Figure 1.
Experimental setup of the target pen.
During a trial, the dolphin positioned herself into the underwater hoop 1 m below the surface of the water. An acoustically opaque metal screen was located inside the target pen, in front of the animal, to prevent her from echolocating prematurely on the targets (acoustic screen). An acoustically transparent, yet visually opaque polyethylene screen was placed in front of the acoustically opaque screen to ensure the subject did not use visual cues during target detection (visual screen). After the target was lowered 1 m into the water, the acoustically opaque screen was moved to reveal the target. The animal ensonified the target located behind the array (positioned at either 2.5, 4 or 7 m) and the hydrophone array recorded the emitted echolocation signals.
Figure 2.
An example trial produced by the dolphin.
Source levels start out around 170–180 dB, increase by 10–20 dB over a few clicks, reach maximum amplitude, and then slowly decrease. Source levels were calculated from the peak-to-peak voltage and reported in dB re: 1 µPa.
Table 1.
Number of data points (clicks) for each averaged click number represented in Figure 3.
Figure 3.
Change in source levels throughout trials according to target distance and condition.
Click source levels (mean and standard error) as a function of click number for 2.5, 4 and 7 m target distance for the unpredictable targets (3a, top) and predictable targets (3b, bottom). Clicks within a trial were indexed by click number and then the values of each click number were averaged across all trials. Source levels were calculated from the peak-to-peak voltage and are reported in dB re: 1 µPa. Because some click numbers towards the end of the click train consisted of less than 5 values, we only further analyzed click numbers at the end of the click trains that had 5 or more values. See Table 1 for the number of clicks that make up each data point.
Table 2.
Summary statistics of one-way ANOVA for all the clicks produced by the dolphin for the unpredictable and predictable sessions.
Figure 4.
Bootstrapped 95% confidence intervals to demonstrate the effect of learning on initial click source level for the predictable trials.
Bootstrapped 95% confidence intervals for the first click between the first ten trials and the last ten trials of each predictable session. Compared to the first ten trials, the dolphin produced significantly (p<0.05) lower source levels for the last ten trials at the 2.5 m distance. The dolphin also produced lower source levels for the last ten trials at the 4 m distance, but due to the large variation this change was not significant (p>0.05).
Figure 5.
Change in source levels throughout trials according to target distance and presence.
Click source levels (mean and standard error) as a function of click number for the unpredictable trials (A), and the 2.5 m (B), 4 m (C), and 7 m (D) predictable target sessions. Clicks within a trial were indexed by click number and then the values of each click number were averaged across all trials. Source levels were calculated from the peak-to-peak voltage and are reported in dB re: 1 µPa.
Figure 6.
Cumulative energy over elapsed time for unpredictable targets (5a, top) and predictable targets (5b, bottom).
Time was estimated based on the two-way-travel time plus a “lag” time of 30 ms. The vertical axis is (amplitude2 × time), which is a measure proportional to energy. This analysis further highlights the increased energy for the 7 m predictable target. For the predictable condition, the slope of the total energy versus time is much steeper than the other target distances, indicating a faster rate of source energy and therefore potential received information from the target echo.