Investigations on Cetacea (1983)

Abstract

This monograph may be regarded as an important supplement to the book ┬╗Echolocation in Whales and Dolphins┬╗ by Purves and Pilleri, published by the Academic Press, London, 1983.
It contains new findings which substantially corroborate the concept of an exclusively laryngeal origin of the echolocation sounds in cetacea.
The study was focused on those structures which are located around and rostrally to the sound source (epiglottic spout and M.palatopharyngeus) and on their function: the pterygoid air sacs of the skull base as derivatives of the middle ear cavity, and the rostrum.
Owing to the extreme impedance differences between air and tissue, the former cause a virtually total reflection of the soundwaves in the proximal part of the rostrum. In the first place, this causes the sound to be conducted to the water with the lowest possible energy loss, and in the second place the proximal part is of paramount importance for the shaping of the sonar field.
The epicranial screening of the rostrum is effected by two diverticula of the upper nares, the premaxillary air sacs.
Owing to the excellent pneumatic screening, a secondary sound source of fixed dimensions is produced within the rostrum. It may be compared with the free surface of a high frequency vibrating piston, in which radiation takes place according to Huygens’s diffraction principle.
If we apply this principle to the rostral secondary sound source, we obtain a theoretical emitting field as a polar diagram at a given frequency.
Comparing the polar diagram with the empirical sonar field (i.e. far field) measured on the living dolphin, we find that they match. It is thus possible to construct emitting fields for any species of cetacean – including fossil forms. As an example of the latter, the emitting field of Zarhachis flagellator, a Miocene species, is being reconstructed.
In some cetaceans with open pterygoids (pterygoschisis) the sonar field also radiates in the ventral direction. Applying the law of diffraction on aperture, the horizontal dimension of the ventral field sector can be calculated on the skull of these species; it matches the width of the ventral field in the live animal.
In the proximal part of the rostrum, anterior to the secondary sound source, the trabecular bone structure displays a peculiar fan-shaped pattern. This fan expresses the acoustic interference phenomenon in the near region of the sonar field and represents a clear proof of the presence of ultrasound in the rostrum.
The melon plays absolutely no part whatever in the sonar apparatus of cetacea.
In the same way as for the far field, it is possible to apply the principle of diffraction on aperture to construct, for all species and frequency ranges and according to the dimensions of the secondary sound source, the interference field (i.e. near field), which coincides with the anatomical findings with drawingboard precision.
The free rostrum of cetacea is an extremely variable structure and has no conceivable effect either on the form of the emitting field or on that of the echolocation click.
Biophysical measurements (sound pressure transmission coefficient, sound power transmission coefficient, sound reduction index, density, sound velocity, impedance and attenuation) measured on the rostrum tissues show that its sole acoustic function consists in conducting the sound produced by the larynx to the water without appreciable energy loss.
With few exceptions, the echolocation clicks display very slight intraspecific characteristics. In the same way as the emitting field, they are stereotyped and fall according to the available data into four categories. The physical properties of the clicks show no correlation with the ecosystems of the various species. On the other hand, there would seem to be a correlation between the signal type and the feeding habits (plankton) in the case of the Mysticeti.
It would seem improbable that echolocation clicks in species which allegedly only produce such clicks also contain symbols’ used for intraspecific communication as in human speech in view of the stereotyped and relatively isomorphic characteristics of the signals, coupled with their comparative lack of specificity.
There seems to be no doubt that whales and dolphins have language. We can confidently use the word language for this animal group without putting it in quotation marks, since, as Karl von Frisch has proved, the bees also have a language. However, the language which John Lilly seeks in cetacea has nothing in common with human speech. Moreover, we are bound to dismiss the idea that cetacea hear with the lower jaw and that the entire middle ear is nonfunctional as pure fantasy.