9. Echolocation in Dolphins
393
response was recorded. In the first discrimination task with aluminum
and steel cylinders, the dolphin was able to achieve the 90% correct
response criterion at the 0° aspect but was unable to reach the criterion for
the 45° and 90° aspects. The dolphin's performance was 80% and 82%
correct at the 45° and 90° aspect angles, respectively. Therefore, probe sessions were not conducted with this set of targets, but additional sessions
were conducted to obtain the animal's performance at 10° and 80° aspect
angles. The dolphin's performance was 100% and 92% correct at the 10°
and 80° aspect angles, respectively. The results indicate that the dolphin
could easily discriminate between the aluminum and steel cylinders at different aspect angles. The worst performance occurred at an aspect angle of
45°, but even at this angle the dolphin overall performance, which is the
average between the performance for the individual targets, was above 80%
correct.
An easier discrimination task between the aluminum and coral rock
cylinders was next attempted. The 220 trials of baseline conditions per angle
were obtained and the dolphin's performance was at least 89% correct.
Novel aspect angles were used in a minimum of 25 probe trials except at
60°, where only 10 probe trials were conducted. The animal's performance
was almost perfect with the probe trials. These results indicated that for a
simple discrimination there are aspect-independent cues that a dolphin can
utilize in discriminating material composition.
Echoes from the three targets obtained with simulated dolphin sonar
signals for three aspect angles, 0°, 45°, and 90° are shown in Figure 9.19.
Difference between the echoes can be clearly seen, however, at 45° and
90° the echo structure was very complex. At the 0° aspect the highlights
occurred at different times for the aluminum and steel cylinders. The echoes
for the aluminum cylinder also did not decay as rapidly as for the steel cylinder. At the 45° and 90° aspect angles, the time of occurrence of the different highlights were similar for the aluminum and steel cylinders. Differences
in the amplitude and shape of the highlights can be seen. The difference
between the echoes from the aluminum and coral rock cylinders are even
more obvious. Secondary highlights tended to attenuate quickly for the
coral rock target so that the echo structure from this target was not very
complex.
3.4.3 Wall Thickness of Cylinders
The ability of an echolocating T. truncatus to discriminate differences in the
wall thickness of hollow steel cylinders was studied by Titov (1972). The
outer diameter and length of the cylinders were 50mm and wall thickness
varied between 0.1 to 2mm. The dolphin was trained to choose the thinner
of two cylinders presented simultaneously at a range of 5 m. The dolphin
was able to discriminate a wall thickness difference of 0.2mm at the 75%
correct response level.
393
response was recorded. In the first discrimination task with aluminum
and steel cylinders, the dolphin was able to achieve the 90% correct
response criterion at the 0° aspect but was unable to reach the criterion for
the 45° and 90° aspects. The dolphin's performance was 80% and 82%
correct at the 45° and 90° aspect angles, respectively. Therefore, probe sessions were not conducted with this set of targets, but additional sessions
were conducted to obtain the animal's performance at 10° and 80° aspect
angles. The dolphin's performance was 100% and 92% correct at the 10°
and 80° aspect angles, respectively. The results indicate that the dolphin
could easily discriminate between the aluminum and steel cylinders at different aspect angles. The worst performance occurred at an aspect angle of
45°, but even at this angle the dolphin overall performance, which is the
average between the performance for the individual targets, was above 80%
correct.
An easier discrimination task between the aluminum and coral rock
cylinders was next attempted. The 220 trials of baseline conditions per angle
were obtained and the dolphin's performance was at least 89% correct.
Novel aspect angles were used in a minimum of 25 probe trials except at
60°, where only 10 probe trials were conducted. The animal's performance
was almost perfect with the probe trials. These results indicated that for a
simple discrimination there are aspect-independent cues that a dolphin can
utilize in discriminating material composition.
Echoes from the three targets obtained with simulated dolphin sonar
signals for three aspect angles, 0°, 45°, and 90° are shown in Figure 9.19.
Difference between the echoes can be clearly seen, however, at 45° and
90° the echo structure was very complex. At the 0° aspect the highlights
occurred at different times for the aluminum and steel cylinders. The echoes
for the aluminum cylinder also did not decay as rapidly as for the steel cylinder. At the 45° and 90° aspect angles, the time of occurrence of the different highlights were similar for the aluminum and steel cylinders. Differences
in the amplitude and shape of the highlights can be seen. The difference
between the echoes from the aluminum and coral rock cylinders are even
more obvious. Secondary highlights tended to attenuate quickly for the
coral rock target so that the echo structure from this target was not very
complex.
3.4.3 Wall Thickness of Cylinders
The ability of an echolocating T. truncatus to discriminate differences in the
wall thickness of hollow steel cylinders was studied by Titov (1972). The
outer diameter and length of the cylinders were 50mm and wall thickness
varied between 0.1 to 2mm. The dolphin was trained to choose the thinner
of two cylinders presented simultaneously at a range of 5 m. The dolphin
was able to discriminate a wall thickness difference of 0.2mm at the 75%
correct response level.