Hydroacoustical regularities of food behavior of dolphins

Abstract

Hydroacoustic regularities of food behavior of dolphins are determined by characteristics and functionality of their acoustic signals. All the acoustic signals of dolphins are classified depending on their physical characteristics by the theory of signals and echolocation as follows: sequences of ultrashort ultrawideband coherent pulses – ‘clicks’, frequency-modulated (FM) simultones with uniformly distributed tones – ‘whistles’, packets of mutually coherent pulses (CI), packets of mutually noncoherent pulses (NI) and packets of versatile pulses (VI). They play the role of probing signals of six dolphin sonars optimized for solving various echolocation tasks. The possibilities of using the signals by dolphins in searching and classifying food objects by echolocation have been studied in this work on the basis of both experimental data obtained by the researcher and those available in literature. The dolphins can use sequences of ‘clicks’ to detect food objects (individual fish at the distance up to 70–110 m, a school of fish at the distance up to 450–600 m) and conspecifics (dolphins) (at the distance up to 450–600 m) and to classify food objects. The dolphins can use ‘whistles’ to detect food objects (individual fish at the distance up to 2 km, a school of fish at the distance up to 9–13 km) and conspecifics (dolphins) (at the distance up to 9–13 km) and to determine their range and relative radial velocity. ‘Whistles’ provide higher accuracy and the range of echolocation of food objects and conspecifics compared to ‘clicks’ (by more than an order of magnitude). Furthermore, the FM sonar provides the measure of the radial velocity of approaching or distance removing of underwater object to or from a dolphin. However, an acute analysis of the amplitude-time regularities of the fish echo for the purpose of their classification is the advantage of sonar using ‘clicks’. The dolphins can use the packs of CI at the distances shorter than 2.5 m for tracing the position dynamics of the prey aiming at accurate capture. The high hearing resolution of the dolphin in time is about 0.02 ms; it allows processing the subtle temporal dynamics of the echoes. Packs of NI (signals of spoken language) can be used by dolphins to organize various types of association and complex cooperation between themselves when hunting and catching fish. Packets of VI can be used by dolphins to expand the echolocation of the survey area around the dolphin in order to improve the quality of food objects monitoring, regardless of the position of the dolphin’s head, as well as to determine the range, relative radial velocity and class of food objects, at short distances. Evolution and perfection of various types of acoustic signals, sonars and various methods of processing echo signals in dolphins is caused, first of all, by optimization of hydroacoustic regularities of their food behavior, along with the need for orientation in three-dimensional space. One can assume the presence of similar hydroacoustic regularities of food behavior in Odontoceti based on the similarity of their acoustic signals and morphology. The acoustic regularities of food behavior of dolphins and bats are similar, despite the fact that they have different habitats (water and terrestrial-air), and these mammals belong to different orders of the animal kingdom (Сetacea and Chiroptera).

Keywords: dolphin, bat, feeding behavior, hydroacoustics, regularities, sonar, echolocation, signal