Development and Habituation of Cardiac and Behavioral Responses in Young Red Deer Calves (Cervus elaphus) Exposed to Alarm Stimuli

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Abstract
Heart rate was recorded by radiotelemetry in six red deer calves ranging in age from 6 h to 7 days. A few hours after birth the mean heart rate of undisturbed resting calves was 170 beats/min (BPM). During the first week of life the rate declined to 155 BPM. The calves responded by bradycardia, startle responses, and heart blocks to three kinds of alarm stimuli; the intensity and rate of the responses varied with stimulus type and age of the calves. At about one week of age the calves displayed flight responses when disturbed by human approach. Calves with low birth weights retained the freezing response significantly longer than calves with higher birth weights. The onset of the flight response was more dependent on the calf's physical development than on its age. Habituation to repeated stimulations, shown by weakening of the bradycardia response, less frequent startle responses and heart blocks, and shorter recovery time, was recorded for all types of stimuli. The tendency for older calves to display weaker cardiac responses than neonatal calves was probably due to maturation rather than habituation. The results are discussed from an adaptive and evolutionary viewpoint.

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