92
DR Ketten
occasionally have only bony structures available for analysis. In many
studies of both fossil and extant species, interlaminar widths have been construed as synonymous with basilar membrane widths. Careful examination
of membrane attachment points (see Fig. 2.6) shows that interlaminar distances do not equal basilar membrane widths. In the apical turn in odontocetes, using laminae to cochlear wall distances as an indicator of basilar
membrane position overestimates membrane width by approximately
26%; in mysticetes at the basal end, interlaminar distances overestimate
membrane widths by approximately 110%. While the construction of the
laminae are certainly an important feature of basilar membrane support
and the absence/presence of robust outer laminae alone may be a useful
diagnostic of generic high- versus low-frequency hearing, the separation of
inner and outer laminae per se, particularly relatively friable laminae, is not
a valid alternative measure of membrane dimensions from which frequency
characteristics can be accurately calculated.
5.3.4 Neural Morphometry
Auditory fiber and ganglion cell counts are remarkable in all cetaceans, particularly considering, as noted earlier, that many counts are based on residual neural populations from stranded animals (Table 2.2). Before describing
neural distributions and morphometry, one curious feature about cetacean
eighth nerves is worth noting. While the acousto-vestibular nerve is clearly
important to cetaceans, it is also remarkably vulnerable. The extracranial
position of the periotic, whether it came about for hydrodynamic or acoustic
reasons, requires the eighth nerve to cross the retro-bullar space without
the protection of bony canals before entering the brain case. In some
species, this means the nerve is exposed along a path of 3cm or more. This
"externalization" of the auditory nerve may be unique in cetaceans. In
odontocetes, the nerve has a dense fibrous sheath covering its exposed segments as well as thick, fibrous gaskets at its entry to the periotic, but, curiously, not at its entry point in the basi-cranium (Ketten 1992).
Whale auditory fiber diameters range from 2 to 40llm, with a mean of
12 11m in odontocetes and 511m in mysticetes, compared to a land mammal
range of 1 to 151lm and an average of 311m (Morgane and Jacobs 1972;
Ketten 1984,1992; Nado11988; Gao and Zhou 1992,1995). Ridgway et al.
(1981) suggested that these diameters are consistent with shorter latencies
in dolphin auditory brain stem responses (ABRs). Spiral ganglion cell
bodies are also larger in cetaceans than in other mammals. The largest spiral
ganglion cells, with axial lengths 50 11m by 31 11m, are found in the sperm
whale (Physeter catodon) (Ketten, unpublished data). One of the smallest
cetaceans, P phocoena, has spiral ganglion cells that average 351lm by
25 11m. In delphinids, most auditory ganglion cells are 40llm by 25 11m. There
is no clear correlation of auditory nerve fibers and ganglion cells with Type
I or Type II ears. Instead, the numbers hint at a correlation with body size,
but this has not been explicitly demonstrated in any mammal.
DR Ketten
occasionally have only bony structures available for analysis. In many
studies of both fossil and extant species, interlaminar widths have been construed as synonymous with basilar membrane widths. Careful examination
of membrane attachment points (see Fig. 2.6) shows that interlaminar distances do not equal basilar membrane widths. In the apical turn in odontocetes, using laminae to cochlear wall distances as an indicator of basilar
membrane position overestimates membrane width by approximately
26%; in mysticetes at the basal end, interlaminar distances overestimate
membrane widths by approximately 110%. While the construction of the
laminae are certainly an important feature of basilar membrane support
and the absence/presence of robust outer laminae alone may be a useful
diagnostic of generic high- versus low-frequency hearing, the separation of
inner and outer laminae per se, particularly relatively friable laminae, is not
a valid alternative measure of membrane dimensions from which frequency
characteristics can be accurately calculated.
5.3.4 Neural Morphometry
Auditory fiber and ganglion cell counts are remarkable in all cetaceans, particularly considering, as noted earlier, that many counts are based on residual neural populations from stranded animals (Table 2.2). Before describing
neural distributions and morphometry, one curious feature about cetacean
eighth nerves is worth noting. While the acousto-vestibular nerve is clearly
important to cetaceans, it is also remarkably vulnerable. The extracranial
position of the periotic, whether it came about for hydrodynamic or acoustic
reasons, requires the eighth nerve to cross the retro-bullar space without
the protection of bony canals before entering the brain case. In some
species, this means the nerve is exposed along a path of 3cm or more. This
"externalization" of the auditory nerve may be unique in cetaceans. In
odontocetes, the nerve has a dense fibrous sheath covering its exposed segments as well as thick, fibrous gaskets at its entry to the periotic, but, curiously, not at its entry point in the basi-cranium (Ketten 1992).
Whale auditory fiber diameters range from 2 to 40llm, with a mean of
12 11m in odontocetes and 511m in mysticetes, compared to a land mammal
range of 1 to 151lm and an average of 311m (Morgane and Jacobs 1972;
Ketten 1984,1992; Nado11988; Gao and Zhou 1992,1995). Ridgway et al.
(1981) suggested that these diameters are consistent with shorter latencies
in dolphin auditory brain stem responses (ABRs). Spiral ganglion cell
bodies are also larger in cetaceans than in other mammals. The largest spiral
ganglion cells, with axial lengths 50 11m by 31 11m, are found in the sperm
whale (Physeter catodon) (Ketten, unpublished data). One of the smallest
cetaceans, P phocoena, has spiral ganglion cells that average 351lm by
25 11m. In delphinids, most auditory ganglion cells are 40llm by 25 11m. There
is no clear correlation of auditory nerve fibers and ganglion cells with Type
I or Type II ears. Instead, the numbers hint at a correlation with body size,
but this has not been explicitly demonstrated in any mammal.