By Nicole Flannigan
Originally written for Compass Education Group
A year ago, while vacationing in Hawaii, I was lucky enough to encounter one of the world’s most beloved yet poorly understood animals: Chelonia mydas, the green sea turtle. Though nearly ubiquitous in some areas of the globe (and completely ubiquitous in the gift shops that dot the Big Island), green sea turtles remain enigmatic. Little is known about their basic biology, and even rudimentary concepts such as the mechanism by which they hear remain shrouded in mystery. However, researchers have recently completed a study that may forever advance our understanding of these fascinating creatures.
The green sea turtle was first described in 1758 by the Swedish botanist and zoologist Carl Linnaeus. Its range extends throughout tropical and subtropical waters, including the Pacific, Atlantic, and Indian oceans, but individuals have been seen as far north as Canada. Despite its large range and numerous subpopulations, C. mydas has been listed as endangered by the International Union for Conservation of Nature since 1982. According to IUCN, imminent threats to turtle populations include overexploitation of eggs and habitat destruction. Mortality associated with fishing is also a threat, and certain fishing methods such as drift netting, shrimp trawling, and dynamite fishing are thought to be responsible for the majority of fatalities.
Given the challenges facing C. mydas, it is of critical importance that scientists understand its biology. Experts have a basic knowledge of the turtle’s sight and breathing patterns; they even sequenced its genome in 2013. However, one area of turtle physiology that has remained frustratingly elusive is the functional morphology of the turtle ear. The sea turtle lacks external pinnae, or ear canals, and its ears are covered by tissue called the tympanum as well as a thick layer of insulating fat. Because of these factors, it can be difficult to comprehend how a turtle’s auditory processing works. Does this structure allow turtles to “hear” in the same way humans do, or are they merely responding to vibrations in their environment? Is there a difference between a sea turtle’s hearing above and below water? Although scientists have hypothesized that sea turtles’ hearing may be superior underwater, turtles perform vital biological functions, such as laying eggs, on land. Thus, the answers to these questions would open an important window onto issues that are key to C. mydas’ survival.
Recently, a team of researchers set out to get some answers. Wendy E. D. Piniak, an ecologist in the Department of Environmental Studies at Gettysburg College in Pennsylvania, and her colleagues David A. Mann of the University of South Florida, Craig A. Harms of North Carolina State University, T. Todd Jones of the University of British Columbia, and Scott A. Eckert of Principa College, performed a study in which they measured the hearing thresholds of five Atlantic juvenile green sea turtles both above and below water.
The researchers measured the turtles’ hearing via the Auditory Evoked Potentials method, in which electrodes are connected to an animal’s head and used to measure the brain activity resulting from various aural stimuli. The turtles were lightly restrained using cloth bags and put into an indoor tank, isolated from outside noises and vibrations. While the turtles were immobilized, a speaker played a series of tones that varied in frequency and sound pressure. All the while, subdermal electrodes measured the AEP waveforms produced in response to the sounds. For each turtle, the experiment was run twice: once with the animal’s ears and speaker above water and once with both below.
The results of the experiment appear to confirm much of what scientists previously surmised. Underwater, the juvenile green sea turtles responded to signals between 50 and 1,600 Hz, with maximum sensitivity between 200 and 400 Hz. In the air, they responded to signals between 50 and 800 Hz, with maximum sensitivity at 400 Hz. Thus, the turtles responded to a greater rage of frequencies underwater. Moreover, Piniak and her team discovered that when the responses to aerial and underwater stimuli were compared, C. mydas’ sound intensity level thresholds were 2-39 decibels lower underwater, especially at frequencies below 400 Hz. Which is to say, the sounds needed to be less intense for them to be heard. Because this is especially true for lower frequency sounds, it appears that sea turtle hearing may be optimized for underwater environments rich in low frequency noises.
Crucially, scientists still don’t understand the mechanism by which turtle’s hear. Piniak and her team hypothesize that turtles may respond to both the pressure and particle motion of a sound, but follow up experiments are needed. Regardless, this study represents a large step forward in our understanding of C. mydas and its aquatic cousins. Hopefully, this knowledge will help us fight back against the forces of extinction before its too late.