Tonic immobility and high‐intensity calls in a precocial chick as a function of age, diet, and time of day

Tonic immobility (TI) and high‐intensity vocalizations are two antipredator behaviors employed by domestic chicks (Gallus gallus) which vary in their function and the proximate mechanisms that govern them. In the present study, we sought to determine the influence of age (3 or 10 days old), diet (control or tryptophan‐free), and time of testing (A.M. or P.M.) on the duration of TI and the number of calls produced in domestic chicks. Older chicks remained immobile significantly longer than younger chicks as did subjects on the control diet or those tested at night. However, 3‐day‐old chicks had significantly shorter TI durations only when tested in the A.M.: TI did not differ between age groups when subjects were tested in the P.M. Three‐day‐olds called significantly more than 10‐day‐olds when given the control diet and when tested in the P.M. but vocalization frequencies did not differ between age groups during the A.M. testing or when given the tryptophan‐free diet (T‐). Vocalization rate (calls/TI duration), however, was significantly greater during the A.M. testing. These results emphasize the importance of considering circadian rhythms and behavioral development. Furthermore, an integrative view of the proximate mechanisms and adaptive functions of TI and high‐intensity vocalizations is presented.©1994 John Wiley & Sons, Inc.