Delay in the Onset of Thermal Tachypnoea in Shorn Sheep Exposed to 42°C in Winter and Summer

Abstract

Shorn sheep were routinely exposed to ambient temperatures (Ta) of 14 and 42°C in a climatic chamber in winter and in summer. Rectal temperature (Tre) and skin temperature (Tsk) were measured at 14°C and the time elapsing between raising the Ta to 42°C and the onset of thermal tachypnoea was recorded. It was found that the onset of thermal tachypnoea in the heat was dependent upon the Ta to which the sheep had been previously exposed in the animal house and, in summer, the degree of fleece regrowth. There was an inverse linear relationship between the mean Tsk recorded immediately prior to raising the Ta and the delay in onset of thermal tachypnoea both in winter and summer. Thus, for either season, the inhibition of panting could be related to the degree of cold stimulation received from the periphery. However, a comparison of winter and summer data indicated that other factors were also involved. Tsk was significantly higher, and Tre significantly lower, in winter than in summer. These observations point towards possible seasonal changes in basal metabolic rate and thermoregulatory set-points. The results are discussed with regard to current theories concerning the mechanisms leading to the inhibition of thermal tachypnoea in shorn sheep.