Thermorespiratory responses of shorn and unshorn sheep to mild heat stress

Three 3-year-old ewes with surgically exterized carotid loops were twice exposed to ambient temperatures (Ta) of 25, 30, 35 and 40 C (at constant 40% relative humidity) tor 120 min in the presence (6.6 cm) and absence (less than 0.3 cm) of fleece Thermoregulatory responses were evaluated during the last 30 min of each exposure by measurements of rectal (Tre), 6 skin surface temperatures (Ts), respiratory frequency (f), oxygen consumption ( o 2 and respiratory evaporative heat (E r): arterial blood samples were analyzed for pH, P co 2 and HCO 3 concentration. Both shorn and unshorn sheep exhibited a progressive increase in f, E r and Ts as Ta was elevated, with the unshorn group show ing a higher E r than the shorn sheep at each Ta without a significant change in heat production. E r was found to be the principal avenue of heat loss. accounting for as much as 59% the total in the shorn sheep compared to 65% for the unshorn sheep. Increases in E r were accompanied by a decline in the arterial P co 2 which was linearly related to the Ta in both shorn and unshorn sheep. These data suggest that in defending against hyperthermia, sheep appear unable to increase E r without a concurrent elevation in V a at nearly all stages of acute heat stress.