EFFECTS OF THERMAL ACCLIMATION ON THE HEAT BALANCE OF TERRESTRIAL LOCOMOTION IN THE WHITE-CROWNED SPARROW ZONOTRICHIA LEUCOPHRYS GAMBELII (THERMOREGULATION, ENERGY BUDGET)

White-crowned sparrows acclimated to warm (25 C) or cold (5 C) were trained to locomote on a circular treadmill. Metabolic rate (MR), intraperitoneal temperature (Tb), and latent heat loss (LE) were measured in resting and locomoting birds at three treadmill speeds (0.25, 0.50, and 0.75 km/h), six air temperatures (Ta; 35, 25, 12, 0, -10, and -17 C), and four durations of locomotion (3, 6, 10, and 15 min). MR, Tb, LE, and total thermal resistance at rest all showed moderate effects of thermal acclimation. Acclimation to warm temperature favored water economy but entailed increased thermostatic cost at cold temperatures. Conversely, acclimation to cold temperature favored energy economy in the cold, but involved relatively greater water loss at warm temperatures. In locomoting birds, MR in relation to treadmill speed was strongly affected by acclimation state at intermediate Ta's (12 and 0 C), which may result mainly from acclimation of thermal resistance. Thermal acclimation also had a significant effect on LE at Ta = 35 C. Warm-acclimated birds when locomoting loss 82-84% (at 0.25 - 0.50 km/h) or 92% (0.75 km/h) as much water by evaporation as cold-acclimated birds at this Ta.;The difference (('(DELTA))Y) between MR at rest and the Y-intercept of the regression of MR on treadmill speed decreased as Ta increased, and was nearly zero when Ta = ('(DELTA))Y. This implied that ('(DELTA))Y is mainly an added thermoregulatory cost in locomoting birds. A heat-balance equation for locomoting birds was devised, and used to examine the hypothesis that no thermal substitution occurred in locomoting birds. Data from this study disproved this hypothesis and showed that, at low Ta's, the thermostatic energy requirement of locomoting birds acclimated both to warm and cold temperature was supplied to a great extent by the heat produced by muscular activity. At Ta = 12 and 0 C, cold-acclimated birds used more heat of locomotion for thermoregulation than warm-acclimated ones did.