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Chapter eleven: Acoustics
11.2.2.3 Family Ursidae
Similar to sea otters, polar bears have a relatively short evolutionary history in the marine
environment, having diverged from terrestrial bears <2 million years ago (Cahill et al.
2013). Little research has been conducted on the auditory system of the polar bear (as an
aside, the polar bear is the only bear for which hearing data are available). The only known
modifications to the auditory apparatus relative to that of terrestrial carnivores are prominent fur inside of the pinna and a reduction in pinna size relative to terrestrial bears,
suggesting thermoregulatory adaptations for reducing heat loss in the extreme cold of the
Arctic (Stirling 1988).
Behavioral and electrophysiological measurements of aerial hearing sensitivity have
demonstrated that polar bears are sensitive to aerial sound over a frequency range similar to that of pinnipeds and mustelids: approximately 125 Hz to 32 kHz (Nachtigall et al.
2007; Owen and Bowles 2011). There are currently no data on underwater hearing for polar
bears, but their capabilities are probably poor given the lack of apparent adaptations of the
auditory system and the suspected low importance of underwater sound in communication, foraging, and navigation. Future measurements of underwater hearing in the polar
bear would, however, be of interest in order to make amphibious comparisons in the most
terrestrial marine mammal.
11.2.3 Order Sirenia
The sirenians comprise manatee species and the dugong. This order of marine mammals
is relatively divergent from the marine carnivores and cetaceans in terms of evolution,
sharing their closest phylogenetic relatives with elephants and hyraxes. The sirenians
likely became distinct from terrestrial ancestors in the Eocene Epoch (along with cetaceans), and their auditory systems display the high level of modification of the terrestrial mammalian ear that is consistent with a fully aquatic existence (Thewissen and
Nummela 2007).
Like cetaceans, sirenians lack pinnae and have a small pin-sized opening that leads
to an ear canal that is probably occluded with cellular debris (Bullock et al. 1980; Ketten
et al. 1992; Chapla et al. 2007). The middle-ear ossicles are large, also like cetaceans
and phocids, and have a high bone density (Robineau 1969; Fleisher 1978; Chapla et al.
2007). Unlike the phocids, however, the middle ear likely remains air-filled without the
engorgement of cavernous tissue. The malleus is strongly fused to the tympanic bone
and the incus fused to the periotic bone, resulting in a middle ear chain that is not activated through a typical mammalian pathway via the tympanic membrane. A striking
feature of the sirenian ear is that the malleus presses on the tympanic membrane, forcing
it outwards. This feature is believed to be unique among mammals, although its function in aquatic hearing is unknown (Nummela 2007). The cochlea is essentially similar
to that of other non-echolocating mammals (Ketten et al. 1992). Despite some knowledge
of the morphological configuration of the sirenian ear, the manner in which sound is
transferred to the inner ear is not currently understood. Some studies have suggested a
pathway could exist through fatty tissue located in the zygomatic process (Bullock et al.
1980; Ketten et al. 1992; Chapla et al. 2007).
The auditory electrophysiological voltages that are recorded at the skin surface of
manatees are small compared to those of odontocetes, a result of the lack of hypertrophy
of neural structures (Klishin et al. 1990). Electrophysiological measures of auditory system function have shown that sirenians are roughly comparable to pinnipeds in terms of
their ability to track the envelope of rapidly presented acoustic stimuli, a result that is not
Chapter eleven: Acoustics
11.2.2.3 Family Ursidae
Similar to sea otters, polar bears have a relatively short evolutionary history in the marine
environment, having diverged from terrestrial bears <2 million years ago (Cahill et al.
2013). Little research has been conducted on the auditory system of the polar bear (as an
aside, the polar bear is the only bear for which hearing data are available). The only known
modifications to the auditory apparatus relative to that of terrestrial carnivores are prominent fur inside of the pinna and a reduction in pinna size relative to terrestrial bears,
suggesting thermoregulatory adaptations for reducing heat loss in the extreme cold of the
Arctic (Stirling 1988).
Behavioral and electrophysiological measurements of aerial hearing sensitivity have
demonstrated that polar bears are sensitive to aerial sound over a frequency range similar to that of pinnipeds and mustelids: approximately 125 Hz to 32 kHz (Nachtigall et al.
2007; Owen and Bowles 2011). There are currently no data on underwater hearing for polar
bears, but their capabilities are probably poor given the lack of apparent adaptations of the
auditory system and the suspected low importance of underwater sound in communication, foraging, and navigation. Future measurements of underwater hearing in the polar
bear would, however, be of interest in order to make amphibious comparisons in the most
terrestrial marine mammal.
11.2.3 Order Sirenia
The sirenians comprise manatee species and the dugong. This order of marine mammals
is relatively divergent from the marine carnivores and cetaceans in terms of evolution,
sharing their closest phylogenetic relatives with elephants and hyraxes. The sirenians
likely became distinct from terrestrial ancestors in the Eocene Epoch (along with cetaceans), and their auditory systems display the high level of modification of the terrestrial mammalian ear that is consistent with a fully aquatic existence (Thewissen and
Nummela 2007).
Like cetaceans, sirenians lack pinnae and have a small pin-sized opening that leads
to an ear canal that is probably occluded with cellular debris (Bullock et al. 1980; Ketten
et al. 1992; Chapla et al. 2007). The middle-ear ossicles are large, also like cetaceans
and phocids, and have a high bone density (Robineau 1969; Fleisher 1978; Chapla et al.
2007). Unlike the phocids, however, the middle ear likely remains air-filled without the
engorgement of cavernous tissue. The malleus is strongly fused to the tympanic bone
and the incus fused to the periotic bone, resulting in a middle ear chain that is not activated through a typical mammalian pathway via the tympanic membrane. A striking
feature of the sirenian ear is that the malleus presses on the tympanic membrane, forcing
it outwards. This feature is believed to be unique among mammals, although its function in aquatic hearing is unknown (Nummela 2007). The cochlea is essentially similar
to that of other non-echolocating mammals (Ketten et al. 1992). Despite some knowledge
of the morphological configuration of the sirenian ear, the manner in which sound is
transferred to the inner ear is not currently understood. Some studies have suggested a
pathway could exist through fatty tissue located in the zygomatic process (Bullock et al.
1980; Ketten et al. 1992; Chapla et al. 2007).
The auditory electrophysiological voltages that are recorded at the skin surface of
manatees are small compared to those of odontocetes, a result of the lack of hypertrophy
of neural structures (Klishin et al. 1990). Electrophysiological measures of auditory system function have shown that sirenians are roughly comparable to pinnipeds in terms of
their ability to track the envelope of rapidly presented acoustic stimuli, a result that is not