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

Abstract

Sea turtles reside in different acoustic environments with each life history stage and may have different hearing capacity throughout ontogeny. For this study, two independent yet complementary techniques for hearing assessment, i.e. behavioral and electrophysiological audiometry, were employed to (1) measure hearing in post-hatchling and juvenile loggerhead sea turtles Caretta caretta (19-62 cm straight carapace length) to determine whether these migratory turtles exhibit an ontogenetic shift in underwater auditory detection and (2) evaluate whether hearing frequency range and threshold sensitivity are consistent in behavioral and electrophysiological tests. Behavioral trials first required training turtles to respond to known frequencies, a multi-stage, time-intensive process, and then recording their behavior when they were presented with sound stimuli from an underwater speaker using a two-response forced-choice paradigm. Electrophysiological experiments involved submerging restrained, fully conscious turtles just below the air-water interface and recording auditory evoked potentials (AEPs) when sound stimuli were presented using an underwater speaker. No significant differences in behavior-derived auditory thresholds or AEPderived auditory thresholds were detected between post-hatchling and juvenile sea turtles. While hearing frequency range (50-1000/1100 Hz) and highest sensitivity (100-400 Hz) were consistent in audiograms pooled by size class for both behavior and AEP experiments, both post-hatchlings and juveniles had significantly higher AEP-derived than behavior-derived auditory thresholds, indicating that behavioral assessment is a more sensitive testing approach. The results from this study suggest that post-hatchling and juvenile loggerhead sea turtles are low-frequency specialists, exhibiting little differences in threshold sensitivity and frequency bandwidth despite residence in acoustically distinct environments throughout ontogeny.

Comparative analysis of the shape and size of the middle ear cavity of turtles reveals no correlation with habitat ecology

“…Some case studies indicate that the frequency spectrum of hearing in turtles is generally similar in air and under water (e.g. Christensen-Dalsgaard et al 2012;Zeyl & Johnston, 2015;Piniak et al 2016, see also Lavender et al 2014). Extremely little is also known about the function of hearing in turtles.…”

Developmental plasticity of hearing sensitivity in red-eared sliderTrachemys scripta elegans

“…Our findings differ somewhat from those reported elsewhere. For instance, 3-year-old Hawksbill turtles ( Eretmochelys imbricata ) were sensitive to a wider frequency range and exhibited a larger amplitude response than the 2-year-olds [18]; loggerhead sea turtles ( Caretta caretta ) exhibited little difference in threshold sensitivity and frequency bandwidth throughout ontogeny [19]; and in subadult Green turtles ( Chelonia mydas ), smaller individuals had a wider hearing sensitivity bandwidth (100–800□Hz) than larger individuals (100–500□Hz) [17]. Our study improves on earlier studies, potentially providing more reliable results, because we used a larger age range, and, more importantly, we first determined the sex of individuals.…”

“…Although there has been considerable research into auditory system development in some groups of vertebrates, much less is known about it in chelonians. At present, we know of only three studies on the developmental plasticity of hearing sensitivity in chelonians, and these reached different conclusions [17–19]. However, these studies had small samples (n < 7), narrow age ranges, and unclear sexual categorization, factors that may affect their findings.…”

A modelling framework for MSP-oriented cumulative effects assessment

“…In this sense, the loggerhead turtle hotspots is in proximity of the most trafficked highway of the sea, with an annual traffic density of about 2400 vessels/km (Fortuna et al, 2015). Although several national and international management measures have been taken to reduce risks for local population, their effectiveness may be reduced in sea areas with intenisve human activities responsible sea noise pollution (Hildebrand, 2009;Ross, 2005;460 Andrew et al, 2002), that can cause communication masking, stress, hearing loss or habitat abandonment (Nowacek et al, 2007;Clark et al, 2009;Lavender et al, 2014;Nelms et al, 2016). In this context underwater noise pollution mapping (Tasker et al, 2010), has been identified as a major research priority (Maccarrone et al, 2015).…”