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Suppression of newly hatched ducklings' distress vocalization by two distinctive stimuli was assessed before and after subjects were imprinted to one of them. Initially, both stimuli strongly suppressed distress calls. Later, only the stimulus involved in imprinting did so. The results suggested that the loss of control by the nonimprinted stimulus was age related and represented the development of fear to insufficiently familiar stimuli. In a second experiment, older ducklings were permitted lengthy exposure to a novel imprinting-type stimulus. It was found that under these circumstances distress vocalization eventually came under stimulus control. The overall pattern of results suggests that imprinting renders stimuli that innately evoke filial behavior familiar and in doing so prevents their later evoking noveltyinduced fear.Among the most salient characteristics of imprinting in ducklings is the range of objects that can serve as the focus of filial attachment. Fabricius (1951aFabricius ( , 1951bFabricius ( , 1955 and Fabricius and Boyd (1954), e.g., have successfully imprinted ducklings to humans, live ducks, stuffed animals, and a variety of other objects such as balloons, boxes, and model railway cars. Data such as these point to the conclusion that for ducklings, the shape, size, and color of the imprinting stimulus, within wide limits, makes little difference in its capacity to elicit filial responses.When, however, imprinting to one stimulus is completed, the subject's tendency to respond filially to other stimuli is often considerably reduced (Sluckin, 1965). Apparently, some process occurs during imprinting that restricts the control initially afforded by a wide range of stimuli to only the imprinting stimulus or the class of stimuli that includes it. Since this differentiating process occurs without prior or concurrent discrimination training, it has been described by Jaynes (1958) as "emergent discrimination."The research reported here was designed to examine the changes in filial reactions

Newly hatched Khaki Campbell ducklings (Anas platyrhynchos domesticus) were initially housed in pairs and subsequently transferred to isolated housing conditions. Subjects living with another bird displayed filial behavior and little aggression upon encountering another duckling in a test arena. In contrast, ducklings housed in isolation exhibited aggressive pecking in addition to filial behavior when another duckling was subsequently encountered. In Experiment 2, ducklings were housed with an imprinting object (i.e., an object that elicits attachment behavior), but they were otherwise isolated from other birds. These subjects displayed little aggression when they were subsequently reunited with a conspecific, indicating that the aggression-precluding effects of social housing are not limited to the particular social stimulus with which the ducklings are housed. In Experiment 3, ducklings were again housed with an imprinting object, but this time the object was behind glass, thereby precluding tactile contact with it. Since these birds also exhibited little aggression when reunited with a conspecific, it is apparent that visual stimulation from an imprinting object is sufficient in itself to preclude subsequent aggression. These findings suggest that stimulation that elicits attachment behavior is the critical factor mediating isolation-induced aggression.Numerous species show increased aggressive tendencies toward conspecifics following a period of social isolation. The effect has been observed with chicks (Beach &

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