The World’s Most Powerful Pair of Glasses—Part 2 of 3
We asked if you had been to seen the movie Finding Dory last week. Our specific interest was the marvelous capability of Bailey the beluga to use sound to locate objects. This bio sonar capability in toothed whales and dolphins has evolved to be so sophisticated that dolphins can detect a steel plate 1 square inch in size 100 meters away. After extolling the virtues of the use of bio sonar in these animals we left the readers with a puzzle.“So, when a whale produces a click for echolocation, it has no idea how far the click will have to travel before being reflected. So the wise thing to do would be to produce the loudest possible click—and so it does. Would that not put the whale’s own hearing in danger?”
In the answer to this question we find the second marvel of biology as it relates to echolocation. Whales and dolphins do indeed protect their own hearing from harmful exposure when producing the echolocation clicks. These animals have evolved the ability to change the gain in their auditory system very rapidly once the echo locating click has been produced. This leads to two questions. First, how do we know that this really happens? And second, how does the animal do it?
We know that there is indeed active gain control in the auditory system in whales and dolphins from many measurements of the auditory brainstem response from these animals during and after the production of echo locating clicks. The brainstem response is measured using a suction cup electrode placed on the animal’s head while the animal performs various experimental maneuvers, usually for a fish reward. These experiments are typically conducted in facilities that house rescued marine mammals or at large aquariums around the world. So the data set is not large but the evidence is irrefutable. The amplitude of the brainstem response is the smallest when the animal is producing the clicks and increases with time after. The observation of increasing brainstem response amplitude suggests that the animal has a way of inhibiting the response during click production, subsequently releasing this inhibition to be able to perceive the reflected echo signal. If that is not amazing enough, these animals seem to know physics well enough to halve the inhibition (double the gain) with every doubling of the distance to the target.
So what is the biological mechanism responsible? The short answer is that we do not know exactly, yet. Some have proposed that this is a simple case of forward masking where a loud sound can influence the response to subsequent sounds even after the initial loud sound. One view could be that forward masking may be the psychophysical outcome of an underlying physiological process. It seems likely that the auditory efferent nervous system with direct connections to the outer hair cells and the ability to change the gain provided by these outer hair cells is involved. One way or the other, here is another example of a specialized auditory system that has evolved to meet the specific needs of an animal operating in a specific environment. Can other animals learn to echo locate? More next week.