348
P.E. Nachtigall et al.
Finally, Renaud and Popper (1975) tested the dolphin's minimum audible
angle (with a 40-kHz signal) when the angles were measured from a reference point not directly in front of the animal, but rotated 15° or 30° to the
left and right. This change in the azimuth of the midpoint was accomplished
by rotating the bite plate so that the animal was now oriented to the left or
right of the buzzer. Auditory localization was found to be azimuth dependent. Smaller audible angles could be discriminated when the midpoint was
located 15° to the right (1.7°) or left (1.4°) of the animal than when the midpoint was directly in front (2.5°). The minimum audible angle was higher,
however, when the midpoint was rotated to 30° to the left (5.2°) or right
(5.3°) of center. In other terrestrial mammals that have been studied, the
best angular discrimination has been found when the head is oriented
directly toward the midpoint.
The dolphin's ability to localize stimuli in the vertical plane with such
high precision is remarkable. Terrestrial mammals appear to use their
pinnae to localize sounds along the vertical plane (Roffler and Butler 1968),
but dolphins do not have such pinnae. Renaud and Popper (1975) attributed this good performance to memorized intensity cues. Another possibility is the complex pathway by which sounds reach the inner ear of the
dolphin. The dolphin's auditory meatus is typically clogged with cellular
debris and dense cerumen (Ketten 1997) and appears to play no functional
role in hearing. Instead, the primary sound reception pathway in the
dolphin appears to be via two fat channels in the dolphin's lower jaw. A
body of fat runs through the lining of the jaw and contains fats whose
impedance closely matches that of seawater. A second, trumpet-shaped,
body of fat lies over the pan bone, a thin ovoid region in the posterior third
of the mandible. The density of the fat in this trumpet matches that of the
fat in the mandibular channel (Ketten 1994, 1997). The two fat channels,
oriented at right angles to each other, have the potential to operate as an
analog of the external pinna of terrestrial mammals. Complex interaction
between the sound patterns received via the two channels on each side of
the dolphin's head may provide cues to the elevation of signals.
There are three major cues to horizontal-plane auditory localization
available to organisms with two ears. Interaural differences in time of
arrival, interaural differences in phase, and interaural differences in loudness. A sound that is located off the main axis of the head will arrive at one
ear earlier than the other, will be phase advanced at one ear relative to the
other, and will be louder at one ear than the other. These cues are known
for terrestrial mammals to be differentially useful depending on the frequency, intensity, and other characteristics of the sound. For example, as
Lord Rayleigh (1907) first speculated, simple low-frequency sounds are
primarily localized on the basis of temporal cues (time of arrival or phase
differences) whereas high-frequency sounds are localized using primarily
intensity differences. Brief transient signals can be localized by time of
arrival, but continuous sounds must use phase differences.
P.E. Nachtigall et al.
Finally, Renaud and Popper (1975) tested the dolphin's minimum audible
angle (with a 40-kHz signal) when the angles were measured from a reference point not directly in front of the animal, but rotated 15° or 30° to the
left and right. This change in the azimuth of the midpoint was accomplished
by rotating the bite plate so that the animal was now oriented to the left or
right of the buzzer. Auditory localization was found to be azimuth dependent. Smaller audible angles could be discriminated when the midpoint was
located 15° to the right (1.7°) or left (1.4°) of the animal than when the midpoint was directly in front (2.5°). The minimum audible angle was higher,
however, when the midpoint was rotated to 30° to the left (5.2°) or right
(5.3°) of center. In other terrestrial mammals that have been studied, the
best angular discrimination has been found when the head is oriented
directly toward the midpoint.
The dolphin's ability to localize stimuli in the vertical plane with such
high precision is remarkable. Terrestrial mammals appear to use their
pinnae to localize sounds along the vertical plane (Roffler and Butler 1968),
but dolphins do not have such pinnae. Renaud and Popper (1975) attributed this good performance to memorized intensity cues. Another possibility is the complex pathway by which sounds reach the inner ear of the
dolphin. The dolphin's auditory meatus is typically clogged with cellular
debris and dense cerumen (Ketten 1997) and appears to play no functional
role in hearing. Instead, the primary sound reception pathway in the
dolphin appears to be via two fat channels in the dolphin's lower jaw. A
body of fat runs through the lining of the jaw and contains fats whose
impedance closely matches that of seawater. A second, trumpet-shaped,
body of fat lies over the pan bone, a thin ovoid region in the posterior third
of the mandible. The density of the fat in this trumpet matches that of the
fat in the mandibular channel (Ketten 1994, 1997). The two fat channels,
oriented at right angles to each other, have the potential to operate as an
analog of the external pinna of terrestrial mammals. Complex interaction
between the sound patterns received via the two channels on each side of
the dolphin's head may provide cues to the elevation of signals.
There are three major cues to horizontal-plane auditory localization
available to organisms with two ears. Interaural differences in time of
arrival, interaural differences in phase, and interaural differences in loudness. A sound that is located off the main axis of the head will arrive at one
ear earlier than the other, will be phase advanced at one ear relative to the
other, and will be louder at one ear than the other. These cues are known
for terrestrial mammals to be differentially useful depending on the frequency, intensity, and other characteristics of the sound. For example, as
Lord Rayleigh (1907) first speculated, simple low-frequency sounds are
primarily localized on the basis of temporal cues (time of arrival or phase
differences) whereas high-frequency sounds are localized using primarily
intensity differences. Brief transient signals can be localized by time of
arrival, but continuous sounds must use phase differences.