When Kelly Martin decided to map out the underwater hearing ability of a loggerhead turtle using behavioural hearing tests, she soon discovered why it had never been done before. When you're training a turtle, she says, ‘you need tremendous patience’. She should know; it took 2 years to train a captive loggerhead turtle at the Mote Marine Laboratory & Aquarium in Sarasota, Florida. But Martin persevered, and was able to show that behavioural hearing tests provide strikingly similar results to physiological testing methods (p. 3001).
We currently know little about turtle hearing, explains Martin, and what is known is based on electrophysiological tests, in which electrodes measure voltages generated by the brain in response to sounds. While these tests are quick and relatively simple, they don't provide a direct measure of hearing ability. But if Martin could show that the results of electrophysiological tests correlate well with behavioural hearing tests, it would provide reassurance that electrophysiological tests are accurate.
Undeterred by the scant success of previous attempts to train reptiles, Martin set to work training a 31 year old loggerhead turtle to respond to sounds. With the turtle positioned in front of a training platform, Martin activated an LED light suspended from the platform to signal the start of a trial. During a ‘go’ trial, Martin played a sound through an underwater transducer. To indicate that she had heard the sound, the turtle had to swim to a plastic response paddle suspended in the water and press it with her beak. During a ‘no-go’ trial, no sound was played, and the turtle had to indicate that she couldn't hear anything by not pressing the response paddle after the LED light was switched on. If the turtle got it right, she was rewarded with food; if she didn't, Martin briefly switched off the overhead lights. But the turtle's erratic behaviour proved to be problematic, especially when she lost her appetite in the months before the nesting season. ‘It was a frustrating period’, Martin recalls. ‘It took months to develop behavioural consistency.’
Eventually, Martin was ready to test how sensitive the turtle's hearing really was. By playing sounds of varying frequencies and incrementally decreasing or increasing the volume between trials, she found that the turtle was able to hear sounds between 50 and 800 Hz. This is fairly low frequency hearing compared with humans, who can typically hear between 20 and 20,000 Hz. But would electrophysiological tests produce the same results? To find out, Martin inserted electrodes under the turtle's skin just above the brain, played sounds at different frequencies, and recorded the signals from the electrodes. To her astonishment, the results correlated closely with the range established during the behavioural tests. Given the difficulties training the turtle, Martin says, ‘It was a big surprise that the results aligned so well’.
Martin concludes that electrophysiological methods can be used to conduct simple and fast hearing tests for wild or untrained marine turtles. She acknowledges that other loggerhead turtles will need to be tested to confirm this conclusion. But, she says, ‘This is a first step towards a better understanding of marine turtle hearing abilities’.