2. Cetacean Ears
57
4.1 Cetacean Ear Morphometries: A Zeeohrplan?
How well do marine mammals mesh with the generalist versus specialist
land mammal hearing schema outlined above? Considering the problems
implicit in an aquatic habitat for an unmodified air-adapted ear, can there
be any commonalities? Despite the fact that there are substantial adaptations in all cetacean ears related to coping with increased sound speed, large
pressures, and a host of other aquatic demands, whales retained the essentials of air-adapted ears, such as a spiral cochlea and discrete middle ear
cavity with a three-part ossicular chain. Consequently, what we see today is
a fascinating admixture: highly specialized pressure adaptations, subtle
structure-frequency-habitat correlations, leviathan-scale ear structures, and
extensive peripheral remodeling, all overlaying a sophisticated but fundamentally mammalian ear.
Most cetaceans are large, massive animals that, by the generalist metric, should have low- to very-low-frequency hearing. The largest whales
(Mysticeti; Type M) are acoustically consistent with their extreme size. They
produce infrasonic frequencies, and we expect to find they have middle and
inner ear adaptations consistent with predominately low-frequency hearing.
As such, mysticetes may be simply an extreme of the generalist format. The
majority of odontocetes, although smaller than mysticetes, are still very
large animals by land mammal standards, and their gross ear dimensions,
particularly cochlear length, scale to body size exactly like those of land
mammals (Table 2.1, Fig. 2.1). Prior to the first major publication of research
on dolphin echolocation (Kellogg 1959), it would have been reasonable to
assume from their sonic range signals and size that these large animals had
mid- to low-frequency hearing capacities similar to cows. Today, it is clear
from their audiograms and sounds that virtually all odontocetes, including
the sperm whale, not only hear some range of ultrasonic frequencies despite
their size but that ultrasonic analyses dominate their auditory systems.
Therefore, Type I and Type II species are acoustically inconsistent with
the mid- to low-frequency ear predicted by generalist land mammal ear
models, making odontocetes in particular prime candidates for having
anisometric ears.
What is the appropriate functional reference for cetacean ears since body
mass obviously is not? Water is a dense medium in which light attenuates
faster than sound. Consequently, marine mammals are de facto crepuscular species. If we look at cetaceans in terms of hearing fitness for their
habitat, good high-frequency hearing is logical and consistent with a similar
trend on land where high-frequency hearing is common in nocturnal
species, particularly among predators. Mysticetes live under the same lowlight conditions, but they are primarily diurnally active, opportunistic
feeders. On land, most dusk, dawn, and nocturnal species are small. How
do odontocetes manage ultrasonic ears despite their size? Like specialist
mammals, they have structural adaptations that override the generalist size-
57
4.1 Cetacean Ear Morphometries: A Zeeohrplan?
How well do marine mammals mesh with the generalist versus specialist
land mammal hearing schema outlined above? Considering the problems
implicit in an aquatic habitat for an unmodified air-adapted ear, can there
be any commonalities? Despite the fact that there are substantial adaptations in all cetacean ears related to coping with increased sound speed, large
pressures, and a host of other aquatic demands, whales retained the essentials of air-adapted ears, such as a spiral cochlea and discrete middle ear
cavity with a three-part ossicular chain. Consequently, what we see today is
a fascinating admixture: highly specialized pressure adaptations, subtle
structure-frequency-habitat correlations, leviathan-scale ear structures, and
extensive peripheral remodeling, all overlaying a sophisticated but fundamentally mammalian ear.
Most cetaceans are large, massive animals that, by the generalist metric, should have low- to very-low-frequency hearing. The largest whales
(Mysticeti; Type M) are acoustically consistent with their extreme size. They
produce infrasonic frequencies, and we expect to find they have middle and
inner ear adaptations consistent with predominately low-frequency hearing.
As such, mysticetes may be simply an extreme of the generalist format. The
majority of odontocetes, although smaller than mysticetes, are still very
large animals by land mammal standards, and their gross ear dimensions,
particularly cochlear length, scale to body size exactly like those of land
mammals (Table 2.1, Fig. 2.1). Prior to the first major publication of research
on dolphin echolocation (Kellogg 1959), it would have been reasonable to
assume from their sonic range signals and size that these large animals had
mid- to low-frequency hearing capacities similar to cows. Today, it is clear
from their audiograms and sounds that virtually all odontocetes, including
the sperm whale, not only hear some range of ultrasonic frequencies despite
their size but that ultrasonic analyses dominate their auditory systems.
Therefore, Type I and Type II species are acoustically inconsistent with
the mid- to low-frequency ear predicted by generalist land mammal ear
models, making odontocetes in particular prime candidates for having
anisometric ears.
What is the appropriate functional reference for cetacean ears since body
mass obviously is not? Water is a dense medium in which light attenuates
faster than sound. Consequently, marine mammals are de facto crepuscular species. If we look at cetaceans in terms of hearing fitness for their
habitat, good high-frequency hearing is logical and consistent with a similar
trend on land where high-frequency hearing is common in nocturnal
species, particularly among predators. Mysticetes live under the same lowlight conditions, but they are primarily diurnally active, opportunistic
feeders. On land, most dusk, dawn, and nocturnal species are small. How
do odontocetes manage ultrasonic ears despite their size? Like specialist
mammals, they have structural adaptations that override the generalist size-