Adult chicken alarm calls enhance tonic immobility in chicks.

An experimenter induced tonic immobility (TI) in parentally naive chicks (G. gallusdomesticus). The chicks remained in TI longer when they were exposed to a conspecific adult fear squawk alarm call than when exposed to an equally novel attraction call or white noise. In a second experiment, both aerial-predator and ground-predator alarm calls enhanced TI similarly to the fear squawk call which is elicited by capture. These results support the hypotheses that TI is an antipredator defense mechanism and that alarm calls evolved through kin-selection.