The Control of Distress Vocalization By an Imprinted Stimulus

Twenty two newly hatched ducklings (A. platyrhynchos) were either exposed to a moving imprinting stimulus under one of several experimental conditions or exposed to an empty stimulus compartment. During these procedures special equipment was used to record distress calls. Ss individually exposed to a moving stimulus emitted first more and then fewer distress calls than Ss individually exposed to an empty stimulus compartment. Ss individually exposed to the moving stimulus while in their cages emitted distress calls whenever the stimulus disappeared from the visual field, but not otherwise. Ss exposed to the moving stimulus while in the company of other Ss failed to emit distress calls during the imprinting procedures. In subsequent tests for the effects of the several procedures the imprinting stimulus was periodically presented and withdrawn. It was found that when tested in isolation: 1. Regardless of the conditions during imprinting procedures, Ss previously exposed to the moving stimulus emitted distress calls when the imprinted stimulus was withdrawn, but they seldom emitted distress calls when the stimulus was present. In general, Ss that had been exposed to the moving stimulus while in their cages (i.e., with subject movement restricted) as well as Ss that had been exposed to the stimulus in the company of other Ss displayed the same vocalization pattern as Ss that had been exposed to the stimulus in isolation. These findings indicated that neither freedom to move about nor social isolation during exposure to an imprinting stimulus are necessary conditions for the imprinting stimulus to acquire control over S's distress calls. 2. Ss previously exposed to an empty stimulus compartment emitted more distress calls in the presence of the imprinted stimulus than in its absence. This implied that prior exposure to an imprinting stimulus was a necessary condition for stimulus withdrawal subsequently to evoke distress calls. 3. Additional tests were concerned with the factors that played a role in the behavioral control exhibited by the imprinted stimulus. For example, in the previous tests during stimulus withdrawal the imprinted stimulus was stationary and hence silent. In Test 3, during periods of stimulus withdrawal the stimulus continued to move under conditions in which S could hear but not see it. As in the initial tests, Ss emitted many distress calls during stimulus withdrawal, but they emitted very few calls during stimulus presentation. This finding suggested that distress vocalization was primarily under the control of visual rather than auditory stimulation. Test 4 examined the effects of stimulus presentation and withdrawal when Ss were in their own cages versus out of them during testing. More distress calls were emitted when S's cages were removed than when Ss were tested while in their cages. This finding suggested that in part, the control over vocalization exerted by the imprinted stimulus was mediated by the familiarity of the stimulus configuration that prevailed at a given time. Test 5 examined the effects of stimulus presentation and withdrawal when S was tested in isolation versus in the company of a second duckling. Withdrawal of the imprinted stimulus yielded many distress calls when S was alone, but not when S was accompanied by a second duckling. This finding suggested that in isolated Ss the high incidence of distress calls in the absence of the imprinted stimulus was a reflectieon of the withdrawal of social stimulation.