The site and physiologic mechanism(s) responsible for the generation of odontocete biosonar signals have eluded investigators for decades. To address these issues we subjected postmortem toothed whale heads to interrogation using medical imaging techniques. Most of the 40 specimens (from 19 species) were examined using x-ray computed tomography (CT) and/or magnetic resonance imaging (MR). Interpretation of scan images was aided by subsequent dissection of the specimens or, in one case, by cryosectioning. In all specimens we described a similar tissue complex and identified it as the hypothetical biosonar signal generator. This complex includes a small pair of fatty bursae embedded in a pair of connective tissue lips, a cartilaginous blade, a stout ligament, and an array of soft tissue air sacs. Comparing and contrasting the morphologic patterns of nasal structures across species representing every extant odontocete superfamily reveals probable homologous relationships, which suggests that all toothed whales may be making their biosonar signals by a similar mechanism.

Functional morphology and homology in the odontocete nasal complex: implications for sound generation

Mead, J. G., and R. E. Fordyce. The Therian Skull: A Lexicon with Emphasis on the Odontocetes. Smithsonian Contributions to Zoology, number 627, 248 pages, frontispiece, 2 diagrams, 32 figures, 9 tables, 2009. — Cetaceans form one of the most unique groups in the evolutionary history of mammals. They have returned to the sea and modified their tail as an efficient means of locomotion. As they adapted to the limited visibility in the aquatic environment, the odontocetes developed a system of echolocation that resulted in extensive modifications to the skull bones. This made descriptive comparisons very difficult and early anatomists unwittingly composed new terms for anatomical structures that had already been named in other taxa. This made anatomical comparisons, based on the literature, extremely tenuous. This lexicon is an attempt to remedy that situation in that it provides headwords and definitions for all the terms that have been used in describing the mammal skull and notes the synonymous terms. The lexicon includes the human nomenclature (Nomina Anatomica), the veterinary nomenclature (Nomina Anatomica Veterinaria) and the nomenclature that is used in descriptive comparative anatomy. The lexicon covers not only extant but extinct mammalian groups and is extensively indexed. Cover: Detail from Frontispiece, illustration of a dolphin skull (cf. Delphinus sp.) presented by

The Therian Skull: A Lexicon with Emphasis on the Odontocetes

Traditionally, sperm whale clicks have been described as multipulsed, long duration, nondirectional signals of moderate intensity and with a spectrum peaking below 10 kHz. Such properties are counterindicative of a sonar function, and quite different from the properties of dolphin sonar clicks. Here, data are presented suggesting that the traditional view of sperm whale clicks is incomplete and derived from off-axis recordings of a highly directional source. A limited number of assumed on-axis clicks were recorded and found to be essentially monopulsed clicks, with durations of 100 micros, with a composite directionality index of 27 dB, with source levels up to 236 dB re: 1 microPa (rms), and with centroid frequencies of 15 kHz. Such clicks meet the requirements for long-range biosonar purposes. Data were obtained with a large-aperture, GPS-synchronized array in July 2000 in the Bleik Canyon off Vesteralen, Norway (69 degrees 28' N, 15 degrees 40' E). A total of 14 h of sound recordings was collected from five to ten independent, simultaneously operating recording units. The sound levels measured make sperm whale clicks by far the loudest of sounds recorded from any biological source. On-axis click properties support previous work proposing the nose of sperm whales to operate as a generator of sound.

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The monopulsed nature of sperm whale clicks

SUMMARY Toothed whales ( Cetacea, odontoceti ) emit sound pulses to probe
their surroundings by active echolocation. Non-invasive, acoustic Dtags were
placed on deep-diving Blainville9s beaked whales ( Mesoplodon
densirostris ) to record their ultrasonic clicks and the returning echoes
from prey items, providing a unique view on how a whale operates its biosonar
during foraging in the wild. The process of echolocation during prey capture
in this species can be divided into search, approach and terminal phases, as
in echolocating bats. The approach phase, defined by the onset of detectable
echoes recorded on the tag for click sequences terminated by a buzz, has
interclick intervals (ICI) of 300-400 ms. These ICIs are more than a magnitude
longer than the decreasing two-way travel time to the targets, showing that
ICIs are not given by the two-way-travel times plus a fixed, short lag time.
During the approach phase, the received echo energy increases by
10.4(±2) dB when the target range is halved, demonstrating that the
whales do not employ range-compensating gain control of the transmitter, as
has been implicated for some bats and dolphins. The terminal/buzz phase with
ICIs of around 10 ms is initiated when one or more targets are within
approximately a body length of the whale (2-5 m), so that strong echo returns
in the approach phase are traded for rapid updates in the terminal phase. It
is suggested that stable ICIs in the search and approach phases facilitate
auditory scene analysis in a complex multi-target environment, and that a
concomitant low click rate allows the whales to maintain high sound pressure
outputs for prey detection and discrimination with a pneumatically driven,
bi-modal sound generator.

Biosonar performance of foraging beaked whales (Mesoplodon densirostris).

The Sierra Club handbook of whales and dolphins (book review)

Sound generation in odontocetes takes place in the upper nasal passage, powered by air pressure created within the bony nares (Norris et al., 1971; Diercks et al., 1971; Hollien, 1976; Dormer, 1979; Ridgway et al., 1980; Amundin and Andersen, 1983; Cranford, 1988). Cranford (1988) describes two pairs of fatty bodies, labelled dorsal bursae, situated in the lateral corners of the spiracular cavity in the spinner dolphin, which may function as “sound transducers and/or conductors of sound” produced at the bursae or in their vicinity. Two posterior branches of the melon connect to the anterior bursae. It also has been suggested that air sacs around the spiracular cavity may guide the emitted sonar pulses rostrally (Norris, 1964; Dormer, 1974; Giro and Dubrovskii, 1975; Alcuri, 1980). Additional sonar beam formation is thought to take place in the melon, as a result of the sound velocity topography of special “acoustic” fats (Norris and Harvey, 1974; Litchfield et al., 1979; Varanasi et al., 1982).

Ted W. Cranford

Papers

Functional morphology and homology in the odontocete nasal complex: implications for sound generation

Forehead Anatomy of Phocoena Phocoena and Cephalorhynchus Commersonii : 3-Dimensional Computer Reconstructions with Emphasis on the Nasal Diverticula