2. Cetacean Ears
97
case of helicotrema size, are a major influence on the acoustic filter characteristics at the apex. While there are no comparable data for cetaceans at
this time, it is intriguing that the l.5-turn cochlea of a small Type I odontocete, P phocoera, has a scala vestibuli area approximately equal to that of
a 2.S-turn cochlea of Megaptera novaeargliae (the humpback whale) at an
equivalent position. This general description is consistent with Dallos's
assessment and with the projected differences in the low-frequency abilities of a Type I (poor low-frequency sensitivity) versus Type M (good
low-frequency sensitivity) ear. This also suggests that the fundamental
mechanics for low-frequency hearing are similar in cetaceans and land
mammals.
6.2 Cross-Boundary Comparisons
Cochlear formats and frequency ranges in cetaceans coincide with habitats
and feeding behaviors. Type I formats are found in inshore phocoenids and
riverine platanistids. These species live in turbid waters and use ultrahighfrequency, short wavelength signals consistent with analyzing fine details of
nearby objects. Type II formats are common in offshore and pelagic delphinids. Their slightly broader, less rigid membranes suggest better mid-to
low sonic range hearing than Type I ears as well as lower frequency ultrasonic ranges. These hearing characteristics are consistent with highly social
species that use 1 to 10 kHz communication signals and lower frequency,
longer wavelength ultrasonic signals that can resolve predators and prey at
greater distances than the Type I signals.
Are these format differences uniquely aquatic? Structurally, yes; functionally, perhaps not. If sound use is correlated with habitat, and in turn
with function, structural adaptations found in one medium should be found
in parallel in animals that use similar sounds and at some level similar
behavior in a different medium. Put simply, ears should parallel habitat and
signal types. Echolocators offer the chance to make multispecies crossmedia comparisons.
Superficially, bat and dolphin echolocation signals and processing appear
to have little in common. Dolphin echolocation signals are generally
shorter, broader band waveforms with higher peak spectra (-SOils, 40 to
150kHz) than most bat signals (several milliseconds, 16 to 80kHz). Bats
and dolphins are comparable at discriminating shape and size, but dolphins
are superior at detecting target range and composition and may be better
at detection in noise (Au, Chapter 9). However, if we put performance
data together with anatomy, habitat, and hunting characteristics, there are
several intriguing parallels.
Basic echolocation frequency differences between the groups are consistent with wavelength differences in the two media and with prey sizes; that
is, the frequencies used by dolphins are only two- to threefold higher than
those of most bats, not 4.5-fold, but moth wing profiles are, acoustically, proportionately smaller than most fish profiles. Source energy flux density (efd)
97
case of helicotrema size, are a major influence on the acoustic filter characteristics at the apex. While there are no comparable data for cetaceans at
this time, it is intriguing that the l.5-turn cochlea of a small Type I odontocete, P phocoera, has a scala vestibuli area approximately equal to that of
a 2.S-turn cochlea of Megaptera novaeargliae (the humpback whale) at an
equivalent position. This general description is consistent with Dallos's
assessment and with the projected differences in the low-frequency abilities of a Type I (poor low-frequency sensitivity) versus Type M (good
low-frequency sensitivity) ear. This also suggests that the fundamental
mechanics for low-frequency hearing are similar in cetaceans and land
mammals.
6.2 Cross-Boundary Comparisons
Cochlear formats and frequency ranges in cetaceans coincide with habitats
and feeding behaviors. Type I formats are found in inshore phocoenids and
riverine platanistids. These species live in turbid waters and use ultrahighfrequency, short wavelength signals consistent with analyzing fine details of
nearby objects. Type II formats are common in offshore and pelagic delphinids. Their slightly broader, less rigid membranes suggest better mid-to
low sonic range hearing than Type I ears as well as lower frequency ultrasonic ranges. These hearing characteristics are consistent with highly social
species that use 1 to 10 kHz communication signals and lower frequency,
longer wavelength ultrasonic signals that can resolve predators and prey at
greater distances than the Type I signals.
Are these format differences uniquely aquatic? Structurally, yes; functionally, perhaps not. If sound use is correlated with habitat, and in turn
with function, structural adaptations found in one medium should be found
in parallel in animals that use similar sounds and at some level similar
behavior in a different medium. Put simply, ears should parallel habitat and
signal types. Echolocators offer the chance to make multispecies crossmedia comparisons.
Superficially, bat and dolphin echolocation signals and processing appear
to have little in common. Dolphin echolocation signals are generally
shorter, broader band waveforms with higher peak spectra (-SOils, 40 to
150kHz) than most bat signals (several milliseconds, 16 to 80kHz). Bats
and dolphins are comparable at discriminating shape and size, but dolphins
are superior at detecting target range and composition and may be better
at detection in noise (Au, Chapter 9). However, if we put performance
data together with anatomy, habitat, and hunting characteristics, there are
several intriguing parallels.
Basic echolocation frequency differences between the groups are consistent with wavelength differences in the two media and with prey sizes; that
is, the frequencies used by dolphins are only two- to threefold higher than
those of most bats, not 4.5-fold, but moth wing profiles are, acoustically, proportionately smaller than most fish profiles. Source energy flux density (efd)