Not Just Rocked, but Delighted: Rhythmic Passive Movement Induces Positive States in Chicks

Rhythm is a fundamental organizing principle in biological systems, shaping physiological regulation, affect, and social interaction across species. In caregiving, rhythmic passive movement, such as carrying and rocking, has long been recognized as a soothing strategy in mammals, reducing crying, heart rate, and arousal while promoting sleep. Anecdotal and behavioral evidence in humans also suggest that some forms of rhythmic motion, such as sliding or tossing infants, can elicit overt enjoyment. Yet, no study has directly investigated the effects of passive rhythmic movement beyond the mammalian lineage or in species lacking maternal carrying, nor assessed its potential to evoke pleasurable states. Here, we examined the effects of rhythmic passive movement in domestic chicks (Gallus gallus), a precocial bird that walks immediately after hatching and is never carried by the mother. Chicks were exposed to rocking (horizontal, vertical) and carrying-like motion at different frequencies. Slow rhythmic movements reduced contact calls, replicating the soothing effects known in mammals, while faster movements increased brood calls, indicating a transition from distress reduction to positive arousal. These results demonstrate that rhythmic passive motion modulates affective states in birds, revealing both calming and pleasurable dimensions and suggesting deeply conserved mechanisms for rhythm-based affect regulation across vertebrates.