72
D.R. Ketten
cavities with loosely articulated, massive ossicles. Small low-frequency
animals often add mass to the system or have disproportionately large, softwalled middle ear spaces.
5.2. Cetacean Middle Ear Anatomy
Odontocete middle ears appear to be a compromise between enhancing
high-frequency sensitivity and strength. In microchiropteran bats, highfrequency sensitivity in the middle ear is achieved by lightening the ossicles and stiffening their attachments (Reysenbach de Haan 1956; Sales and
Pye 1974). In odontocetes, the ossicles are more massive than in most land
mammals but have multiple stiffening elements as well (McCormick 1970;
Ketten 1984,1992). As noted earlier, there is no conventional, discoid membranous tympanum as is commonly found in land mammals. Instead, the
tympanic membrane is a highly derived structure comprised of a strip of
hyaline membrane backed by the tympanic conus, a compressed, fibrous,
partly calcified funnel that attaches to the body of the malleus with a straplike ligament. A bony ridge, the processus gracilis, connects the malleus to
the outer wall of the tympanic bulla. This is the one ossicle that is clearly
normally fixed in all cetaceans. The other two major interossicular joints are
stiffened with ligaments and a membranous sheath but are flexible. The
incus is freely mobile. The stapes is mobile with a moderately thick but conventional annular ligament. A substantial stapedial ligament attaches to the
head of the stapes; the crus is generally closed.
The consensus of the anatomical data is that odontocete middle ear
anatomy is fundamentally consistent with a middle ear system tuned to high
frequencies. There are no obviously degenerated or aberrant structures that
suggest the middle ear cavity or ossicular chain is vestigial or dysfunctional
in any cetacean species documented to date. Sporadic reports in the
cetacean literature that indicate odontocete ossicles are either wholly or
partially fused are occasionally cited as evidence that odontocete middle
ears are dysfunctional, but there are good reasons to be cautious about
these conclusions. Whale research is plagued by small sample sizes and lack
of comprehensive, controlled subject histories, and the possibility that the
ear under study is abnormal must always be considered. In humans, ossicular ankylosis occurs in -5% of the population (Schuknecht 1993). It
has recently been shown that dolphins are subject to presbycusic
(age-related) hearing loss and several forms of irrecoverable ear disease
(Brill et al. 1997; Ketten et al. 1997; Ridgway and Carder 1997). If odontocetes have incidences similar to humans of other hearing related pathologies, we can expect that 1 in 20 adult animals may have a compromised
middle ear.
Mysticete middle ears contrast sharply with the high-frequency odontocete model. Mysticete ossicles are larger and more massive with none of
the high-frequency-related specializations common among odontocetes.
D.R. Ketten
cavities with loosely articulated, massive ossicles. Small low-frequency
animals often add mass to the system or have disproportionately large, softwalled middle ear spaces.
5.2. Cetacean Middle Ear Anatomy
Odontocete middle ears appear to be a compromise between enhancing
high-frequency sensitivity and strength. In microchiropteran bats, highfrequency sensitivity in the middle ear is achieved by lightening the ossicles and stiffening their attachments (Reysenbach de Haan 1956; Sales and
Pye 1974). In odontocetes, the ossicles are more massive than in most land
mammals but have multiple stiffening elements as well (McCormick 1970;
Ketten 1984,1992). As noted earlier, there is no conventional, discoid membranous tympanum as is commonly found in land mammals. Instead, the
tympanic membrane is a highly derived structure comprised of a strip of
hyaline membrane backed by the tympanic conus, a compressed, fibrous,
partly calcified funnel that attaches to the body of the malleus with a straplike ligament. A bony ridge, the processus gracilis, connects the malleus to
the outer wall of the tympanic bulla. This is the one ossicle that is clearly
normally fixed in all cetaceans. The other two major interossicular joints are
stiffened with ligaments and a membranous sheath but are flexible. The
incus is freely mobile. The stapes is mobile with a moderately thick but conventional annular ligament. A substantial stapedial ligament attaches to the
head of the stapes; the crus is generally closed.
The consensus of the anatomical data is that odontocete middle ear
anatomy is fundamentally consistent with a middle ear system tuned to high
frequencies. There are no obviously degenerated or aberrant structures that
suggest the middle ear cavity or ossicular chain is vestigial or dysfunctional
in any cetacean species documented to date. Sporadic reports in the
cetacean literature that indicate odontocete ossicles are either wholly or
partially fused are occasionally cited as evidence that odontocete middle
ears are dysfunctional, but there are good reasons to be cautious about
these conclusions. Whale research is plagued by small sample sizes and lack
of comprehensive, controlled subject histories, and the possibility that the
ear under study is abnormal must always be considered. In humans, ossicular ankylosis occurs in -5% of the population (Schuknecht 1993). It
has recently been shown that dolphins are subject to presbycusic
(age-related) hearing loss and several forms of irrecoverable ear disease
(Brill et al. 1997; Ketten et al. 1997; Ridgway and Carder 1997). If odontocetes have incidences similar to humans of other hearing related pathologies, we can expect that 1 in 20 adult animals may have a compromised
middle ear.
Mysticete middle ears contrast sharply with the high-frequency odontocete model. Mysticete ossicles are larger and more massive with none of
the high-frequency-related specializations common among odontocetes.