88
D.R. Ketten
than other segments of the membrane, a characteristic profile or envelope
develops for the signal.
Based on length alone, cetacean basilar membranes are highly differentiated, anisotropic structures capable of exceptionally wide frequency
responses. However, it is well established that multiple basilar membrane
paramenters are functional correlates of hearing characteristics (von
B6kesy 1960; Manley 1972; Ketten 1984; West 1985). Peak spectra and
hearing ranges have been shown to correlate (with varying degrees of
robustness) with length, width, thickness, etc., but the key to interpreting
these relationships is to determine to what extent and how anyone parameter relates to function. Thickness and width both have distinct gradients
in mammalian basilar membranes. The combination of the two appear to
give the highest correlation with hearing characteristics (Ketten 1984).
Cetaceans, as a group, have the most extreme range of basilar membrane
developments of any known mammal and are therefore excellent subjects
for basilar membrane functional analyses.
Humans have an unspecialized, mid-range, generalist ear; average basilar
membrane length is 33.5 mm with an approximately fivefold increase in
width (125 to 500J.lm) and three-fold decrease in thickness (7 to 2J.lm) base
to apex (Schuknecht 1993; Ketten et al. 1998). In the typical odontocete,
width increases 10-fold (35 to 350J.lm) while thickness decreases fivefold
from 25 to 5 J.lm base to apex. Mysticete basilar membranes display as much
or more base to apex variation (100 to 2,2ooJ.lm wide, 10 to 2.5J.lm thick)
but are consistently thinner at each point than their odontocete counterparts. In comparison to human membranes, we obviously expect odontocetes to have significantly higher and mystictes, significantly lower,
functional hearing.
Thickness to width (TIW) ratios are consistent with the maximal high and
low frequencies each cetacean species hears and with differences in their
peak spectra (Ketten and Wartzok 1990) (Table 2.1; Figs. 2.7B, 2.8). For
example, P phocoena, a Type I odontocete, has a basal T/W ratio of 0.83
and a peak frequency of 120 to 130kHz. T. truncatus, a Type II odontocete,
has a T/W ratio of 0.71 and a peak signal of -80kHz; Rhinolophus, a CFIFM
bat, a 0.44 TIW ratio and a 40 kHz echolocation signal with significant harmonics near 80 kHz. All three echolocators have terminal apical ratios of
0.01 to 0.02. Mysticete (Type M) TIW apical ratios are commonly 0.001, that
is, mysticete membrane ratios start at the basal end at a point equivalent to
middle or low apical ratios in the ultrasonic species and decrease steadily
to a value a full magnitude lower at the apex than odontocetes. B. mysticetus has a basal ratio of 0.062 and produces calls with peak spectra of
-150 Hz. The high TIW ratio areas in bats and dolphins are accompanied
by other cochlear duct stiffening elements, creating a high-frequency resonating complex that is entirely independent of membrane length. The mysticete basal ratio is only slightly lower than that of human membranes,
implying some mysticetes and humans have similar functional high-frequency limits, but the exceptionally low apical ratios of mysticetes are con-
D.R. Ketten
than other segments of the membrane, a characteristic profile or envelope
develops for the signal.
Based on length alone, cetacean basilar membranes are highly differentiated, anisotropic structures capable of exceptionally wide frequency
responses. However, it is well established that multiple basilar membrane
paramenters are functional correlates of hearing characteristics (von
B6kesy 1960; Manley 1972; Ketten 1984; West 1985). Peak spectra and
hearing ranges have been shown to correlate (with varying degrees of
robustness) with length, width, thickness, etc., but the key to interpreting
these relationships is to determine to what extent and how anyone parameter relates to function. Thickness and width both have distinct gradients
in mammalian basilar membranes. The combination of the two appear to
give the highest correlation with hearing characteristics (Ketten 1984).
Cetaceans, as a group, have the most extreme range of basilar membrane
developments of any known mammal and are therefore excellent subjects
for basilar membrane functional analyses.
Humans have an unspecialized, mid-range, generalist ear; average basilar
membrane length is 33.5 mm with an approximately fivefold increase in
width (125 to 500J.lm) and three-fold decrease in thickness (7 to 2J.lm) base
to apex (Schuknecht 1993; Ketten et al. 1998). In the typical odontocete,
width increases 10-fold (35 to 350J.lm) while thickness decreases fivefold
from 25 to 5 J.lm base to apex. Mysticete basilar membranes display as much
or more base to apex variation (100 to 2,2ooJ.lm wide, 10 to 2.5J.lm thick)
but are consistently thinner at each point than their odontocete counterparts. In comparison to human membranes, we obviously expect odontocetes to have significantly higher and mystictes, significantly lower,
functional hearing.
Thickness to width (TIW) ratios are consistent with the maximal high and
low frequencies each cetacean species hears and with differences in their
peak spectra (Ketten and Wartzok 1990) (Table 2.1; Figs. 2.7B, 2.8). For
example, P phocoena, a Type I odontocete, has a basal T/W ratio of 0.83
and a peak frequency of 120 to 130kHz. T. truncatus, a Type II odontocete,
has a T/W ratio of 0.71 and a peak signal of -80kHz; Rhinolophus, a CFIFM
bat, a 0.44 TIW ratio and a 40 kHz echolocation signal with significant harmonics near 80 kHz. All three echolocators have terminal apical ratios of
0.01 to 0.02. Mysticete (Type M) TIW apical ratios are commonly 0.001, that
is, mysticete membrane ratios start at the basal end at a point equivalent to
middle or low apical ratios in the ultrasonic species and decrease steadily
to a value a full magnitude lower at the apex than odontocetes. B. mysticetus has a basal ratio of 0.062 and produces calls with peak spectra of
-150 Hz. The high TIW ratio areas in bats and dolphins are accompanied
by other cochlear duct stiffening elements, creating a high-frequency resonating complex that is entirely independent of membrane length. The mysticete basal ratio is only slightly lower than that of human membranes,
implying some mysticetes and humans have similar functional high-frequency limits, but the exceptionally low apical ratios of mysticetes are con-