Abstract
ABSTRACTThe acoustic startle reflex is an oligo-synaptic reflex arc elicited by rapid-onset sounds. Odontocetes evolved a range of specific auditory adaptations to aquatic hearing and echolocation, e.g. the ability to downregulate their auditory sensitivity when emitting clicks. However, it remains unclear whether these adaptations also led to changes of the startle reflex. We investigated reactions to startling sounds in two bottlenose dolphins (Tursiops truncatus) and one false killer whale (Pseudorca crassidens). Animals were exposed to 50 ms, 1/3 octave band noise pulses of varying levels at frequencies of 1, 10, 25 and 32 kHz while positioned in a hoop station. Startle responses were quantified by measuring rapid muscle contractions using a three-dimensional accelerometer attached to the dolphin. Startle magnitude increased exponentially with increasing received levels. Startle thresholds were frequency dependent and ranged from 131 dB at 32 kHz to 153 dB at 1 kHz (re. 1 µPa). Startle thresholds only exceeded masked auditory AEP thresholds of the animals by 47 dB but were ∼82 dB above published behavioural audiograms for these species. We also tested the effect of stimulus rise time on startle magnitude using a broadband noise pulse. Startle responses decreased with increasing rise times from 2 to 100 ms. Models suggested that rise times of 141–220 ms were necessary to completely mitigate startle responses. Our data showed that the startle reflex is conserved in odontocetes and follows similar principles as in terrestrial mammals. These principles should be considered when assessing and mitigating the effects of anthropogenic noise on marine mammals.
Highlights
The acoustic startle response is a rapid contraction of flexor muscles which is mediated by an oligo-synaptic reflex arc in the brainstem (Koch and Schnitzler, 1997)
There are three possible explanations for this discrepancy: (1) the hearing thresholds were masked due to the noise caused by snapping shrimp in the test pens (Au and Banks, 1998) and actual hearing thresholds under quiet conditions would be lower and sensation levels higher; (2) hearing thresholds were obtained with an electro-physiological method which typically yields thresholds that are approximately 20 dB higher than those obtained with traditional psycho-physical methods; and (3) the test subjects in our study showed some age-related hearing loss at higher frequencies
The basic physiology followed similar principles as in terrestrial mammals, i.e. startle magnitude was positively correlated with sound pressure levels (SPLs) and negatively correlated with rise time
Summary
The acoustic startle response is a rapid contraction of flexor muscles (flinch) which is mediated by an oligo-synaptic reflex arc in the brainstem (Koch and Schnitzler, 1997). The reflex arc is used as a model for simple learning mechanisms, the functioning of sensory motor gating and emotional processing (Koch and Schnitzler, 1997; Lang et al, 1998) The latter is the result of startle magnitude being modulated by emotional state, i.e. watching pleasant/unpleasant images (humans) or inducing conditioned fear in animal models (Lang et al, 1998). In a study that investigated follow-up responses after repeated startle elicitation, the majority of tested grey seals (Halichoerus grypus) sensitised to the stimulus, exhibited flight responses, developed place avoidance and showed signs of temporary fear conditioning (Götz and Janik, 2011) These findings kindled research interest in the startle reflex in the context of practical applications such as target-specific predator deterrence from farmed fish stocks and fisheries (Götz and Janik, 2015, 2016; Schakner et al, 2017). This reflex arc is relevant for understanding the physiological mechanisms underlying aversive responses to anthropogenic ocean noise in marine mammals (Harris et al, 2018), but empirical data is lacking
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