2. Cetacean Ears
77
In fact, at this moment, middle ear functions are unresolved for all
cetaceans. Curiously, middle ears are more obtainable than many other
whale ear structures. They simply have not been well explored, and, in the
absence of new data, the debate goes on.
5.3 The Inner Ear
The cetacean inner ear is subdivided into the auditory and vestibular
systems.
5.3.1 The Vestibular System
The vestibular system is not generally considered part of the auditory
system, although it has been implicated in low-frequency hearing (Yeowart
1976) and there are some special features of cetacean vestibular systems
that are worth noting briefly. Size is not a criterion for a functional vestibular system, but cetaceans have semicircular canals that are "disproportionately minute" compared to cochlear canal diameters and volumes
(Boenninghaus 1903; Gray 1951). This reduction is most extreme in odontocetes but it is true also for mysticetes. Semicircular canals in some individuals are compressed, the ampullae are nearly acellular, and the
vestibular fiber counts are commensurately small (Ketten 1992; Gao and
Zhou 1995). Both the average cell count for Scarpa's ganglia «4,100) and
the proportion of eighth nerve fibers that are vestibular «5%) are exceptionally low compared to an average of 30% in most mammals (Table 2.2).
No other mammal, including pinnipeds, is known to have similar vestibular reductions, which argues that attenuated semicircular canals are related
to an obligate aquatic lifestyle. One possibility is that the fusion of the cervical vertebrae that occured in whales limited head movement, reducing
inputs to the vestibular system and decreasing its utility, which led to an
evolutionary diminution. The modern cetacean vestibular system may act
therefore purely as van Bergeijk (1967) suggested, that is, as a "vehicleoriented accelerometer," obtaining only linear acceleration and gravity cues
but no rotational or three-dimensional acceleration inputs. Studies of
labyrinthectomized cats and congenitally alabyrinthine humans found that
the absence of functional semicircular canals eliminates motion sickness
(Graybiel 1964). An attenuated vestibular system may therefore be highly
adaptive for cetaceans, permitting high-flying spins without "spacesickness" side-effects.
Two cetacean groups have been found to have noticeably larger vestibular systems. Bowhead whales (Balaena mysticetus) and Northern right
whales (Eubalaena glacialis) have large semicircular canals and very similar
bullar shapes (Fig. 2.4) (Yamada and Yoshizaki 1959). While the vestibular
system in these whales is still much smaller than in most land mammals, it
is approximately double the size of vestibular systems in other mysticetes.
77
In fact, at this moment, middle ear functions are unresolved for all
cetaceans. Curiously, middle ears are more obtainable than many other
whale ear structures. They simply have not been well explored, and, in the
absence of new data, the debate goes on.
5.3 The Inner Ear
The cetacean inner ear is subdivided into the auditory and vestibular
systems.
5.3.1 The Vestibular System
The vestibular system is not generally considered part of the auditory
system, although it has been implicated in low-frequency hearing (Yeowart
1976) and there are some special features of cetacean vestibular systems
that are worth noting briefly. Size is not a criterion for a functional vestibular system, but cetaceans have semicircular canals that are "disproportionately minute" compared to cochlear canal diameters and volumes
(Boenninghaus 1903; Gray 1951). This reduction is most extreme in odontocetes but it is true also for mysticetes. Semicircular canals in some individuals are compressed, the ampullae are nearly acellular, and the
vestibular fiber counts are commensurately small (Ketten 1992; Gao and
Zhou 1995). Both the average cell count for Scarpa's ganglia «4,100) and
the proportion of eighth nerve fibers that are vestibular «5%) are exceptionally low compared to an average of 30% in most mammals (Table 2.2).
No other mammal, including pinnipeds, is known to have similar vestibular reductions, which argues that attenuated semicircular canals are related
to an obligate aquatic lifestyle. One possibility is that the fusion of the cervical vertebrae that occured in whales limited head movement, reducing
inputs to the vestibular system and decreasing its utility, which led to an
evolutionary diminution. The modern cetacean vestibular system may act
therefore purely as van Bergeijk (1967) suggested, that is, as a "vehicleoriented accelerometer," obtaining only linear acceleration and gravity cues
but no rotational or three-dimensional acceleration inputs. Studies of
labyrinthectomized cats and congenitally alabyrinthine humans found that
the absence of functional semicircular canals eliminates motion sickness
(Graybiel 1964). An attenuated vestibular system may therefore be highly
adaptive for cetaceans, permitting high-flying spins without "spacesickness" side-effects.
Two cetacean groups have been found to have noticeably larger vestibular systems. Bowhead whales (Balaena mysticetus) and Northern right
whales (Eubalaena glacialis) have large semicircular canals and very similar
bullar shapes (Fig. 2.4) (Yamada and Yoshizaki 1959). While the vestibular
system in these whales is still much smaller than in most land mammals, it
is approximately double the size of vestibular systems in other mysticetes.