Habituation indicates a decrement in response to a specific stimulus by repeated exposure to that stimulus.
Responding to or ignoring sound stimuli, which are common background distractors, is important for the
undisturbed functioning of the organism, efficient animal management and safe handling. We habituated
20 warmblood horses to a set of distracting sounds and assessed the possibility of reversing this process
through simple modifications in sound playback. The study involved three stages: habituation (Stage 1) and
two trials of dishabituation (Stage 2, 3). In Stage 1, five different sounds of animals, each on a separate day,
were repeatedly (every 20 minutes (min)) played during a stay on an experimental paddock. The sounds
were selected out of 40 sounds of different origin based on their distracting effect on the horses (e.g.,
interruption of previous activity, increase in emotional arousal) during the first exposure to them three
months earlier. In Stage 2, four new and unknown sounds (futuristic characteristics) were additionally
introduced to the set of habituated sounds, whereas in Stage 3, habituated sounds (from Stage 1) were
played from an altered location. Behavioral (e.g., disruption of feeding, duration of feeding, locomotion and
standing alert) and cardiac (heart rate and heart rate variability) variables were assessed. Generally, horses
quickly habituated to the initially distracting sounds, but great variability was observed in the number of
needed playbacks (1–7) for each horse. After completion of Stage 1, the horses responded with an increase
in vigilance, locomotion, refeeding frequency and a 1-minute increase in mean heart rate to novel but not to
known sounds. None of the modifications resulted in a recovery of previously habituated reactions. After
introduction of playback modifications, only maximum heart rate (in Stage 2, Stage 3) and time of standing
alert (in Stage 3) increased after re-exposure to sounds horses had been habituated to, suggesting some
level of vigilance. However, this response was too weak to conclude about dishabituation. The results of the
study confirm sound recognition and the fast and persistent audio habituation in horses.
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