activity.

Could we come up with a way to find this sufficient evidence to demonstrate these associations?

They note, however, that this does not presuppose that the dolphins were labeling these objects.

What do they mean by this?

To what extent might these age differences have influenced the results of the study?

• They found ball-ring combination

whistles and, more frequently, ring-ball combination whistles.

?

?

• A nonambiguous production of a ball facsimile was recorded and analyzed by the researchers
• The production was found to have the same relative frequency as the computer-generated ball whistles

Anecdotal Evidence for Short-Term and

Long-Term Auditory

Memory:

• the dolphins had been exposed

to the computer-generated ball

whistle only once between Years 1 and 2.

Pan's Facsimile Productions

• Pan produced two ring facsimiles and two ball facsimiles before producing the ring key or ball key hits on the keyboard during year

• He was able to effectively produce the whistles without first hearing them produced by the computer-generator

The computer generated whistles were designed to be similar to sounds that dolphins make naturally. The whistles were similar to those of the dolphins in range and duration. Yet they were distinct whistles from the dolphins’ repertoire.

Production was said to have occurs if the dolphin produced a sound similar to those generated by the computer but it was performed more than 0.5 seconds after hearing the computer generated model.

Mimicry was said to have occurred if the dolphin produced a sound similar to those generated by the computer and if it was performed at least 0.5 seconds after hearing the computer generated whistle.

Figure 2

The researchers hoped that similar whistles might facilitate signal processing, learning, and reproduction in the dolphins.

The computer generated whistles were characteristically distinct from those of the dolphins because:

*Researchers wanted the signals to be easily perceived and reproduced by the dolphins.

*The computer generated whistles needed to be distinct from the dolphins' own repertoire so that vocal mimicry could be established. Vocal mimicry has been defined as copying of an otherwise improbable act or utterance, rather than just producing their own whistles.