The acoustic signals produced by dolphins Yana and Yasha wer

The acoustic signals produced by dolphins Yana (1) and Yasha (2) were recorded with no special training and no food reward for the dolphins. The periods of time used was when the dolphins habitually swam to walkway 5 (located 0.1m above the water level), and remained afloat at the water surface with almost no motion (quasi-stationary) (Fig. 1b). The signals were registered by a two-channel recording system, which detected the moment when each signal arrived at the hydrophone of its channel. Each signal was ascribed to the specific dolphin and NPs were identified during the analysis of the two-channel recording, taking into account the inter-channel time delays for each signal, the inter-channel amplitude difference for the sound pressure of the given signal, as well as the known distances between the hydrophones, the dolphins and the borders of the pool. The distance between the hydrophones of channel I and channel II (a recording VE821 of 3.5m) has been chosen so as to obtain the inter-channel difference of the time delays and the SPL amplitudes of each acoustic signal at the hydrophones that was required for subsequent analysis. The distance between the dolphins was about 1m. The hydrophones were located in the far acoustic field of the dolphins at ∼1.5m. The hydrophones were immersed to a depth of 1m in order to, as far as possible, reduce the probability that the signals going toward the hydrophones located further from each dolphin would be shielded by the body of the other animal. Moreover, to assess the effect of pool reverberation on the recorded signals, hydrophones were positioned so that one of them (4) was located near the pool wall, and the other (3) in the center of the pool (see Fig. 1a). The distance from wall 4 to hydrophone 6 was 0.45m, and 3m from hydrophones 3 and 4 to wall 7. The specifications of the equipment used are listed in Table. 1.
Experimental results One of the typical recordings that display a sequence of mutually noncoherent pulses produced by Yana and Yasha is shown in Fig. 2. The dolphins emitted pulsed sounds in packs with time intervals between the packs greatly exceeding the interpulse intervals in a pack. Pulse duration in packs varied from 0.08 to 0.60ms and its average value was about 0.25ms. The slight difference in the inter-channel SPLs of each signal (less than 16dB) indicates the absence of a high directivity for NP radiation. Each pulse in the pack has a characteristically complex shape which varies from one pulse to another in the NP pack. Because of this, the spectrum of each pulse also varied from one pulse to another. As an example, Fig. 3 shows the forms and the spectra of the first four NP from pack 5 (see Fig. 2), produced by Yana. These signals were classified in accordance with the theory of signals and echolocation as mutually noncoherent pulses (NP) [25–27]. The amplitude spectrum of these pulses had many maxima and minima and covered the entire frequency range of the dolphin\'s hearing, from 6–15 to 160–200kHz. However, the amplitude spectra of the signals (see Fig. 3) are only shown up to 160kHz, since the auditory thresholds of the bottlenose dolphin start to increase significantly at frequencies above 135kHz. The SPL of the pulses (Fig. 2) ranged from 15 to 330Pa; NP packs contained from 4 to 27 pulses; interpulse intervals ranged from 19 to 300ms.
Discussion of the results The analysis of numerous NPs registered in our experiments showed that the dolphins took turns in producing pulse packs and did not interrupt each other, which gives reason to believe that each of the dolphins listened to the other\'s NPs before producing its own. In this case, the directions of the arrows next to the numbers in Fig. 2 indicating the number of each NP pack mark the direction of message transfer (from Yasha to Yana or vice versa), i.e., an exchange of NPs. Essentially, this exchange resembles a conversation between two people. The fundamental difference between the dolphin exchange of information and the human conversation is in the characteristics of the acoustic signals of their spoken language. Each pulse in the NP packs that is produced by dolphins is different from another by its appearance in the time domain and by the set of spectral components in the frequency domain. In this regard, we can assume that each pulse represents a phoneme or a word of the dolphin\'s spoken language.