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    • Rapalink-1 Many species of dolphins produce long frequency m

    2018-11-05

    Many species of dolphins produce long frequency-modulated (FM) acoustic signals containing a large number of harmonics that occur at n-tuples of the fundamental frequency (n is an integer). Such signals have come to be called whistles because this is how humans perceive them. The repertoire of context-specific whistles which vary according to the situation or to animal activity was studied and described almost immediately after the whistles were first recorded [11–13]. Most species of dolphins producing whistle signals are gregarious animals and live in large groups, so it was suggested that whistles play an important role in their social communication [14–16]. The manner in which such signals can be used for communications is the most common subject of discussion in the scientific literature. Recent studies have established that the fundamental frequency of the majority of whistles covers the frequency range of 2–35kHz and up to 100kHz for the harmonic frequencies [7,17–19]. However, both the necessity and the function of the harmonics which make up an integral part of the whistle are currently unexplained. The signature-whistle hypothesis claiming that dolphins use these whistles to inform the community about their identity and about the location of other members of the social group [20,21] has been discussed recently and has found support in numerous papers (see, e.g., [16,22,23]). It was also suggested that whistles have a communicative function, i.e., they are used for establishing connections, coordinating actions and maintaining Rapalink-1 in a group of animals scattered around the water area [16]. The maximum distance at which the dolphins can communicate with whistling signals was calculated based on the data on the maximum SPL of the whistle, the sensitivity of the dolphin\'s hearing, the level of ambient noise and sound attenuation with distance. It amounted to about 10.5km. [24] The material of the brief review presented above indicates the great interest of researchers in studying the acoustic signals of dolphins. At the same time, only echolocation clicks were the most extensively investigated in the frequency band up to 200kHz, with a known position of the dolphin relative to the hydrophone. The vast majority of other types of dolphin signals were detected and described in the frequency band up to 20kHz. Additionally, acoustic signals were recorded using equipment with insufficient dynamic range; the pulsed character of the sounds and the position of the dolphins relative to the hydrophone (the animals were swimming freely) were not taken into account. Perhaps that is why the authors of these studies failed to clearly identify which acoustic signals of the animals could be regarded as communication. At the same time, a promising new technique for studying the functions of the acoustic signals of dolphins by registering the signals of two quasi-stationary dolphins using a two-channel recording system was described in Refs. [25–27]. This technique has allowed for the first time to ascribe each signal to a specific animal, to record the sequence for the exchange of different types of signals between the dolphins, the dynamics of the changes in the characteristic of the radiation pattern and the signal waveform, to classify and interpret the functionality of the signals in view of the theory of signals and echolocation. Dolphins signals were divided into the following classes:
    The results of the studies give reason to regard all acoustic signals of dolphins as sounding signals of not one sonar (as discussed earlier) but at least six different sonar types. At the same time, it was suggested in Refs. [25–27] that NPs are the signals of a highly advanced spoken language of dolphins.
    Experimental subjects and procedures The experiments were performed on two adult Black Sea bottlenose dolphins (Tursіps truncatus), named Yasha (male) and Yana (female), in a closed concrete pool with the dimensions 27m×9.5m×4.5m, located at the T.I. Vyazemsky Karadag Scientific Station – Nature Reserve of RAS. The dolphins have lived in the pool for about 20 years and have normal hearing. The water level in the pool is 4m. The configuration of the experiment is shown in Fig. 1a.