Imprinting in older ducklings: Some tests of a reinforcement model

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Ducklings (Anas Platyrhynchos domesticus) older than the so-called critical period (Days 1 and 2 posthatch) were exposed to an imprinting stimulus after various experimental histories. The first study found that in previously isolated 10-day-old subjects the stimulus exhibited the same capacity to reinforce an operant response and to generate a burst-like pattern of responding as in ducklings imprinted to it on Day 1. In Experiment 2, an imprinting stimulas exhibited reinforcing capacities in 5 to 10-day-old ducklings that had previously been imprinted to a different stimulus. Most of these ducklings came to prefer the second imprinting stimulus over the first. Experiment 3 revealed that in 5-day-old ducklings with a prior history of imprinting to a different stimulus, the otherwise neutral features of a new imprinting stimulus acquired the same kind of persistent control over distress vocalization that they acquire in younger, naive subjects. In all these studies, the only difference between imprinting in older vs. younger subjects was that in older subjects a novel imprinting stimulus initially evoked fear reactions rather than filial behavior. These findings contradict the traditional view of imprinting as an irreversible process that occurs only during a brief critical period, but are entirely consistent with a reinforcement model of imprinting.