66
D.R. Ketten
speed of sound, attenuating at 1/r rather than 1/r
2
and therefore has relatively little attenuation within the first kilometer. Elephants clearly respond
to infrasonic stimuli within several hundreds of meters of Rayleigh sources.
O'Connell et al. (1997) suggest elephants detect the technically subsonic
energy via bone and soft tissue conduction. Reuter et al. (1998) commenting on O'Connell et al.'s findings noted that elephants have massive ossicles with extensive soft tissue associations. Elephants also have temporal
bone complexes that have both partially and fully ossified skull attachments
(Meng et al. 1997). Similar bony attachments to the temporal bullae are
common in modern ungulates and are thought to have been a "preadaptive
feature" for aquatic hearing that was present in the ungulate condylarts
ancestors of whales (Thewissen 1998). Reuter et al. proposed that elephants
have a dual reception system of bone/soft tissue conduction for ultra-low
signals and pinnal-aerial channels for high-frequency sound reception.
Their hypothesis could explain the apparent contradiction in the audiometric and behavioral data. More important, it is an intriguing idea in light
of the independently proposed multichannel odontocete sound reception
scheme (Fig. 2.3) because Reuter et al. suggest that to accurately determine
elephant low-frequency sensitivity, it may be necessary to provide a nonaerial, substrate-coupled source. This is, in effect, what happens in water.
5.1.2 The Tympano-Periotic Complex
In modern Cetacea, the ear bone consists of two connected bullae, properly called the "tympano-periotic complex," that differ from temporal bone
complexes of other mammals in form, construction, position, and, possibly,
overall function. In all whales, the periotic bulla is dorsal and slightly medial
to the tympanic bulla (Fig. 2.2). It houses the inner ear and is partly fused
to the tympanic bulla (the "resonant" middle ear bulla) at one or more
points on its lateral and posterior faces. The shell-like, hollow tympanic
bulla encloses the middle ear space and ossicular chain. Tympanic and periotic dimensions are strongly correlated with animal size (r = 0.9) (Ketten
and Wartzok 1990). To put the size range of cetacean temporal bones into
perspective, a blue whale (B. musculus) periotic bulla is approximately the
size of a human brain, and with the tympanic bone attached, the complete
blue whale ear complex weighs well over a kilogram. The entire tympanoperiotic complex of the harbor porpoise (P. phocoena) weighs about 16 gm;
its periotic bulla would fit reasonably well into the blue whale round
window niche.
The tympano-periotic complex resides outside the skull in an extensive
peribullar cavity. The extracranial position of the tympano-periotic substantially increases the functional separation of the ears, which is a crucial
factor in underwater localization and is discussed in detail later in this
section. The peribullar cavity is bounded by the mandible, squamosal, pterygoid, and basi- and exoccipital bones (Figs. 2.2,2.4). In odontocetes, a specialized spongy, vascularized epithelium, the peribullar plexus, fills the
D.R. Ketten
speed of sound, attenuating at 1/r rather than 1/r
2
and therefore has relatively little attenuation within the first kilometer. Elephants clearly respond
to infrasonic stimuli within several hundreds of meters of Rayleigh sources.
O'Connell et al. (1997) suggest elephants detect the technically subsonic
energy via bone and soft tissue conduction. Reuter et al. (1998) commenting on O'Connell et al.'s findings noted that elephants have massive ossicles with extensive soft tissue associations. Elephants also have temporal
bone complexes that have both partially and fully ossified skull attachments
(Meng et al. 1997). Similar bony attachments to the temporal bullae are
common in modern ungulates and are thought to have been a "preadaptive
feature" for aquatic hearing that was present in the ungulate condylarts
ancestors of whales (Thewissen 1998). Reuter et al. proposed that elephants
have a dual reception system of bone/soft tissue conduction for ultra-low
signals and pinnal-aerial channels for high-frequency sound reception.
Their hypothesis could explain the apparent contradiction in the audiometric and behavioral data. More important, it is an intriguing idea in light
of the independently proposed multichannel odontocete sound reception
scheme (Fig. 2.3) because Reuter et al. suggest that to accurately determine
elephant low-frequency sensitivity, it may be necessary to provide a nonaerial, substrate-coupled source. This is, in effect, what happens in water.
5.1.2 The Tympano-Periotic Complex
In modern Cetacea, the ear bone consists of two connected bullae, properly called the "tympano-periotic complex," that differ from temporal bone
complexes of other mammals in form, construction, position, and, possibly,
overall function. In all whales, the periotic bulla is dorsal and slightly medial
to the tympanic bulla (Fig. 2.2). It houses the inner ear and is partly fused
to the tympanic bulla (the "resonant" middle ear bulla) at one or more
points on its lateral and posterior faces. The shell-like, hollow tympanic
bulla encloses the middle ear space and ossicular chain. Tympanic and periotic dimensions are strongly correlated with animal size (r = 0.9) (Ketten
and Wartzok 1990). To put the size range of cetacean temporal bones into
perspective, a blue whale (B. musculus) periotic bulla is approximately the
size of a human brain, and with the tympanic bone attached, the complete
blue whale ear complex weighs well over a kilogram. The entire tympanoperiotic complex of the harbor porpoise (P. phocoena) weighs about 16 gm;
its periotic bulla would fit reasonably well into the blue whale round
window niche.
The tympano-periotic complex resides outside the skull in an extensive
peribullar cavity. The extracranial position of the tympano-periotic substantially increases the functional separation of the ears, which is a crucial
factor in underwater localization and is discussed in detail later in this
section. The peribullar cavity is bounded by the mandible, squamosal, pterygoid, and basi- and exoccipital bones (Figs. 2.2,2.4). In odontocetes, a specialized spongy, vascularized epithelium, the peribullar plexus, fills the