Pre and post hatching associative habituation in chicks

Habituation is considered an example of non-associative learning, depending on stimulus characteristics1. This form of learning is documented in birds even prenatally, with effects persisting a few hours posthatching2. However, an associative model of habituation suggests that during the stimulus repetition a stimulus-context association is established3: habituation to the same stimulus is less likely to transfer to a different context4. We considered 120 chicks (Gallus gallus) whose stops of the wheel-running behavior were scored as measure of habituation to 2 sequences of five 250ms-bursts of 90dB white-noise, presented one hour apart, during their second day of life (D2). Chicks exposed to the noise within the same running-wheel the day before (D1), on D2 showed only a residual freezing response (long-term habituation). Conversely, chicks that heard the noise on D1 within the incubator, on D2 showed a more intense freezing performance, showing that habituation was at least partially context specific. Indeed, chicks tested at the same age but never exposed to the stimuli on D1, showed a significantly stronger freezing response although habituating across the two sequences of the same day (short-term habituation). Furthermore, chicks exposed to the noise before hatching, within the egg, showed the same performance of chicks stimulated on D1 within the incubator. Hence, our results indicate that in chicks habituation of the freezing response varies depending on the history of stimulation, showing that associative mechanisms underlying context-specific habituation5 are active also before hatching, and that this form of learning can last for at least 48 hours posthatching