Conspecific calls evoke characteristic cardiac responses in mallard ducklings

The cardiac response of mallard ducklings, Anas platyrhynchos, has a characteristic pattern, which varies according to which of three conspecific calls, or an artificial sound, is presented. Ducklings were played one of four acoustic stimuli, each of which has well-documented behavioural efects: maternal alarm calls, maternal assembly calls, duckling distress calls and a synthetic control stimulus. The evoked changes in heart rate consisted of brief phasic responses to the playbacks of each call type, together with a cumulative (tonic) change with repeated stimulus presentations. The phasic response consisted of an acceleration during the stimulus in every case, but, while all three conspecific calls elicited a profound post-stimulus deceleration, characteristic of an orienting response, the control stimulus did not. Two of the stimuli also had significant tonic effects; the assembly call evoked a prolonged increase in heart rate, while the alarm call caused a prolonged decrease. These tonic changes may be related to the locomotor responses characteristically elicited by maternal calls. When both phasic and tonic changes in heart rate are considered, it is apparent that each of the stimuli elicited a unique response. The cardiac response thus provides an assay of sufficient sensitivity to examine both recognition of species-typical calls and discrimination between conspecific vocalizations.