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Chapter eleven: Acoustics
11.2.2 Order Carnivora
The marine carnivores comprise five familial groups: Phocidae (true or earless seals),
Otariidae (sea lions and fur seals), Odobenidae (walruses), Mustelidae (sea otters), and
Ursidae (polar bears). With regards to acoustic ecology, the defining characteristic of
the marine carnivores is their amphibious nature; while all species forage on prey in the
aquatic environment, each carries out some essential life-history functions in air. The
degrees to which their auditory systems have modified the ear of their terrestrial ancestors
for underwater function reflect the importance of underwater function relative to aerial
function in the life of each species, as some trade-offs exist with specific adaptation for a
particular physical medium.
11.2.2.1 The Pinnipeds: Families Phocidae, Otariidae, and Odobenidae
Of the five Carnivore families which contain marine species, the three pinniped families
are the only ones in which all species are amphibious (although the degrees of marine
versus terrestrial activity and adaptation varies greatly across pinniped species). The pinnipeds are thought to represent a monophyletic group, having evolved from a terrestrial
dog or bear-like ancestor more than 25 million years ago in the Oligocene epoch. This long
evolutionary history in the marine environment relative to the other marine carnivores
has resulted in the greatest degree of adaptation of the auditory system (Thewissen and
Nummela 2007). Auditory anatomy and processing capabilities are thought to be generally
similar among otariid species. In contrast, the phocids display greater interspecies differences in these qualities, a result of their greater diversity in morphologies and ecological
roles. Odobenids are thought to be more closely related to otariids but possess an auditory
system that appears in many ways to be intermediate between those of otariids and phocids (Nummela 2007).
The external pinnae of the pinnipeds are notable for their degrees of reduction when
compared to those of terrestrial carnivores. In the otariids, the pinnae have been reduced
to small folds, while the phocids lack external pinnae altogether (Ramprashad et al. 1972;
Repenning 1972). The reduction in pinna size is likely a result of selection for an increasingly hydrodynamic, fusiform shape when swimming underwater (see Chapter 1). This
adaptation for underwater locomotion likely has ramifications, however, for pinnipeds
while listening in air. In air, the pinnae direct and filter sound from the external environment into the ear canal toward the tympanic membrane. Their directional filtering properties also help individuals resolve ambiguities of the location of sound sources when large
interaural differences are not present, for example, when sounds arrive from directly in
front of or behind a subject, or when a sound source is moved only in the vertical plane
(Yost 2007). Underwater, the impedances of the tissues of the pinnae are similar to water,
and the absence of these structures is likely of relatively little importance to hearing.
The various morphologies of pinniped ears have marked impacts on species-specific
hearing sensitivities (Figure 11.4). The ear canals of otariids are generally similar to those
of terrestrial mammals (Ramprashad et al. 1972; Repenning 1972). In air, they appear to
have hearing that is generally as sensitive as terrestrial carnivores. Hearing sensitivity is also quite good underwater, although otariids are not as sensitive as and lack the
broad frequency range of odontocetes and phocids. These findings support the view that
otariids are adapted primarily for aerial hearing (Reichmuth et al. 2013). Phocid species
display a variety of outer ear morphologies that diverge from the basic mammalian construction. In the extreme case of the deep-diving elephant seals, the external auditory
meatus is reduced to nearly the size of a pinhole, and there is probably not a continuous
Chapter eleven: Acoustics
11.2.2 Order Carnivora
The marine carnivores comprise five familial groups: Phocidae (true or earless seals),
Otariidae (sea lions and fur seals), Odobenidae (walruses), Mustelidae (sea otters), and
Ursidae (polar bears). With regards to acoustic ecology, the defining characteristic of
the marine carnivores is their amphibious nature; while all species forage on prey in the
aquatic environment, each carries out some essential life-history functions in air. The
degrees to which their auditory systems have modified the ear of their terrestrial ancestors
for underwater function reflect the importance of underwater function relative to aerial
function in the life of each species, as some trade-offs exist with specific adaptation for a
particular physical medium.
11.2.2.1 The Pinnipeds: Families Phocidae, Otariidae, and Odobenidae
Of the five Carnivore families which contain marine species, the three pinniped families
are the only ones in which all species are amphibious (although the degrees of marine
versus terrestrial activity and adaptation varies greatly across pinniped species). The pinnipeds are thought to represent a monophyletic group, having evolved from a terrestrial
dog or bear-like ancestor more than 25 million years ago in the Oligocene epoch. This long
evolutionary history in the marine environment relative to the other marine carnivores
has resulted in the greatest degree of adaptation of the auditory system (Thewissen and
Nummela 2007). Auditory anatomy and processing capabilities are thought to be generally
similar among otariid species. In contrast, the phocids display greater interspecies differences in these qualities, a result of their greater diversity in morphologies and ecological
roles. Odobenids are thought to be more closely related to otariids but possess an auditory
system that appears in many ways to be intermediate between those of otariids and phocids (Nummela 2007).
The external pinnae of the pinnipeds are notable for their degrees of reduction when
compared to those of terrestrial carnivores. In the otariids, the pinnae have been reduced
to small folds, while the phocids lack external pinnae altogether (Ramprashad et al. 1972;
Repenning 1972). The reduction in pinna size is likely a result of selection for an increasingly hydrodynamic, fusiform shape when swimming underwater (see Chapter 1). This
adaptation for underwater locomotion likely has ramifications, however, for pinnipeds
while listening in air. In air, the pinnae direct and filter sound from the external environment into the ear canal toward the tympanic membrane. Their directional filtering properties also help individuals resolve ambiguities of the location of sound sources when large
interaural differences are not present, for example, when sounds arrive from directly in
front of or behind a subject, or when a sound source is moved only in the vertical plane
(Yost 2007). Underwater, the impedances of the tissues of the pinnae are similar to water,
and the absence of these structures is likely of relatively little importance to hearing.
The various morphologies of pinniped ears have marked impacts on species-specific
hearing sensitivities (Figure 11.4). The ear canals of otariids are generally similar to those
of terrestrial mammals (Ramprashad et al. 1972; Repenning 1972). In air, they appear to
have hearing that is generally as sensitive as terrestrial carnivores. Hearing sensitivity is also quite good underwater, although otariids are not as sensitive as and lack the
broad frequency range of odontocetes and phocids. These findings support the view that
otariids are adapted primarily for aerial hearing (Reichmuth et al. 2013). Phocid species
display a variety of outer ear morphologies that diverge from the basic mammalian construction. In the extreme case of the deep-diving elephant seals, the external auditory
meatus is reduced to nearly the size of a pinhole, and there is probably not a continuous