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Marine Mammal Physiology: Requisites for Ocean Living
The submersion of the pinniped head in water has profound implications for hearing. Upon submersion, the impedance mismatch between the external environment
and biological tissues is reduced, resulting in an increased potential for bone conduction of sound to the cochlea along pathways other than that of the outer/middle ear
(although work by Møhl and Ronald 1975 demonstrated best reception of sound near
the opening of the ear canal in harbor seals). Underwater bone-conduction pathways
and the 4–4.5 times increase in sound speed relative to air reduce the ability to detect
and localize sound; however, anatomical evidence, including the partial separation of
the auditory bullae from the skull, suggests anatomical modifications may aid pinnipeds in sound localization. Indeed, underwater sound localization experiments with
pinnipeds have shown that, while inferior to the capabilities of odontocete cetaceans,
pinnipeds are still able to acutely localize underwater sound (Gentry 1967; Moore and
Au 1975; Babushina and Poliakov 2004; Bodson et al. 2006; Bodson et al. 2007). Many
critical aspects of underwater hearing pathways in pinnipeds, however, remain poorly
understood.
Most psychological measurements of auditory capabilities that are independent of
the physical media in which they occur (i.e., at the level of cochlear and neural processes) have been limited to measurements with simple acoustic stimuli. Studies have
shown that some features of auditory processing, such as the abilities to detect changes
in tonal frequency and amplitude, are inferior to those of odontocetes and generally
similar to those of other mammals (Møhl 1967; Moore and Schusterman 1976; Terhune
and Ronald 1976; Schusterman and Moore 1978). In contrast, recent measurements have
demonstrated that pinnipeds appear to be able to more efficiently extract tones from
noise than other mammalian species, potentially an adaptation of the auditory system
for function in noisy marine environments (Southall et al. 2000, 2003; Sills et al. 2014).
Investigations utilizing more complex real-world stimuli have generally been limited to
the realm of playback studies in field settings, but there is increasing interest in studying
the perception of complex stimuli in controlled laboratory settings (e.g., Cunningham
et al. 2014).
11.2.2.2 Family Mustelidae
Having diverged from non-marine ancestors approximately 3–5 million years ago, the
sea otter has a much shorter evolutionary history in the marine environment than pinnipeds (Thewissen and Nummela 2007). Sea otters do, however, display some of the adaptations of the outer and middle ear seen in pinnipeds, including reductions in pinna size,
increases in the size of middle-ear ossicles, and a thickening of the tympanic membrane
(Solntseva 2007). Also like the pinnipeds, these adaptations may have been primarily
driven by needs related to swimming and diving as opposed to hearing, although there
are almost certainly effects of these morphological changes on auditory capabilities relative to non-marine species.
Behavioral measurements of hearing sensitivity with the sea otter have shown that
aerial hearing is similar to that of otariid pinnipeds, while underwater hearing is apparently inferior to that of otariids, phocids, and most terrestrial mammals (Ghoul and
Reichmuth 2014). Additionally, sea otters are apparently similar to terrestrial mammals
in terms of their ability to detect tones in simultaneous noise (Ghoul and Reichmuth
2014). These findings reinforce the notion that due to their relatively recent transition to
the marine environment, the hearing capabilities of sea otters are still primarily adapted
for aerial function, although some adaptations of the ear for diving may have been at the
cost of aerial hearing sensitivity.
Marine Mammal Physiology: Requisites for Ocean Living
The submersion of the pinniped head in water has profound implications for hearing. Upon submersion, the impedance mismatch between the external environment
and biological tissues is reduced, resulting in an increased potential for bone conduction of sound to the cochlea along pathways other than that of the outer/middle ear
(although work by Møhl and Ronald 1975 demonstrated best reception of sound near
the opening of the ear canal in harbor seals). Underwater bone-conduction pathways
and the 4–4.5 times increase in sound speed relative to air reduce the ability to detect
and localize sound; however, anatomical evidence, including the partial separation of
the auditory bullae from the skull, suggests anatomical modifications may aid pinnipeds in sound localization. Indeed, underwater sound localization experiments with
pinnipeds have shown that, while inferior to the capabilities of odontocete cetaceans,
pinnipeds are still able to acutely localize underwater sound (Gentry 1967; Moore and
Au 1975; Babushina and Poliakov 2004; Bodson et al. 2006; Bodson et al. 2007). Many
critical aspects of underwater hearing pathways in pinnipeds, however, remain poorly
understood.
Most psychological measurements of auditory capabilities that are independent of
the physical media in which they occur (i.e., at the level of cochlear and neural processes) have been limited to measurements with simple acoustic stimuli. Studies have
shown that some features of auditory processing, such as the abilities to detect changes
in tonal frequency and amplitude, are inferior to those of odontocetes and generally
similar to those of other mammals (Møhl 1967; Moore and Schusterman 1976; Terhune
and Ronald 1976; Schusterman and Moore 1978). In contrast, recent measurements have
demonstrated that pinnipeds appear to be able to more efficiently extract tones from
noise than other mammalian species, potentially an adaptation of the auditory system
for function in noisy marine environments (Southall et al. 2000, 2003; Sills et al. 2014).
Investigations utilizing more complex real-world stimuli have generally been limited to
the realm of playback studies in field settings, but there is increasing interest in studying
the perception of complex stimuli in controlled laboratory settings (e.g., Cunningham
et al. 2014).
11.2.2.2 Family Mustelidae
Having diverged from non-marine ancestors approximately 3–5 million years ago, the
sea otter has a much shorter evolutionary history in the marine environment than pinnipeds (Thewissen and Nummela 2007). Sea otters do, however, display some of the adaptations of the outer and middle ear seen in pinnipeds, including reductions in pinna size,
increases in the size of middle-ear ossicles, and a thickening of the tympanic membrane
(Solntseva 2007). Also like the pinnipeds, these adaptations may have been primarily
driven by needs related to swimming and diving as opposed to hearing, although there
are almost certainly effects of these morphological changes on auditory capabilities relative to non-marine species.
Behavioral measurements of hearing sensitivity with the sea otter have shown that
aerial hearing is similar to that of otariid pinnipeds, while underwater hearing is apparently inferior to that of otariids, phocids, and most terrestrial mammals (Ghoul and
Reichmuth 2014). Additionally, sea otters are apparently similar to terrestrial mammals
in terms of their ability to detect tones in simultaneous noise (Ghoul and Reichmuth
2014). These findings reinforce the notion that due to their relatively recent transition to
the marine environment, the hearing capabilities of sea otters are still primarily adapted
for aerial function, although some adaptations of the ear for diving may have been at the
cost of aerial hearing sensitivity.