COLD-INDUCED CALLING AND SHIVERING IN YOUNG AMERICAN WHITE PELICANS - HONEST SIGNALING OF OFFSPRING NEED FOR WARMTH IN A FUNCTIONALLY INTEGRATED THERMOREGULATORY SYSTEM

Vocal regulation of body temperature in thermally dependent offspring can occur when cold-induced vocalizations elicit appropriate brooding and rewarming by a parent or surrogate. I tested the ability of young American white pelicans to regulate vocally body temperature in the laboratory by exposing them to moderate chilling (20-23 degrees C), either continuously, or punctuated by brief (usually 2 min) periods of warmth in response to cold-induced calling. When continuously chilled, they began to call almost immediately (median latency 35 s), and body temperature began to drop. At 1 and 2 weeks of age, visible shivering also occurred, but was not sufficient to arrest the decline in body temperature. When chilling was then ameliorated by brief periods of warmth in response to cold-induced calling, body temperature shifted upwards, indicating a vocal thermoregulatory ability similar to, but somewhat faster, than that previously reported for late-stage embryos in this species. At 1 day of age, cold-induced calling occurred with a shorter latency and at a higher rate at lower body temperatures, as expected in a compensatory thermoregulatory system. After the onset of shivering thermogenesis at 1 and 2 weeks of age, the compensatory response decreased, but calling still occurred in response to chilling, as required by a simple on-off regulatory system. Begging for food was inhibited at body temperatures low enough to elicit shivering, suggesting that vocal maintenance of a relatively high body temperature may be of importance to normal begging and food acquisition prior to the full development of endothermy in this species. Effective functioning of the vocal thermoregulatory system requires that information about offspring body temperature be accurate. This requirement suggests a strong parallel between vocal thermoregulation and the concept of honest signalling in animal communication. Honest signalling of need by offspring has received recent theoretical support, but is controversial and has rarely been tested experimentally. I used the occurrence of shivering, which is the main source of endothermic heat in birds, as a standard against which to test empirically the honesty of cold-induced calls as signals of need for warmth. When 1 and 2-week old chilled, shivering chicks began to warm up during vocal regulation sessions, shivering eventually became periodic, starting each time a bird was chilled and ending during each subsequent period of rewarming. Calling at this time invariably (84 of 84 cases) began only after the onset of shivering, then ended while shivering was still in progress. To the extent that shivering is an honest, perhaps unfakable indicator of a biologically relevant need for warmth, the vocalizations associated with it appear to be honest signals indicating the presence of that need. Results are also consistent with a recent theoretical model of honest signalling of need by offspring, but it remains to be determined whether honest solicitation of warmth prevails when chicks are in naturally competitive situations at the nest.