2. Cetacean Ears
99
jected 120 to 130kHz region of the P. phocoena estimated basilar membrane frequency distribution map (Ketten 1994; Ketten et al. 1997).
At the moment, we cannot affirm or deny any these proposed bat-dolphin
commonalities. Recent anatomical studies in bats are heavily weighted
towards CF acoustic foveal mechanisms. In the last 10 years, more than
70% of papers on bat periphery dealt with neural and basilar membrane
specializations of Pteronotus parnelli (the mustache bat) and R. ferrumequinum. In these bats, normal basilar membrane tapering is disrupted by
one or more constant cross-section segments where spatial and neural representation of a narrow frequency band is grossly expanded (Kossl and
Vater 1995). The consensus is that this adaptation provides exceptionally
narrow tuning and enhancement of CF and CF2 in noise, consistent with
the ability of these bats to handle clutter. Ironically, there are fewer broad
interspecies comparative studies of bat inner ears than of whale ears and
almost no studies that address functional cochlear structure in less specialized FM bat species. If the necessary data are obtained for both groups,
comprehensive cross-species/cross-media/cross-ear comparisons focusing
on task-dependent adaptations could provide not only a better understanding of echolocation but also a new way to think about hearing from a
task in habitat perspective.
6.3 Deep Ears
Type M inner ear formats are known only in large, pelagic whales. A
specific use for infrasonic frequencies by whales has not yet been demonstrated, although several possibilities exist. Low frequencies could be used
to communicate over long distances and even to echolocate seabed and
coastal topographic details as aids for offshore navigation and long-range
migrations. Whatever the present function, ultra-low-frequency hearing in
mysticetes may simply have evolved as an outgrowth of mechanical constraints imposed by larger ear size.
The ears of mysticetes are less derived than those of odontocetes because
their bullar and inner ear proportions are consistent with their mass. Put
simply, these ears are huge. All middle and inner ear structures scale to body
size, which suggests that ear configurations dominated by low-frequency
characteristics is a morphometric by-product of being large and was not
fundamentally driven by a special advantage from infrasonic detection. If
so, their hearing capacities are a secondary effect of rather than in defiance
of their body size, as is speculated to be the case in odontocetes. However,
even if the theory is correct that a bigger ear came after rather than before
the baleen body and that infrasonic hearing abilities were a de facto result,
that does not preclude a subsequent sophisticated exploitation of the mysticete ear's low-frequency capacity.
Because of the extreme divergence in the ears and in their associated
skull features between extant mysticetes and odontocetes, even fragmen-
99
jected 120 to 130kHz region of the P. phocoena estimated basilar membrane frequency distribution map (Ketten 1994; Ketten et al. 1997).
At the moment, we cannot affirm or deny any these proposed bat-dolphin
commonalities. Recent anatomical studies in bats are heavily weighted
towards CF acoustic foveal mechanisms. In the last 10 years, more than
70% of papers on bat periphery dealt with neural and basilar membrane
specializations of Pteronotus parnelli (the mustache bat) and R. ferrumequinum. In these bats, normal basilar membrane tapering is disrupted by
one or more constant cross-section segments where spatial and neural representation of a narrow frequency band is grossly expanded (Kossl and
Vater 1995). The consensus is that this adaptation provides exceptionally
narrow tuning and enhancement of CF and CF2 in noise, consistent with
the ability of these bats to handle clutter. Ironically, there are fewer broad
interspecies comparative studies of bat inner ears than of whale ears and
almost no studies that address functional cochlear structure in less specialized FM bat species. If the necessary data are obtained for both groups,
comprehensive cross-species/cross-media/cross-ear comparisons focusing
on task-dependent adaptations could provide not only a better understanding of echolocation but also a new way to think about hearing from a
task in habitat perspective.
6.3 Deep Ears
Type M inner ear formats are known only in large, pelagic whales. A
specific use for infrasonic frequencies by whales has not yet been demonstrated, although several possibilities exist. Low frequencies could be used
to communicate over long distances and even to echolocate seabed and
coastal topographic details as aids for offshore navigation and long-range
migrations. Whatever the present function, ultra-low-frequency hearing in
mysticetes may simply have evolved as an outgrowth of mechanical constraints imposed by larger ear size.
The ears of mysticetes are less derived than those of odontocetes because
their bullar and inner ear proportions are consistent with their mass. Put
simply, these ears are huge. All middle and inner ear structures scale to body
size, which suggests that ear configurations dominated by low-frequency
characteristics is a morphometric by-product of being large and was not
fundamentally driven by a special advantage from infrasonic detection. If
so, their hearing capacities are a secondary effect of rather than in defiance
of their body size, as is speculated to be the case in odontocetes. However,
even if the theory is correct that a bigger ear came after rather than before
the baleen body and that infrasonic hearing abilities were a de facto result,
that does not preclude a subsequent sophisticated exploitation of the mysticete ear's low-frequency capacity.
Because of the extreme divergence in the ears and in their associated
skull features between extant mysticetes and odontocetes, even fragmen-