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The below mentioned article provides a study note on cetaceans.
Cetaceans are true aquatic mammals. There are two lineages among extant cetaceans. One lineage is Mysticeti which includes baleen whales, and the other lineage is Odontoceti that contains the toothed whales (dolphins, porpoises and other toothed whales).
The baleen whales are mainly zooplankton feeders (Zooplanktonophagous) and filter their food through the baleen. They are not known to use echolocation or sonar-like phenomenon in finding their prey.
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The baleens produce intricate vocal songs which are associated with their complex social behaviour that indicates emotional condition, sexual or territorial advertisement and also can communicate distress and excitement. The other function is also communication used both in baleen and toothed whales.
The vocalizations of baleen whales are of low frequency and the sound is of below 3 kilohertz (kHz) in most and is below 35 kilo-hertz. The vocal sounds or songs of baleen whales travel hundreds miles through the sea.
The calls of baleens have been recognised by a variety of names and these are bellows, moans, rumbles, belches, grunts, rasps and chirps, etc. Moans are more common than other calls and are of 12-500 Hertz and of pure tone.
The dolphins emit two kinds of sound; one is a shrill-whistle type whose auditory range is located between frequencies from 7000-15000 cycles per second with a continuously changing pitch and the other odontocetes emit a whistle of lower pitch as low as 500 cycles per second.
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The members of toothed whales are smaller in size as compared with the size of baleen whales. Some of them live near the coastal areas of the sea and many of them have penetrated into the fresh water and live in the Ganges River in India, Indus River of Pakistan, Amazon and Orinoco Rivers of South America, Yangtze River and Dongting Lake in China.
Major portion of the toothed whales consume squids and allied cephalopods and fish. Coastal zones are the productive zones because maximum amount of planktons are formed and the water remains more or less turbid due to extreme tides and waves. For the maximum abundance of food major portion of toothed whales are found near about the coastal areas.
River water is also murky; so many odontocetes are forced to prey in limited vision. Again, the sperm whales (Physeter) are found in deep oceans, rarely venturing near the coast, and consume squids, octopi and different fish species. They generally prey below about 400 m water depth.
Some species (physeter macrocephalus) are known to dive to 1100 m. In this region light does not penetrate. In all cases of toothed whales they use the sonar system (echolocation) for navigation, communication and also in finding the prey in dark in gloomy waters in the same way as bats.
The principle of the echolocation is that clicks of sounds or structural pulses produced by the animal are reflected off by any object in their paths and are directed back as echo’s. The form of echo’s including loudness can detect and inform the toothed whales the shape, size, and movement of the object.
Some of the low frequency clicks (0.25-1 kilohertz) can detect the general profile of the surroundings but 2-220 kilohertz clicks can identify detail of the object. The distance is considered in between the whale and the object by the time lapse between emitted signal and receiving it’s as echo.
By applying this extra-ordinary echolocation system, the toothed whales can detect, chase and catch the moving prey. It has been observed that a bottle-nosed dolphin, Tursiops, by using this system, can avoid an obstacle in total darkness.
The exact organ which is associated with the production of sound in cetaceans is not clearly known. Some species of dolphin produce two types of sounds with separate frequencies at a time which indicate that two different cites or organs are associated with the sound production. The nares that connect the blow hole, and larynx are the most likely sound-producing organs.
They have no vocal cords but they produce sound by blowing air through the larynx. Of course in baleen whales the sound producing organ is larynx. But Norris (1974) suggested that the site of sound production in cetaceans has shifted from larynx to complex nasal passages that open by a single opening on the top of the head.
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Echolocation clicks of dolphins are quite different from groans and whistles which are used to communicate with each other. The clicks are very short and are less than one millisecond in duration and repeated many times in a second, resulting in the sound like a door creaking to the human ear. In sperm whales, the clicks are long and repeated less often.
Cetologists are divided in question of the origin of sounds. Some believe that sound of whales is produced by the nasal plug around the blow hole (Mead, 1972). Supporters of the nasal plug theory believe that focusing is done by the ‘melon’ (Fig. 10.93), a structure of fatty tissue in the front part of the upper jaw.
This structure is found only in dolphins, porpoises and other toothed whales but not in baleen whales. The changing of focal length is dependent on the change of the muscles in the head. The shape of melon is changed in some cetaceans during echolocation.
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Other group of cetologists is in favour of the theory that sound is produced in the larynx which is situated far forward and focusing is achieved by bouncing off the parts of the skull.
The larynx possesses complex folds and muscles but no vocal cords. Recent workers on acoustics suggest that some species can simultaneously produce two types of sounds of different frequencies and, therefore, suggest that both theories may be correct.
