44
D.R. Ketten
(Fay 1992). If we extend the analyses to ultra- and infrasonic animals, we
can learn substantially more about how hearing ranges are determined as
well as how to detect and use physical cues that are normally imperceptible to us (e.g., Hinchcliffe and Pye 1968; Webster and Webster 1975). Using
ears adapted to different media, we can begin to explore how the auditory
system deals with physical features of acoustic cues.
Whales and dolphins fit all three criteria for productive analyses. Even
more important, cetacean ears are derived from land mammal auditory
systems but may now be more acoustically and physically diverse than any
related land mammal group. Whales originally had air-adapted ears. All
cetaceans are descended from mesonychid condylarths, catlike, carnivorous,
land-based ungulates that became amphibious in the Eocene, probably to
exploit food-rich near-shore waters (Thewissen 1998). In the intervening 50
to 60 million years, as these condylarths gradually transformed from hoofed
waders into full-fledged, flippered whales, every portion of their anatomy
was physically and functionally reshaped to accommodate life in water.
Their air-adapted high-frequency mammalian ears had to be coupled to
water-borne sound for hearing to remain functional, but ear evolution took
place in tandem with other body reconfigurations. Just as the physical
demands of operating in water exacted a structural price in the locomotory
and thermoregulatory systems of marine mammals, the physics of swimming, diving, and resting on the surface reshaped the head. Modern
cetaceans have the most derived cranial structure of any mammal (Thewissen 1998; Cranford, Chapter 3). "Telescoping," a term coined by Miller
(1923), refers to the evolutionary revamping of the cetacean face. As the
anterior cranial structures pushed up and back, one bone sliding over
another, every facet of the auditory periphery was modified: pinnae and
external auditory canals were lost, the middle and inner ear capsules fused,
and a new ear complex with the bone density of enamel erupted from its
intracranial position to settle into a newly formed, cavernous peribullar
sinus. Today, all cetaceans are absolute aquatics, unable to move, reproduce,
or feed on land, and their ears are so fully adapted to water-borne sound
that they may no longer be able to detect or interpret airborne signals. Consequently they have ancestral ear elements in common with land mammals
but have added hydro-related specializations that hold clues to mediadependent hearing mechanisms.
Currently, there are 76 extant species of whales, ranging in size from
the harbor porpoise (Phocoena phocoena, 1m, 55 kg) to the blue whale
(Balaenoptera musculus,40m, 93,869 kg) (Nowak 1991). Most are members
of the suborder Odontoceti (65 species), all of which produce ultrasonic
signals and are presumed to echolocate (Nachtigall et al., Chapter 8; Au,
Chapter 9). The second suborder, the Mysticeti (rorquals, right, and baleen
whales; 11 species) are pelagic omnivores that produce intense infrasonic
signals, the function of which remains unknown. Therefore, as a group, they
D.R. Ketten
(Fay 1992). If we extend the analyses to ultra- and infrasonic animals, we
can learn substantially more about how hearing ranges are determined as
well as how to detect and use physical cues that are normally imperceptible to us (e.g., Hinchcliffe and Pye 1968; Webster and Webster 1975). Using
ears adapted to different media, we can begin to explore how the auditory
system deals with physical features of acoustic cues.
Whales and dolphins fit all three criteria for productive analyses. Even
more important, cetacean ears are derived from land mammal auditory
systems but may now be more acoustically and physically diverse than any
related land mammal group. Whales originally had air-adapted ears. All
cetaceans are descended from mesonychid condylarths, catlike, carnivorous,
land-based ungulates that became amphibious in the Eocene, probably to
exploit food-rich near-shore waters (Thewissen 1998). In the intervening 50
to 60 million years, as these condylarths gradually transformed from hoofed
waders into full-fledged, flippered whales, every portion of their anatomy
was physically and functionally reshaped to accommodate life in water.
Their air-adapted high-frequency mammalian ears had to be coupled to
water-borne sound for hearing to remain functional, but ear evolution took
place in tandem with other body reconfigurations. Just as the physical
demands of operating in water exacted a structural price in the locomotory
and thermoregulatory systems of marine mammals, the physics of swimming, diving, and resting on the surface reshaped the head. Modern
cetaceans have the most derived cranial structure of any mammal (Thewissen 1998; Cranford, Chapter 3). "Telescoping," a term coined by Miller
(1923), refers to the evolutionary revamping of the cetacean face. As the
anterior cranial structures pushed up and back, one bone sliding over
another, every facet of the auditory periphery was modified: pinnae and
external auditory canals were lost, the middle and inner ear capsules fused,
and a new ear complex with the bone density of enamel erupted from its
intracranial position to settle into a newly formed, cavernous peribullar
sinus. Today, all cetaceans are absolute aquatics, unable to move, reproduce,
or feed on land, and their ears are so fully adapted to water-borne sound
that they may no longer be able to detect or interpret airborne signals. Consequently they have ancestral ear elements in common with land mammals
but have added hydro-related specializations that hold clues to mediadependent hearing mechanisms.
Currently, there are 76 extant species of whales, ranging in size from
the harbor porpoise (Phocoena phocoena, 1m, 55 kg) to the blue whale
(Balaenoptera musculus,40m, 93,869 kg) (Nowak 1991). Most are members
of the suborder Odontoceti (65 species), all of which produce ultrasonic
signals and are presumed to echolocate (Nachtigall et al., Chapter 8; Au,
Chapter 9). The second suborder, the Mysticeti (rorquals, right, and baleen
whales; 11 species) are pelagic omnivores that produce intense infrasonic
signals, the function of which remains unknown. Therefore, as a group, they