2. Cetacean Ears
55
4. Fundamental Ear Morphometries: Generalist Versus
Specialist Bauplans
Audible ranges and thresholds vary dramatically from one species to the
next. Analyses of how hearing abilities, habitat, and ear anatomy are linked
in different species, particularly in animals from diverse habitats, provide
insights into how each component in the auditory periphery functions and
how different hearing capacities evolved. By observation, we know that
many species hear sounds inaudible to humans. Most mammals have some
ultrasonic hearing, and some, like African (Loxodonta africana) and Asian
elephants (Elephas maximus) appear to detect infrasonic signals (Payne et
al. 1986; O'Connell et al. 1997). Theoretically, "hearing" could extend arbitrarily high, but there are practical limits both in terms of the utility of the
information and in the physics of the receptor.
Hearing ranges are related to both animal size and niche. In general,
smaller animals have good high-frequency hearing while larger animals
tend to have better low-frequency hearing and a lower top frequency.
For example, mice have a functional high-frequency limit of approximately
90kHz; cats, 70kHz; humans, 20kHz; cows, 16kHz; elephants, 12kHz
(Fay 1988).A functional relationship between cochlear length and a species'
hearing range has been assumed in several mammalian ear modeling
efforts, but this is a shibboleth. Mammalian ear structures, particularly the
size of the temporal bone and inner ear canals, scale with body size, but
hearing does not (Fig. 2.1) (Ketten 1984). Body mass and cochlear length
are strongly correlated because both are products of body scaling processes,
but there is no direct,functional relationship between cochlear length alone
and an animal's hearing range.
A primary assumption of some inner ear models is that all mammalian
basilar membranes are constructed of similar components that have a
common stiffness gradient (e.g., Greenwood 1990). Think of a megamembrane composed of graded modules from which each species selected
a contiguous set proportional to its body mass. That set dictated its hearing
range, which in most mammals covers about nine octaves. The human nineoctave subset lies near the middle of this hypothetical mega-array. Smaller
animal ears would be constructed largely of shorter, narrower, stiffer
modules towards the high-frequency mega-membrane base and therefore
have a higher maximal and higher minimal frequency than larger mammals.
Large species would have longer membranes but the span would be composed primarily of broad, thin modules from the lower-frequency apical
end, where the blue whale, of course has the corner on the last module. For
many land mammals, the assumption appears correct, but only because
length is an indirect correlate of the real functional feature for basilar membrane resonances, stiffness. For ears that follow this regular modular distribution, termed "generalists" (Echteler et al. 1994), basilar membrane
55
4. Fundamental Ear Morphometries: Generalist Versus
Specialist Bauplans
Audible ranges and thresholds vary dramatically from one species to the
next. Analyses of how hearing abilities, habitat, and ear anatomy are linked
in different species, particularly in animals from diverse habitats, provide
insights into how each component in the auditory periphery functions and
how different hearing capacities evolved. By observation, we know that
many species hear sounds inaudible to humans. Most mammals have some
ultrasonic hearing, and some, like African (Loxodonta africana) and Asian
elephants (Elephas maximus) appear to detect infrasonic signals (Payne et
al. 1986; O'Connell et al. 1997). Theoretically, "hearing" could extend arbitrarily high, but there are practical limits both in terms of the utility of the
information and in the physics of the receptor.
Hearing ranges are related to both animal size and niche. In general,
smaller animals have good high-frequency hearing while larger animals
tend to have better low-frequency hearing and a lower top frequency.
For example, mice have a functional high-frequency limit of approximately
90kHz; cats, 70kHz; humans, 20kHz; cows, 16kHz; elephants, 12kHz
(Fay 1988).A functional relationship between cochlear length and a species'
hearing range has been assumed in several mammalian ear modeling
efforts, but this is a shibboleth. Mammalian ear structures, particularly the
size of the temporal bone and inner ear canals, scale with body size, but
hearing does not (Fig. 2.1) (Ketten 1984). Body mass and cochlear length
are strongly correlated because both are products of body scaling processes,
but there is no direct,functional relationship between cochlear length alone
and an animal's hearing range.
A primary assumption of some inner ear models is that all mammalian
basilar membranes are constructed of similar components that have a
common stiffness gradient (e.g., Greenwood 1990). Think of a megamembrane composed of graded modules from which each species selected
a contiguous set proportional to its body mass. That set dictated its hearing
range, which in most mammals covers about nine octaves. The human nineoctave subset lies near the middle of this hypothetical mega-array. Smaller
animal ears would be constructed largely of shorter, narrower, stiffer
modules towards the high-frequency mega-membrane base and therefore
have a higher maximal and higher minimal frequency than larger mammals.
Large species would have longer membranes but the span would be composed primarily of broad, thin modules from the lower-frequency apical
end, where the blue whale, of course has the corner on the last module. For
many land mammals, the assumption appears correct, but only because
length is an indirect correlate of the real functional feature for basilar membrane resonances, stiffness. For ears that follow this regular modular distribution, termed "generalists" (Echteler et al. 1994), basilar membrane