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Martin, K. J. (2011). Underwater hearing in the loggerhead turtle (caretta caretta): a comparison of behavioral and auditory evoked potential audiograms. Unpublished thesis , University of South Florida. 
Added by: Admin (29 Jan 2012 12:38:57 UTC)
Resource type: Thesis/Dissertation
BibTeX citation key: Martin2011
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Categories: General
Keywords: akustische Kommunikation = acoustic communication, Caretta, Caretta caretta, Cheloniidae, Physiologie = physiology, Schildkröten = turtles + tortoises, Verhalten = ethology
Creators: Martin
Publisher: University of South Florida
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URLs     http://scholarcommons.usf.edu/etd/3233
Abstract     
Caretta caretta Results from this study indicate that loggerhead turtles detect low frequency sound with a functional hearing range of 50 - 1131 Hz and peak sensitivity between 100 - 400 Hz. These results are consistent with previous work done in other marine and terrestrial turtles and establish the first underwater audiogram of a loggerhead turtle. Behavioral results confirm that auditory evoked potential measurements function as good indicators of marine turtle hearing. Threshold calculations differed by an average of 8 dB between the two methods. Frequency ranges detected by the loggerhead were similar to those found in green turtles and Kemp’s ridley turtles by Bartol and Ketten (2006). Green turtles had a frequency range of 100 – 800 Hz with peak sensitivity occurring between 200 - 400 Hz in one group tested and 600 - 700 Hz in another group. Kemp’s ridley turtles had a frequency range of 100 - 500 Hz with highest sensitivity between 100 and 200 Hz. Bartol et al. (1999) examined hearing ranges in loggerhead turtles with the animal tested in air. Frequency ranges in that study were 250 - 750 Hz. Peak sensitivity was at 250 Hz, the lowest frequency tested. Threshold levels are difficult to compare because the study utilized a mechanical vibrator attached directly to the tympanum to relay stimuli through the bone and the results of vibrational testing (presented in dB re 1 g rms) cannot be easily compared with results presented in SPL. However, the frequency ranges are similar to those found in the current study. Similarly, Ridgway et al. (1969) found green turtle sensitivity to be 60 - 1000 Hz with a peak between 300 and 500 Hz. Each of these studies indicates that marine turtles have hearing abilities that detect low frequency sound with a narrow band of greatest sensitivity. The current study confirms the results of previous electrophysiological tests through behavioral test comparisons. It would be beneficial to collect behavioral audiograms in other marine turtle species. However, given the similarity between evoked potential results and auditory system anatomy among the species, results from this study indicate that behavioral and evoked potential results should show close correlation in these species as well. The current understanding of marine turtle auditory abilities is based on results from morphological findings and electrophysiological results that have until now, not been substantiated with behavioral results. This study showed that an audiogram collected through operantly conditioned behavioral methods was similar to that measured with auditory evoked potential testing, which can be conducted in just a few hours on an untrained animal. The thresholds calculated in this study could have potentially been masked as a result of tank noise and other sources of background noise, resulting in a threshold above the actual audible level. However, it is likely that similar levels of background noise would be seen in a natural coastal environment with increased levels of low-frequency noise from wind, waves, marine vessels, construction, and other natural and anthropogenic sources. Additional testing with increased sample sizes as well as additional age classes may be needed to account for any ontogenetic changes in auditory function, behavior or habitat usage. While behavioral testing methods provide accurate measures of threshold determination, evoked potential testing is beneficial when time is a factor and multiple individuals need to be tested. With continued improvement in threshold estimations through electrophysiological testing, a more rapid and thorough understanding of marine turtle auditory abilities will aid in future assessments of the impacts of anthropogenic noise and the function of sound in the marine turtle environment.
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