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Newly metamorphosed Western toads, Bufo boreas, are diurnally active and thermoregulate by the behavioral exploitation of available microenvironments including exposure to solar radiation. In the laboratory, toads thermoregulate by basking beneath incandescent lamps when placed within a photothermal gradient. Basking depends on feeding, and when food is withheld, individuals abandon the available heat source and assume lower body temperatures. The preferred temperature of feeding individuals is around 26—27 degrees C as determined from animals both in the laboratory and in nature; fasting animals remain between 15—20 degrees C. Growth of young toads with ad libitum food supply was measured at a variety of temperatures. Energy ingestion (appetite), linear growth, weight increase, and gross conversion efficiencies were all maximal at 27 degrees C and were nearly identical to that of toads allowed to thermoregulate in a photothermal gradient. Weight—specific energy ingestion, weight—specific growth, and gross conversion efficiency decreased with age. Survivorship of starved toads in a thermal gradient was prolonged over that of toads maintained at a constant 27 degrees C. Metabolism increased with body temperature over the range 10—33 degrees C. We have concluded that this terrestrial ectotherm has evolved a behavioral thermoregulatory mechanism which maximizes growth and economic utilization of energy. The diurnal behavior patterns of small toads (compared with the more strictly nocturnal adults) may have evolved to maximize the growth rates of younger individuals, thus shortening their time to adult size.
Lillywhite et al. (Thu,) studied this question.