Perihatch surge of thyroid hormone drives cognitive flexibility in newborn chicks

Early experience in infancy affects cognitive development. Birds, like mammals, acquire cognitive flexibility attributed to a well-developed telencephalon. Precocial chicks acquire imprintability just after hatching when thyroid hormone (T₃) flows into the brain and primes later learning. Here, we show that the perihatch synthesis of T₃ paralleling thyroid development is crucial for imprinting and endows newborn chicks with cognitive flexibility via a mechanism involving the nidopallium dorsocaudale, the avian “prefrontal cortex.” Imprinted chicks showed higher cognitive flexibility than those unimprinted in switching or reversal task experiments. Notably, we discovered that exogenous T₃ endowed similar flexibility in unimprinted chicks. Cognitive stimulation by a surge of thyroid hormone indicates a vertebrate tactic involving high cognitive ability for adapting to environmental changes.