100
D.R. Ketten
tary evidence about squamosal development, bullar proportions, or skull
attachments and level of fixation in a fossil could be surprisingly revealing
about its hearing. Much of the work on fossil whales and their hearing
capacity has focused on the middle and inner ear anatomy, but in some of
the most interesting forms, the bullae are lacking or damaged. In these
cases, looking at the remainders of the ear suite could produce useful
insights into the hearing of archaeocetes and, therefore, help determine
which came first, the clicking or the tympanic egg.
Mysticetes appear geologically near the time new oceans opened in
southern latitudes (Thewissen 1998). Even today, these high-latitude waters
are terrifically productive, but they are also colder than the temperate seas
in which whales first evolved. Surface area increases more slowly than
volume, therefore bigger mammals have a substantial thermal advantage
in cold water; large whales are warmer. Inner ear membranes scale with
animal size. It is likely that increased body size coincided with successful
adaptation to cold seas and, in turn, with large ears. As ears grew, basilar
membranes would, given no counter pressure to retain high-frequency
hearing, simply expand to scale. A lower frequency cochlea would be the
product of this nonspecialized expansion. At the same time the tympanic
bulla grew. Therefore, as larger whales evolved, ear scaling may have forced
inner ear and middle resonance characteristics to progressively lower
frequencies, ultimately reaching the practical and profound limits of the
blue whale.
7. Summary
An underlying assumption of this chapter is that systematic comparisons of
land and cetacean peripheral auditory systems can provide insights into
how whales hear in water. The available data reveal a complex, highly
derived peripheral auditory architecture with specializations for extended
hearing ranges, as well as reception and localization of water-borne sound.
Aquatic influences are most evident at the gross anatomical level. There
are no pinnae. All cetacean periotics, tympanics, and ossicles are constructed
of dense, compact bone. The odontocete tympano-periotic complex is isolated acoustically from the skull, which is adaptive for aquatic echolocation.
The position and isolation of odontocete bullae support the jaw theory of
ultrasonic signal reception via fatty acoustic wave guides in and around the
mandible. Sound reception mechanisms in mysticetes are unknown, but
bony skull connections and a highly derived tympanic membrane (glove
finger) suggest combined bone and soft tissue mechanisms. The extracraniallocation of the ear in all whales is advantageous for underwater sound
localization.
Cetacean middle ears divide grossly into low- versus high-frequency composites that follow the suborders. Inner ear anatomy varies more by species.
Cochlear lengths correlate with animal size, ranging 20 to 70mm. Cochlear
D.R. Ketten
tary evidence about squamosal development, bullar proportions, or skull
attachments and level of fixation in a fossil could be surprisingly revealing
about its hearing. Much of the work on fossil whales and their hearing
capacity has focused on the middle and inner ear anatomy, but in some of
the most interesting forms, the bullae are lacking or damaged. In these
cases, looking at the remainders of the ear suite could produce useful
insights into the hearing of archaeocetes and, therefore, help determine
which came first, the clicking or the tympanic egg.
Mysticetes appear geologically near the time new oceans opened in
southern latitudes (Thewissen 1998). Even today, these high-latitude waters
are terrifically productive, but they are also colder than the temperate seas
in which whales first evolved. Surface area increases more slowly than
volume, therefore bigger mammals have a substantial thermal advantage
in cold water; large whales are warmer. Inner ear membranes scale with
animal size. It is likely that increased body size coincided with successful
adaptation to cold seas and, in turn, with large ears. As ears grew, basilar
membranes would, given no counter pressure to retain high-frequency
hearing, simply expand to scale. A lower frequency cochlea would be the
product of this nonspecialized expansion. At the same time the tympanic
bulla grew. Therefore, as larger whales evolved, ear scaling may have forced
inner ear and middle resonance characteristics to progressively lower
frequencies, ultimately reaching the practical and profound limits of the
blue whale.
7. Summary
An underlying assumption of this chapter is that systematic comparisons of
land and cetacean peripheral auditory systems can provide insights into
how whales hear in water. The available data reveal a complex, highly
derived peripheral auditory architecture with specializations for extended
hearing ranges, as well as reception and localization of water-borne sound.
Aquatic influences are most evident at the gross anatomical level. There
are no pinnae. All cetacean periotics, tympanics, and ossicles are constructed
of dense, compact bone. The odontocete tympano-periotic complex is isolated acoustically from the skull, which is adaptive for aquatic echolocation.
The position and isolation of odontocete bullae support the jaw theory of
ultrasonic signal reception via fatty acoustic wave guides in and around the
mandible. Sound reception mechanisms in mysticetes are unknown, but
bony skull connections and a highly derived tympanic membrane (glove
finger) suggest combined bone and soft tissue mechanisms. The extracraniallocation of the ear in all whales is advantageous for underwater sound
localization.
Cetacean middle ears divide grossly into low- versus high-frequency composites that follow the suborders. Inner ear anatomy varies more by species.
Cochlear lengths correlate with animal size, ranging 20 to 70mm. Cochlear