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

Abstract

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.

Vascularization of Air Sinuses and Fat Bodies in the Head of the Bottlenose Dolphin (Tursiops truncatus): Morphological Implications on Physiology

“…We have observed extensive vascularization in three of the largest acoustic fat bodies of dolphins, namely the melon, intra-mandibular fat body (IMFB), and extra-mandibular fat body (EMFB) (Figures 1 and 6–8) in agreement with the observations of Fraser and Purves (1960), Maxia et al (2007), and Slijper (1936) (for more complete descriptions of the fat bodies see Norris, 1969; Norris and Harvey, 1974; Cranford et al, 1996, 2008a; Koopman et al, 2003; Scano et al, 2005; Harper et al, 2008; McKenna et al, 2011). The melon fat is located on the dorsorostral aspect of the skull, extending from the rostral border of the nasal passage to the apex of the melon.…”

“…At the forefront is their presumed function as analogs to the external pinnae of other mammals, receiving and channeling sound to the ears (Norris, 1969; Norris and Harvey, 1974; Cranford et al, 1996, 2008a,b). This suggested function naturally implicates these structures in any discussion concerning auditory impairment resulting from intense ensonification.…”

Vocalizations of Amazon River Dolphins, Inia geoffrensis: Insights into the Evolutionary Origins of Delphinid Whistles

“…In conclusion, our understanding of the origins of delphinid whistles awaits further advances in dolphin vocal mechanics, for which even the basic question of sound source identity remains unresolved (but see Cranford et al 1996). Identification of distinct vocal sources for Inia vocalizations and delphinid whistles would, for example, indicate an absence of homology among the vocal forms (e.g.…”

“…Further, as noted by Ding et al (1995) and Matthews et al (1999), whistle frequency in delphinids scales with body size, with large species producing whistles with comparatively low frequencies. The structural uniformity and scaling of delphinid whistle features suggest a common vocal mechanism for whistle production, synapomorphic to the group (see Cranford et al 1996). A satisfactory explanation of dolphin whistle origins thus requires attention to taxa outside of the Delphinidae.…”

Left–right asymmetry of the gnathostome skull: Its evolutionary, developmental, and functional aspects

“…Behavior correlated cranial skeletal asymmetry has been well investigated in other marine gnathostomes, in particular with echolocation in odontocete cetaceans (toothed whales; e.g., Cranford et al, 1996Cranford et al, , 2008Fahlkea et al, 2011;Fordyce and Barnes, 1994; Raven and Gregory, 1933) and a Ziphid whale (e, light blue arrowhead, Knox, 1870), are well known. (f) Cranial asymmetries outside of the skeletal elements are also seen in marine mammals, including the sinestral balloon (red arrow) of the male hooded seal, Cystophora cristata.…”