Comparative morphometry of odontocete ears through computerized tomography

Abstract

Describing the auditory periphery of odontocetes is a key conservation issue to further assess the effect of acoustic pollution. Because all odontocetes produce species-specific frequency ranges, differences in echolocation signals should reflect anatomical differences in the auditory pathways. Here, we studied the ears of 15 odontocete species through 3D reconstructions from computerized tomography scans to extract standard measurements (bullae lengths/volumes and cochlear volume) and investigate the discriminatory weight of each of these variables as well as their relation to the species' hearing specificity. Any of the measurements appeared to be a good indicator of the species and could therefore be used to classify them. All the ear lengths and most volumes were strongly linearly correlated (r >0.9) in all species and the proportion between the tympanic and periotic bones appeared to remain constant. This constant ratio could be an indication of a functional relationship between both structures, and might suggest an active role of the odontocete middle ear during target acoustic detection, providing new information on the odontocete sound reception mechanism. Our results are generally consistent with previous studies, although here the coefficients of correlation between animal lengths and the total volume and lengths of the bullae were lower (0.77< r <0.86), indicating that the length of the animals may not be a primary parameter to take into account when defining ear measurements. These results suggest that the measurements described characterize standard ears which could be used as a morphological basis for further species-specific acoustic comparison.

Decalcifying odontocete ears following a routine protocol with RDO®

“…-the tympanic-periotic complex volumes extracted from previous CT scans (Morell et al, 2007) (r = 0.935, n = 12, Fig. 2A) or, Fig.…”

“…2. Correlation between the periotic decalcification time with: A) the tympanic-periotic volume (r = 0.935, n = 12) and B) the mean of T-P volume for the species (Morell et al, 2007; r = 0.891, n = 34) when data were not available. □ Phocoena phocoena, Δ Tursiops truncatus, Ж Stenella frontalis, ○ Stenella coeruleoalba, -Delphinus delphis, ◊ Kogia breviceps, + Steno bredanensis, × Lagenodelphis hosei, • Globicephala melas, ♦ Globicephala macrorhynchus.…”

“…-the average of the tympanic-periotic complex volumes for each species (Morell et al, 2007) when data were not available (r = 0.891, n = 34, Fig. 2B).…”

“…Following a routine protocol with a specific dilution of RDO®, the odontocete ear decalcification time ranged from several hours to a few days (Table 2), depending on the volume of the periotic bone, that, in turn, is highly correlated with the T-P complex volume (Morell et al, 2007). This reduced the decalcification time from a few months using EDTA (Ketten, 1984) to a maximum of a week for the largest T-P complexes.…”

“…The T-P complex, which is suspended through ligaments outside the skull in a peribullar cavity, is surrounded by air filled sinuses, allowing the acoustic isolation from the skull (Reysenbach de Haan, 1957;Fleischer, 1976;Ketten and Wartzok, 1990;Nummela et al, 1999b). While some authors expressed doubts about the functionality of the middle ear (Fraser and Purves, 1954;Reysenbach de Haan, 1957;McCormick et al, 1970;Fleisher, 1978;Ridgway et al, 1997) others presented morphological evidences supporting the active role of the middle ear in sound transmission (Nummela et al, 1999a,b;Hemila et al, 1999Hemila et al, , 2001Ketten, 2000;Morell et al, 2007).…”