Acclimatization of Thermoregulation in the Desert Cottontail, Sylvilagus audubonii

Abstract

Seasonal physiological responses to ambient temperature were determined in Sylvilagus audubonii at Tucson, Arizona. From winter to summer the thermoneutral zone of desert cottontails (700 grams) shifted upwards, basal metabolism decreased by 18 per cent, and standard metabolism decreased above thermoneutrality and increased below. Water loss during summer was higher than in winter below 30° C, began to increase at a higher ambient temperature, and rose less steeply. The per cent of metabolic heat production dissipated by evaporative heat loss did not change seasonally, and equalled 100 at an ambient temperature of 41.4° C. Body temperature was 38.3° C at ambient temperatures below 30° C, and did not differ seasonally. Hyperthermia began within thermoneutrality at a slightly higher ambient temperature during summer resulting in lower hyperthermic body temperatures. Body temperature equalled ambient at 41.9° C during summer and a predicted 42.9° C during winter. The dry heat transfer coefficient was higher during summer below 30° C but there was no apparent seasonal difference at higher ambient temperatures. Sylvilagus audubonii survives in the desert by taking advantage of every possibility to minimize the heat load and water expenditure. A relatively high evaporative cooling capacity and high lethal body temperature of 44.8° C provides a safety factor for desert cottontails if avoidance is not possible.