9. Echolocation in Dolphins
387
clicks can be heard by swimmers observing the behavior. It is also possible
that the dolphins are listening to prey movement, although if this was the
case, the dolphins would probably forage in silence and not continuously
emit clicks. If these dolphins are detecting their prey by echolocation, then
these animals are performing an echolocation task in a highly reverberant
environment. I am not aware of any human-made sonar that can detect and
discriminate small buried objects like these bottlenose dolphins do in the
Bahamas. Another interesting feature of this benthic feeding behavior is
the observation that these animals rarely burrow without capturing a prey.
The Atlantic spotted dolphin (5. frontalis) has also been observed foraging for buried prey (Herzing 1996) in the waters off the Grand Bahama
Island. These animals typically align themselves almost vertically with their
rostrum several centimeters off the bottom. Click signals can be heard while
the S. frontalis forage off the bottom. However, the prey is usually only
buried several centimeters in the bottom so that the dolphins do not need
to burrow more than about 6 to lOcm into the sand.
3.3 Target Range Difference Discrimination
Murchison (1980) conducted a study to determine the ability of an echoloeating dolphin to indicate which of two targets was at a closer range. The
T. truncatus was trained to wear rubber eye cups and station in a chin cup
that could swivel from side to side and echolocate two targets separated in
azimuth by 40° directly in front of the chin cup. The targets were 7.62-cm
polyurethane foam spheres with internal lead weights. A diagram of the
dolphin-target geometry is depicted in the insert of Figure 9.16. The dolphin
was trained to station in the chin cup and begin its sonar scan when an
acoustic screen was lowered out of the way. Upon completing its sonar scan,
the animal backed out of the chin up and responded by touching the paddle
on the same side of the center line as the closer target. The dolphin's relative range acuity was tested for absolute target ranges of 1,3, and 7 m. The
dolphin's performance results are shown in Figure 9.16 with percent correct
plotted as a function of ~R for the different absolute target ranges.
Although the results clearly indicated that the ~R threshold increased
with absolute range, the ~R/R became progressively smaller with range.
The 75% correct response thresholds were at ~Rs of 0.9, 1.5, and 3cm
for absolute target ranges of 1, 3, and 7 m, indicating that the dolphin
could resolve range differences of 0.9% at 1 m, 0.50% at 3 m, and 0.43 % at
7m. At the 1-m absolute range, the dolphin's performance approached the
theoretical performance of a matched-filter (see Section 9.6 of Au 1993).
These results indicate that the temporal resolution capability of T. truncatus is extremely fine. The dolphin is able to resolve time differences of
121ls in 1.3 ms, 20 Ils in 4 ms, and 40 Ils in 9.3 ms. The bat Eptesicus fuscus
typically resolves time differences of 80llS in 2 to 14ms (Simmons 1973;
Au 1993).
387
clicks can be heard by swimmers observing the behavior. It is also possible
that the dolphins are listening to prey movement, although if this was the
case, the dolphins would probably forage in silence and not continuously
emit clicks. If these dolphins are detecting their prey by echolocation, then
these animals are performing an echolocation task in a highly reverberant
environment. I am not aware of any human-made sonar that can detect and
discriminate small buried objects like these bottlenose dolphins do in the
Bahamas. Another interesting feature of this benthic feeding behavior is
the observation that these animals rarely burrow without capturing a prey.
The Atlantic spotted dolphin (5. frontalis) has also been observed foraging for buried prey (Herzing 1996) in the waters off the Grand Bahama
Island. These animals typically align themselves almost vertically with their
rostrum several centimeters off the bottom. Click signals can be heard while
the S. frontalis forage off the bottom. However, the prey is usually only
buried several centimeters in the bottom so that the dolphins do not need
to burrow more than about 6 to lOcm into the sand.
3.3 Target Range Difference Discrimination
Murchison (1980) conducted a study to determine the ability of an echoloeating dolphin to indicate which of two targets was at a closer range. The
T. truncatus was trained to wear rubber eye cups and station in a chin cup
that could swivel from side to side and echolocate two targets separated in
azimuth by 40° directly in front of the chin cup. The targets were 7.62-cm
polyurethane foam spheres with internal lead weights. A diagram of the
dolphin-target geometry is depicted in the insert of Figure 9.16. The dolphin
was trained to station in the chin cup and begin its sonar scan when an
acoustic screen was lowered out of the way. Upon completing its sonar scan,
the animal backed out of the chin up and responded by touching the paddle
on the same side of the center line as the closer target. The dolphin's relative range acuity was tested for absolute target ranges of 1,3, and 7 m. The
dolphin's performance results are shown in Figure 9.16 with percent correct
plotted as a function of ~R for the different absolute target ranges.
Although the results clearly indicated that the ~R threshold increased
with absolute range, the ~R/R became progressively smaller with range.
The 75% correct response thresholds were at ~Rs of 0.9, 1.5, and 3cm
for absolute target ranges of 1, 3, and 7 m, indicating that the dolphin
could resolve range differences of 0.9% at 1 m, 0.50% at 3 m, and 0.43 % at
7m. At the 1-m absolute range, the dolphin's performance approached the
theoretical performance of a matched-filter (see Section 9.6 of Au 1993).
These results indicate that the temporal resolution capability of T. truncatus is extremely fine. The dolphin is able to resolve time differences of
121ls in 1.3 ms, 20 Ils in 4 ms, and 40 Ils in 9.3 ms. The bat Eptesicus fuscus
typically resolves time differences of 80llS in 2 to 14ms (Simmons 1973;
Au 1993).