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Student Research Projects

Hearing Frequencies of Local Semi-Aquatic Turtles

Student Allison Shannon
Faculty Mentor(s) Dr. Soraya Bartol
Department Biology
Course BIO 489: Research in the Natural Sciences

Abstract

Although a great deal of information known about Chelonians focuses on the anatomy of the ear, there have been few investigations on the hearing-range and mechanism of hearing in these animals. The first attempt in turtles to record electrophysiological activity in response to sound stimuli was made by Foa and Peroni in 1930 (The Reptile Ear 1978). In 1956, Wever and Vernon carried out a further series of experiments in which the electrical potentials of the inner ear were used to measure auditory sensitivity in three common turtle species, Chrysemys p. picta, Chrysemys scripta, and Clemmys insculpta. Their studies, as well as those compared with other scientists, consistently showed a high degree of sensitivity in the low frequencies between 100 and 700 Hz. Tones above this range showed a very rapid decline of response (The Reptile Ear 1978).
Two attempts have been successful in collecting electrophysiological data from sea turtles; one study performed on juvenile green sea turtles (Ridgway et al., 1969) and one study on juvenile loggerhead sea turtles (Bartol et al. 1999). In contrast to terrestrial and semi-aquatic species, sea turtles have a thick layer of subtympanal fat under a tympanum that is a continuation of the facial tissue (Bartol et al. 1999). It is understood that the tympanum does not inhibit sounds reception, but instead acts as additional mass loading to the ear, allowing for reduction in the frequency sensitivity. In these two experiments aerial and vibrational stimuli were tested on the sea turtles in frequencies ranging from 30-2000Hz; results for stimuli revealed that green sea turtles detected limited sound frequencies (200-700 Hz) and displayed a high level of sensitivity at the low tone region.
The main goal of my study is to determine whether or not semi-aquatic turtles hear differently in two different medium: air and water. However, collecting hearing data from low frequency underwater sound sources can be problematic. The long waveform sounds will readily bounce off any surface (tank walls, floor, air-water interface, etc.) and cause harmonics in the signal. Recently, Bartol and Ketten (in prep) developed methods to collect auditory brainstem responses (ABRs) from sea turtles underwater. Using a hydrophone and matlab generated waveform, they were able to control the signal, produce tones underwater, and collect ABRs from greens, loggerheads, and Kemp’s ridleys. I plan to use these same methods in order to investigate the hearing frequency of local semi-aquatic turtles, turtles which spend much of their days occupying both aquatic and terrestrial environments. These techniques are well suited for experiments with semi-aquatic turtles because they are noninvasive, rapid and require no additional training of the test subjects. This technique involves the presentation of an acoustic stimulus to the subject and simultaneously recording the evoked neural response from electrodes on the scalp, in this case by using a Tucker Davis Technologies system. The electrodes will be placed on both sides of the frontal parietal plate on the dorsal surface of the head. In addition, a reference electrode will be inserted in the skin immediately behind the skull over the extension of the supraoccipital and a ground electrode will be placed in the inactive skin of the lateral neck. Fortunately, the placement of the electrodes will not require surgery and the animal will not be anesthetized for the trials, and all methods will
follow protocols set up by Bartol et al. (1999). These methods have received
IACUC approval from Virginia Institute of Marine Science, College of William
and Mary and will be subject to review by the new VWC IACUC committee." Stimuli will be generated using a matlab program, presented using a low frequency speaker, and all sounds will be continuously recorded using a hydrophone located next to the turtle’s tympanum. Presentation of an acoustic stimulus to the turtle produces synchronized discharges of large populations of neurons within the auditory pathway. As the stimuli are presented to the turtles, auditory evoked potentials will be extracted from the EEG using signal averaging techniques. These data will be analyzed, auditory brainstem responses will be extracted, and ABRs will tracked to approximate threshold. By collecting data in both air and water, I will be able to determine if these semi-aquatic turtles are listening to different frequencies in the different media. In conducting this experiment, I hope to establish the groundwork for understanding hearing frequencies of semi-aquatic turtles.

Grants

2007 VWC Summer Undergraduate Research Fellowship

 

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