HEARING IN THE GIANT SEA TURTLE, Chelonia mydas

Abstract

Abstracts.-Measurements were made of the cochlear potentials in three specimens of the giant sea turtle, Chelonia mydas, in response to both aerial and mechanical stimulation of the ear. The results show maximum sensitivity in the region of 300 to 400 Hz, with a rather rapid decline in sensitivity for lower and especially for higher tones. The upper limit for the observation of cochlear potentials without injury is 2000 Hz, and a practical limit of usefulness of this ear is probably about 1000 Hz.The presence of hearing in turtles is now well established through both behavioral and electrophysiological observations. However, these observations have been limited to the small and readily available species, and nothing definite is known about the giant marine forms. An early attempt by Foa and Peroni to record auditory potentials in large specimens of Thalassochelys caretta gave no useful results.1 1\Ieasurements of auditory sensitivity in terms of the cochlear potentials were carried out by Wever and Vernon in 1956 on four of the small common species: Chrysemys p. picta, Clemmys insculpta, Pseudemys scripta, and Terrapene c. carolina, all belonging to the family Emydidae.2-4 More recently, the hearing of one of these species, Pseudemys scripta, was tested by a behavioral method by Patterson.5The patterns of auditory sensitivity obtained for all these animals, and by both methods on P. scripta, were closely similar.6 In every instance a fairly high level of sensitivity was observed for low tones, with the best responsiveness in the region of 200 to 700 Hz. For tones below 100 Hz the sensitivity was found to decline slowly in the two species in which tests were made in this region, and for tones above 700 Hz the sensitivity of all species was found to fall off with great rapidity, so that beyond 3000 Hz no tests could be made without injury to the ear.The present experiments were carried out on the giant sea turtle, Chelonia mydas. The specimens were obtained from commercial sources in Hawaii, and thus no doubt represent the Pacific subspecies. Measurements of sensitivity to sounds were made by the cochlear potential method in 3 animals, whose weights were 50, 68, and 75 pounds. Several additional animals were used for anatomical and surgical studies. All the potential measurements were made on the left ear.Procedure.-The animals were held in a saltwater pool until needed for the experiments. While in the pool, they were observed to remain on the bottom at a depth of 8 ft most of the time, and they came to the surface periodically to breathe. The smaller specimens usually surfaced at intervals of ten minutes 884

Specialization for underwater hearing by the tympanic middle ear of the turtle,Trachemys scripta elegans

“…The sea turtle middle ear is more massive than the present turtle ear, but the audiogram of Chelonia mydas shows a best frequency at 400 Hz and a similar shape to Trachemys, with comparable sensitivities to direct vibration of the tympanum [42]. The Chelonia ear also has a middle ear cavity and a long columella.…”

Ontogenetic investigation of underwater hearing capabilities of loggerhead sea turtles (Caretta caretta) using a dual testing approach

“…The AEP-derived hearing range and threshold levels reported here are consistent with previous studies conducted on sea turtles using a variety of approaches. Ridgway et al (Ridgway et al, 1969) surgically implanted electrodes to measure cochlear potentials in the green sea turtle in response to aerial and vibratory stimuli and found that these turtles detect a limited frequency range (200-700 Hz) with best sensitivity in the low tone region of approximately 400 Hz. Bartol et al (Bartol et al, 1999) collected ABRs from juvenile loggerhead sea turtles using vibratory stimuli delivered directly to the dermal plates over the tympanum and found best sensitivity in the low-frequency region of 250-1000 Hz, especially 250 Hz, which was the lowest frequency tested.…”

“…Ridgway et al (Ridgway et al, 1969) used both aerial and vibrational stimuli to obtain auditory cochlear potentials from juvenile green sea turtles (Chelonia mydas) for frequencies ranging from 50 to 2000 Hz. Bartol et al (Bartol et al, 1999) collected auditory brainstem Ontogenetic investigation of underwater hearing capabilities in loggerhead sea turtles (Caretta caretta) using a dual testing approach responses (ABRs) from juvenile loggerhead sea turtles [Caretta caretta (Linnaeus 1758)] presented with vibratory stimuli, finding best sensitivity in the low-frequency region of 250-1000 Hz.…”

Flexibility within the middle ears of vertebrates

“…Marine turtle middle ears are essentially similar to what is described above, except that more fat is present between extrastapes and skin. 31,32 The ear morphology of testudines in general has been regarded as a compromise between aquatic and terrestrial hearing. 33 Bird middle ears Birds emerged from within the reptile group.…”