Date Published: August 25, 2014
Publisher: Public Library of Science
Author(s): Laura N. Kloepper, Adam B. Smith, Paul E. Nachtigall, John R. Buck, James A. Simmons, Aude F. Pacini, Jacob Engelmann.
Echolocating animals adjust the transmit intensity and receive sensitivity of their sonar in order to regulate the sensation level of their echoes; this process is often termed automatic gain control. Gain control is considered not to be under the animal’s cognitive control, but previous investigations studied animals ensonifying targets or hydrophone arrays at predictable distances. To test whether animals maintain gain control at a fixed level in uncertain conditions, we measured changes in signal intensity for a bottlenose dolphin (Tursiops truncatus) detecting a target at three target distances (2.5, 4 and 7 m) in two types of sessions: predictable and unpredictable. Predictable sessions presented the target at a constant distance; unpredictable sessions moved the target randomly between the three target positions. In the predictable sessions the dolphin demonstrated intensity distance compensation, increasing the emitted click intensity as the target distance increased. Additionally, as trials within sessions progressed, the animal adjusted its click intensity even from the first click in a click train, which is consistent with the animal expecting a target at a certain range. In the unpredictable sessions there was no significant difference of intensity with target distance until after the 7th click in a click train. Together, these results demonstrate that the bottlenose dolphin uses learning and expectation for sonar gain control.
Bats and toothed whales are model organisms for the investigation of sensory processing. These two animal groups convergently evolved echolocation, an active sense relying on the integration of auditory, vocal and motor systems. To forage in darkness, these animals emit intense high frequency sounds and use information from the corresponding echoes to locate, discriminate and track prey, often at great distances. Sound propagating through open space is attenuated by 6 dB for each doubling of distance to the object, and echoes returning from a small object are attenuated by a further 6 dB for each doubling of distance . Assuming an ideal reflector, a target at 100 m, the detection limit of the bottlenose dolphin , would return an echo more than 80 dB quieter than the outgoing signal . Processing such a large range of echo intensities poses a challenge for the animal’s auditory system. To compensate, echolocators maintain a constant perceived echo level by changing both the transmit and receive sonar systems –.
The experiments were conducted in March and June of 2013 at the floating pen complex of the Marine Mammal Research Program of the Hawaii Institute of Marine Biology off Coconut Island, Kaneohe Bay, Oahu, Hawaii. The experimental subject was a 27-year-old female bottlenose dolphin (Tursiops truncatus) named BJ, who measured 2.4 m and weighed 186 kg at the time of the experiment. This was a trained, experienced laboratory animal (see – for examples of previous experiments).
A total of 3,782 clicks were collected in the 143 unpredictable trials recorded in March, and 3,358 clicks were collected in the 142 predictable trials recorded in June. Throughout all of the trials the dolphin achieved a high level of echolocation performance, committing only 2 errors out of 300 trials (99.3% correct). Both of these errors were false alarms (reporting the presence of a target when the target was, however, absent) at the 2.5 m target distance for the predictable targets. The dolphin produced up to 80 clicks in each trial, with an average of 21.6±10.5 clicks per trial. Source levels of all clicks ranged from 174 to 199 dB re: 1 µPa.
These results demonstrate experience is an important driver of gain control. Changes in intensity according to target distance occur only with prior knowledge of the target range. This knowledge can occur with two types of learning: within a trial and within a session. Both the predictable and unpredictable sessions are subject to within-trial learning; after receiving echoes from several clicks in the click train the dolphin learns the distance of the target and adjusts its click output accordingly. The second type of learning, within a session, is only present in the predictable sessions. The unpredictable sessions are designed to control against learning within a session.