Do dolphins rehearse show-stimuli when at rest? Delayed matching of auditory memory

Abstract

The mechanisms underlying vocal mimicry in animals remain an open question. Delphinidae are able to copy sounds from their environment that are not produced by conspecifics. Usually, these mimicries occur associated with the context in which they were learned. No reports address the question of separation between auditory memory formation and spontaneous vocal copying although the sensory and motor phases of vocal learning are separated in a variety of songbirds. Here we show that captive bottlenose dolphins produce, during their nighttime resting periods, non-dolphin sounds that they heard during performance shows. Generally, in the middle of the night, these animals produced vocal copies of whale sounds that had been broadcast during daily public shows. As their life history was fully known, we know that these captive dolphins had never had the opportunity to hear whale sounds before then. Moreover, recordings made before the whale sounds started being broadcast revealed that they had never emitted such sounds before. This is to our knowledge the first evidence for a separation between formation of auditory memories and the process of learning to produce calls that match these memories in a marine mammal. One hypothesis is that dolphins may rehearse some special events heard during the daytime and that they then express vocally what could be conceived as a more global memory. These results open the way for broader views on how animals might rehearse life events while resting or maybe dreaming.

Keywords: auditory memory processes; cetacean acoustic plasticity; interspecific vocal copying; sensory–motor-phases separation.

Figure 1

A spectrogram of the Plantète Sauvage dolphins’ (A) common vocal repertoire, (a) whistles and (b) burst-pulsed vocalizations, and (B) their atypical (a) whale-like productions (WLPs) and (b) the “model” humpback whales’ sounds. FFT: (Aa) 1024; (Ab), (Ba), and (Bb) 2282. The time axes for all spectrograms have been standardized; but the (Aa) frequency axis has been extended.

Figure 2

Scatter plot of the discriminant function analysis comparing several acoustic parameters of dolphins’ whistles (asterisks), dolphins’ WLPs (black dots), and humpback whale sounds (white diamonds). Squares indicate the group means. Discriminant function 1 corresponds to minimum frequency (canonical correlation coefficient = 0.949; Wilks-λ = 0.069; P ≤ 0.001); discriminant function 2 corresponds to number of harmonics (canonical correlation coefficient = 0.551; Wilks-λ = 0.696; P = 0.001).

Figure 3

Human evaluation of humpback whale sounds, dolphin whistles, and dolphin WLPs broadcast at different speeds. The classification is given in percent. The upper part (black) represents the classification of sounds as being produced by a whale; the lower part (white) represents the classification of sounds as being produced by a dolphin.