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.

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By using small hydrophones implanted in porpoise tissue and an external sound source, pulsed sound was found to be channeled by the blubber coat and strongly reflected by blubber-muscle interfaces. Implants in the head showed concentration of sound in the proposed sound channel of the throat and jaws, complete acoustic isolation in the peribullary air space, and sound focusing in the melon. Sound-velocity measurements in the melon showed a low-velocity core extending from just below the anterior surface toward the right nasal plug and a graded outer shell of high-velocity tissue. Such structure should beam sound generated or transmitted from the area of the superior narial passages forward and impedance match it closely to the surrounding water.Subject Classification: 80.20, 80.50.Toothed whales (suborder Odontoceti), which include porpoises, dolphins, and a number of moderate to largesized whales, uniformly seek out individual prey items, pursue and catch them, often at night or in the deep sea. Such pursuit and capture is greatly aided by active echolocation, perhaps in all odontoeetes. The requirements of echolocation systems operated by these aquatic animals have resulted in many structural and behavioral modifications.Amongst these are transmission of sound from the forehead and upper jaw, •-3 probably entry of sound into the entire blubber coat and its eanalization into new pathways to the middle ear •'+'ø including the lower jaw and throat region, and the generation of at least some sounds in the soft tissue of the nasal passages superior to the cranium. 7 The lipids of the sound-transmission structures are chemically distinct from nearby lipid deposits. s Thus, both sound-sending and reception systems seem grossly modified from the general mammalian plan. Sounds emitted from the forehead are propagated in highly structured fields with the highest frequencies narrowly beamed directly forward, 3,0, 0 which allows the animal to scan its environment by means of associated head movements. Such scanning movements allow not only selective ensonifieation of the environment but refined reception across echo fields, as the surfaces of the jaws, throat, and sides of the head move and produce frequency-related refractive and refleetire effects. • Internal sound reflection and isolation systems in the form of tissue interfaces and air sacs at the same time promote acoustic isolation between transmission and reception.Seaborne sound hitting a porpoise must first hit the blubber coat except at a few loci where this layer is absent or very thin, such as over flippers, flukes, and the fin. Over the proposed acoustic receptive surfaces of jaws and throat, blubber is well developed. Thus, entering sound first encounters a water-blubber interface.Once inside the animal, it will encounter a complex of underlying tissue layers which we can expect to alter its character. What actually occurs is unknown, though it must be crucial to porpoise hearing. However, sound in the blubber coat appears to become channeled over underlying ...

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It has been established that some dolphins possess well-developed acoustic orientation (echolocation) and information gathering abilities, though substantially less is known about the system of sound generation and beam formation. Dolphins use a narrowly focused sound beam that emanates from the forehead and rostrum during echolocation. The primary objectives of this study were to simulate the effects of anatomical structure on beam formation, and to test the viability of various hypothetical sound source locations. Outlines from parasagittal x-ray CT scans were used to construct a 2-D model of the head of the common dolphin, Delphinus delphis. Finite difference techniques were used to simulate sound propagation through tissues modeled as inhomogeneous fluids. Preliminary simulations confirm that beam formation results primarily from reflection off of the skull and the skullsupported air sac surfaces. For the frequencies tested, beam angles best approximate those measured by experimental methods for a source located in a region of the model referred to as the monkey lip/dorsal bursae (MLDB) complex. The results suggest that: ( 1 ) the skull and air sacs play the central role in beam formation; (2) the geometry of reflective tissue is more important than the exact acoustical properties assigned; (3) a melon velocity profile of the magnitude tested is capable of mild focusing effects; and (4) experimentally observed beam patterns are best approximated at all frequencies simulated when the sound source is placed in the vicinity of the MLDB complex. PACS numbers: 43.80.Ka, 43.80.Lb, 43.80.Nd INTRODUCTION It has been established that some dolphins possess a highly sophisticated and adaptable sonar system, though substantially less is known about the system of sound generation and beam formation. Measurements • of the acoustic field of echolocating dolphins have demonstrated that dolphins emit a rapid series of clicks in a narrowly focused beam which emanates from the forehead and rostrum during echolocation. For an Atlantic bottlenose dolphin, Au et al. (1986) found the major axis of the echolocation beam pattern in the vertical direction to be elevated at 5 deg above the reference axis of the upper jaw, and reported a --3-dB vertical beamwidth of approximately 5-7 deg for the composite broadband pattern. Though details of sound source location and operation remain conjectural, several experimental methodologies have implicated the region of the upper (supranarial) nasal passages as the site of echolocation click production (see Sec. III B). In addition, investigators have studied the role of the skull and/or soft tissues in beam formation using acoustic sources with real tissues (Evans et al., 1964; Norris and Harvey, 1974; Romanenko, 1974), light sources and ray-tracing techniques (Evans et al., 1964; Dubrovkiy and Zaslavskiy, 1975; Litchfield et al., 1979), and topographic chemical and/or acoustic lipid analyses (Norris and Harvey, 1974; Litchfield et al., 1979; Varanasi et al., 1981 ). Experimental procedures for ...

The colors of living amphibians and reptiles have been studied, using a General Electric recording reflectance spectrophotometer. The animals were brought to activity temperature levels and the appropriate surface pressed over the reflectance port of the machine while a color record was taken. Background samples from the localities at which the animals were taken were also recorded. Reptiles and amphibians living on backgrounds of relatively uniform color tend to match that background through superposition with considerable fidelity. The animal's color curve is superimposed over that of the background. Ventral color in most forms tested was lighter than the dorsal surfaces of the same animal. It was darker only in some forest—dwelling salamanders and in desert lava—dwelling species. The difference results primarily from the highly reflective ventral surfaces of these forms. The ventral surfaces of white—bellied amphibia show clear oxyhaemoglobin absorption peaks, as do the dorsal surfaces of some amphibia. These effects are entirely absent in curves recorded from reptiles. It is concluded that the degree of background color—matching is related to: (a) the degree of color uniformity of the animal's background, (b) the degree of exposure of the color—matched species to predator, (c) the illumination level prevalent in the habitat, (d) the size range of the color—matched species, (e) the ecological restriction of the species, (f) the qualities of the visual apparatus of predators upon the species, and (g) the adaptive compromise struck by the species. The size of a color—matched animal, or the size of the part of its body that is normally exposed, is related to the point at which such color—matching breaks down. This point of just noticeable difference between animal and background is also determined by the wave—length discrimination curves of the predators, the closeness of the match involved, the uniformity of the background color and its texture, and the presence of absence of concealing patterns. Background color—matching varies greatly in its degree of perfection. This variation is the result of adaptive compromise and balance between this adaptive characteristic and many others that in one way or another affect its complete expression.