255
Chapter eleven: Acoustics
airspace to the tympanic membrane due to a narrow ear canal that is filled with cellular
debris. This modification for deep diving results in relatively poor aerial hearing sensitivity relative to terrestrial carnivores and other pinniped species, although underwater
hearing appears to be sensitive across a wide frequency range (Kastak and Schusterman
1999). Many phocid species (e.g., harbor seals) possess musculature that can constrict
the outer ear canal, presumably an adaptation for restricting the entrance of water into
the ear canal when diving (Ramprashad et al. 1972; Repenning 1972). In contrast to the
elephant seals, the ear canal can remain open in air, imparting hearing sensitivity in air
that is comparable to that of terrestrial mammals (Reichmuth et al. 2013). These levels
of adaptation reflect the relative degrees to which various pinniped species spend their
time in aerial versus underwater environments: elephant seals are by far the most aquatic,
otariids the most terrestrial, with harbor seals lying somewhere in-between (Kastak and
Schusterman 1998, 1999).
The three middle ear ossicles of pinnipeds appear to function in a similar way to
that of terrestrial mammals in air, with pressure differences between the ear canal and
the middle ear transferred via the tympanic membrane and ossicular chain to a stapes
that is moveable in the cochlear oval window (Ramprashad et al. 1972; Repenning 1972;
Nummela 2007). While the mass and density of otariid middle ear ossicles are essentially
similar to those of terrestrial carnivores, those of phocids (and to a lesser degree odobenids) have a relatively increased mass and density. Additionally present in the pinniped
middle ear and outer ear canal is cavernous tissue that may engorge with blood upon
diving in order to minimize pressure differences between the middle ear and the external
environment (Odend’hal and Poulter 1966; Møhl 1967b, 1968; Repenning 1972).
The pinniped cochlea is essentially the same as that of other mammalian carnivores and notably different from that of the cetaceans that possess structural modifications for very high-frequency underwater hearing. Under water, the basal end of the
cochlea determines high-frequency hearing limit in both otariids and phocids, although
these limits are very different for the two families, with high-frequency hearing limits of
approximately 40 and 100 kHz, respectively (Reichmuth et al. 2013). In air, the increased
mass of the middle ear ossicles of phocids results in an inertial constraint on the transfer
of high-frequency sound from the tympanic membrane to the inner ear, reducing the
high-frequency limit to between 20 and 30 kHz (Hemilä et al. 2006). The only hearing
data that exist for odobenids are underwater thresholds for one individual, which show
a high-frequency limit of approximately 15 kHz (Kastelein et al. 2002). It is unknown
whether this relatively low cutoff is representative of walruses in general, or just the
subject of that study.
The sensory physiology of pinnipeds does not have the same extensive history of comparable research as odontocete cetaceans. This has resulted historically from the lower
scientific interest in the capabilities of pinniped auditory nervous systems, as they lack the
echolocation abilities and the high degree of specialization seen in odontocete cetaceans.
Although some investigators suggested that pinnipeds might echolocate (Poulter 1963,
1966), electrophysiological and behavioral observations have demonstrated that pinnipeds
do not possess a sophisticated biosonar system (Schusterman et al. 2000). For example, the
auditory processing capabilities of odontocetes that are required for echolocation, such
as the ability of the auditory nervous system to follow the envelopes of rapidly presented
acoustic stimuli, are much less developed in pinnipeds and more akin to the capabilities of
terrestrial carnivores (Bullock et al. 1971; Mulsow and Reichmuth 2007). Likewise, anatomical studies of the cerebral cortex have shown that auditory projection areas are essentially
similar to other carnivores (Alderson et al. 1960; Popov et al. 1986).
Chapter eleven: Acoustics
airspace to the tympanic membrane due to a narrow ear canal that is filled with cellular
debris. This modification for deep diving results in relatively poor aerial hearing sensitivity relative to terrestrial carnivores and other pinniped species, although underwater
hearing appears to be sensitive across a wide frequency range (Kastak and Schusterman
1999). Many phocid species (e.g., harbor seals) possess musculature that can constrict
the outer ear canal, presumably an adaptation for restricting the entrance of water into
the ear canal when diving (Ramprashad et al. 1972; Repenning 1972). In contrast to the
elephant seals, the ear canal can remain open in air, imparting hearing sensitivity in air
that is comparable to that of terrestrial mammals (Reichmuth et al. 2013). These levels
of adaptation reflect the relative degrees to which various pinniped species spend their
time in aerial versus underwater environments: elephant seals are by far the most aquatic,
otariids the most terrestrial, with harbor seals lying somewhere in-between (Kastak and
Schusterman 1998, 1999).
The three middle ear ossicles of pinnipeds appear to function in a similar way to
that of terrestrial mammals in air, with pressure differences between the ear canal and
the middle ear transferred via the tympanic membrane and ossicular chain to a stapes
that is moveable in the cochlear oval window (Ramprashad et al. 1972; Repenning 1972;
Nummela 2007). While the mass and density of otariid middle ear ossicles are essentially
similar to those of terrestrial carnivores, those of phocids (and to a lesser degree odobenids) have a relatively increased mass and density. Additionally present in the pinniped
middle ear and outer ear canal is cavernous tissue that may engorge with blood upon
diving in order to minimize pressure differences between the middle ear and the external
environment (Odend’hal and Poulter 1966; Møhl 1967b, 1968; Repenning 1972).
The pinniped cochlea is essentially the same as that of other mammalian carnivores and notably different from that of the cetaceans that possess structural modifications for very high-frequency underwater hearing. Under water, the basal end of the
cochlea determines high-frequency hearing limit in both otariids and phocids, although
these limits are very different for the two families, with high-frequency hearing limits of
approximately 40 and 100 kHz, respectively (Reichmuth et al. 2013). In air, the increased
mass of the middle ear ossicles of phocids results in an inertial constraint on the transfer
of high-frequency sound from the tympanic membrane to the inner ear, reducing the
high-frequency limit to between 20 and 30 kHz (Hemilä et al. 2006). The only hearing
data that exist for odobenids are underwater thresholds for one individual, which show
a high-frequency limit of approximately 15 kHz (Kastelein et al. 2002). It is unknown
whether this relatively low cutoff is representative of walruses in general, or just the
subject of that study.
The sensory physiology of pinnipeds does not have the same extensive history of comparable research as odontocete cetaceans. This has resulted historically from the lower
scientific interest in the capabilities of pinniped auditory nervous systems, as they lack the
echolocation abilities and the high degree of specialization seen in odontocete cetaceans.
Although some investigators suggested that pinnipeds might echolocate (Poulter 1963,
1966), electrophysiological and behavioral observations have demonstrated that pinnipeds
do not possess a sophisticated biosonar system (Schusterman et al. 2000). For example, the
auditory processing capabilities of odontocetes that are required for echolocation, such
as the ability of the auditory nervous system to follow the envelopes of rapidly presented
acoustic stimuli, are much less developed in pinnipeds and more akin to the capabilities of
terrestrial carnivores (Bullock et al. 1971; Mulsow and Reichmuth 2007). Likewise, anatomical studies of the cerebral cortex have shown that auditory projection areas are essentially
similar to other carnivores (Alderson et al. 1960; Popov et al. 1986).