2. Cetacean Ears
45
employ the broadest known acoustic range, spanning low infrasonic (10 Hz)
to high ultrasonic (200kHz) frequencies.
Hearing is arguably the primary sensory and communication channel for
cetaceans, and we expect all whale ears are highly evolved. Comparative
functional anatomy studies may be the only way to understand the breadth
of whale ears because the majority of whale species are not approachable
by conventional audiometry. Only about 13% of all species have ever been
tested, all those tested are from one suborder, and nearly all are from one
family. Given the diversity of habitats, behaviors, and sizes that cetaceans
encompass, it would be naive to expect that data from a few species or one
division will provide a full picture of cetacean hearing. By analyzing the
structure of a broad spectrum of cetacean ears, we can gain insights not only
into whale hearing and aquatic adaptations but also into some basic hearing
issues. First, similarities between land and aquatic mammal ears are likely
to be related to fundamental mammalian ear mechanisms. Second, structures that are common among aquatic species but lacking or significantly
different in land mammals are probably key elements for transducing
water-borne sound. Third, because of extreme variations among whales in
animal size, sound use patterns, and habitats, differences that we see among
whale and dolphin ears that have land mammal parallels can teach us something about how auditory anatomy is shaped by physiologic and environmental factors.
Therefore, the most cogent reason for studying whale ears is simply to
find out how they to do it, that is, how do they hear-at high speed, under
pressure, and underwater.
2. Comparative Acoustics: Sound in Air Versus Water
To understand ears, it is imperative to understand not only how they were
evolutionarily tailored by the fundamental needs of the animal but also how
the information options were constrained by the acoustic properties of the
medium in which each species evolved. To properly assess whale ears and
place them in a general mammalian hearing context, it is necessary to
understand how the physical properties of water vs. air affect acoustic cues.
Because water is denser than air, sound in water travels faster and with
less attenuation than sound in air. Sound speed (c) in moist ambient surface
air is approximately 340m/s. Sound speed in sea water averages 1,530m/s
but will vary with any factor affecting density. The principal physical factors
affecting density in sea water are salinity, temperature, and pressure.
Because these factors act synergistically, the oceans have highly variable
sound profiles that change both seasonally and regionally. This raises the
interesting possibility that a whale during a few thousand meter dive could
experience theoretically a 10% variation in ambient acoustic velocities, but
45
employ the broadest known acoustic range, spanning low infrasonic (10 Hz)
to high ultrasonic (200kHz) frequencies.
Hearing is arguably the primary sensory and communication channel for
cetaceans, and we expect all whale ears are highly evolved. Comparative
functional anatomy studies may be the only way to understand the breadth
of whale ears because the majority of whale species are not approachable
by conventional audiometry. Only about 13% of all species have ever been
tested, all those tested are from one suborder, and nearly all are from one
family. Given the diversity of habitats, behaviors, and sizes that cetaceans
encompass, it would be naive to expect that data from a few species or one
division will provide a full picture of cetacean hearing. By analyzing the
structure of a broad spectrum of cetacean ears, we can gain insights not only
into whale hearing and aquatic adaptations but also into some basic hearing
issues. First, similarities between land and aquatic mammal ears are likely
to be related to fundamental mammalian ear mechanisms. Second, structures that are common among aquatic species but lacking or significantly
different in land mammals are probably key elements for transducing
water-borne sound. Third, because of extreme variations among whales in
animal size, sound use patterns, and habitats, differences that we see among
whale and dolphin ears that have land mammal parallels can teach us something about how auditory anatomy is shaped by physiologic and environmental factors.
Therefore, the most cogent reason for studying whale ears is simply to
find out how they to do it, that is, how do they hear-at high speed, under
pressure, and underwater.
2. Comparative Acoustics: Sound in Air Versus Water
To understand ears, it is imperative to understand not only how they were
evolutionarily tailored by the fundamental needs of the animal but also how
the information options were constrained by the acoustic properties of the
medium in which each species evolved. To properly assess whale ears and
place them in a general mammalian hearing context, it is necessary to
understand how the physical properties of water vs. air affect acoustic cues.
Because water is denser than air, sound in water travels faster and with
less attenuation than sound in air. Sound speed (c) in moist ambient surface
air is approximately 340m/s. Sound speed in sea water averages 1,530m/s
but will vary with any factor affecting density. The principal physical factors
affecting density in sea water are salinity, temperature, and pressure.
Because these factors act synergistically, the oceans have highly variable
sound profiles that change both seasonally and regionally. This raises the
interesting possibility that a whale during a few thousand meter dive could
experience theoretically a 10% variation in ambient acoustic velocities, but