The conventional theory about the snail shell shape of the mammalian cochlea is that it evolved essentially and perhaps solely to conserve space inside the skull. Recently, a theory proposed that the spiral's graded curvature enhances the cochlea's mechanical response to low frequencies. This article provides a multispecies analysis of cochlear shape to test this theory and demonstrates that the ratio of the radii of curvature from the outermost and innermost turns of the cochlear spiral is a significant cochlear feature that correlates strongly with low-frequency hearing limits. The ratio, which is a measure of curvature gradient, is a reflection of the ability of cochlear curvature to focus acoustic energy at the outer wall of the cochlear canal as the wave propagates toward the apex of the cochlea.inner ear ͉ function ͉ mammalian evolution ͉ spiral I t is often thought that mammalian cochleae are coiled to pack a longer organ into a small space inside the skull and that the cochlear coil increases the efficiency of blood and nerve supply through a central shaft (1). Although these spatial advantages of a coiled cochlea have been generally accepted, understanding the effect of shape on hearing itself has been a challenge.Cochlear coiling is absent in reptiles, birds, and monotreme mammals, and it appears to have originated in the marsupial and placental mammal lines (2). Coiling allowed the cochlea to become longer, increasing the potential octave range, whereas uncoiled cochleae have been associated with relatively limited hearing ranges. Earlier studies suggested that the evolution of coiling enhanced high-frequency hearing (3). This suggestion, however, is not wholly satisfactory for several reasons. Above all, increased hearing ranges extended both high-frequency and low-frequency (LF) hearing abilities in mammals compared with birds and reptiles and improved sensitivities compared with even LF specialist fishes (4). Further, the highest-frequency waves are resolved near the base (entrance) before they propagate far enough into the spiral to ''feel'' the cochlear curvature; it is the lowest-frequency waves that propagate along the cochlea's coils.Earlier work on land mammal ear anatomy (5) found a strong correlation between the LF hearing limit of each species and the product of basilar membrane length and number of spiral turns, but did not adduce a mechanistic explanation for this relationship. Other data suggested also that longitudinal curvature of the cochlear duct generates radial fluid pressure gradients (6) and enhances radial movement of hair cells (1, 7).Recently, a new theory proposed that the cochlea's graded curvature actually enhances LF hearing (8), similar to a whispering gallery in which sounds cling to the concave surface of the lateral wall (9). The cochlear spiral shape redistributes wave energy toward the outer wall, particularly along its innermost, tightest, apical turn, and thereby enhances sensitivity to lowerfrequency sounds.In this article, we test this theory morphometrically. W...

SUMMARY Lung compression of vertebrates as they dive poses anatomical and physiological challenges. There has been little direct observation of this. A harbor and a gray seal, a common dolphin and a harbor porpoise were each imaged post mortem under pressure using a radiolucent, fiberglass, water-filled pressure vessel rated to a depth equivalent of 170 m. The vessel was scanned using computed tomography (CT), and supported by a rail and counterweighted carriage magnetically linked to the CT table movement. As pressure increased, total buoyancy of the animals decreased and lung tissue CT attenuation increased, consistent with compression of air within the lower respiratory tract. Three-dimensional reconstructions of the external surface of the porpoise chest showed a marked contraction of the chest wall. Estimation of the volumes of different body compartments in the head and chest showed static values for all compartments except the lung, which showed a pressure-related compression. The depth of estimated lung compression ranged from 58 m in the gray seal with lungs inflated to 50% total lung capacity (TLC) to 133 m in the harbor porpoise with lungs at 100% TLC. These observations provide evidence for the possible behavior of gas within the chest of a live, diving mammal. The estimated depths of full compression of the lungs exceeds previous indirect estimates of the depth at which gas exchange ceases, and concurs with pulmonary shunt measurements. If these results are representative for living animals, they might suggest a potential for decompression sickness in diving mammals.

Cetaceans possess highly derived auditory systems adapted for underwater hearing. Odontoceti (toothed whales) are thought to receive sound through specialized fat bodies that contact the tympanoperiotic complex, the bones housing the middle and inner ears. However, sound reception pathways remain unknown in Mysticeti (baleen whales), which have very different cranial anatomies compared to odontocetes. Here, we report a potential fatty sound reception pathway in the minke whale (Balaenoptera acutorostrata), a mysticete of the balaenopterid family. The cephalic anatomy of seven minke whales was investigated using computerized tomography and magnetic resonance imaging, verified through dissections. Findings include a large, well-formed fat body lateral, dorsal, and posterior to the mandibular ramus and lateral to the tympanoperiotic complex. This fat body inserts into the tympanoperiotic complex at the lateral aperture between the tympanic and periotic bones and is in contact with the ossicles. There is also a second, smaller body of fat found within the tympanic bone, which contacts the ossicles as well. This is the first analysis of these fatty tissues' association with the auditory structures in a mysticete, providing anatomical evidence that fatty sound reception pathways may not be a unique feature of odontocete cetaceans. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.