Studies on the adaptability of three breeds of sheep to a tropical environment modified by altitude III. The response of mature and young rams to a thermal burden induced by exercise

Abstract

Mature and young Merino, Persian and Native rams were compelled to run 1 mile in 10–12 min. The response to the thermal burden induced by this exercise was measured in terms of body and skin temperatures, rate of respiration and length and temperature of the scrotum.1. Marked increases were recorded in every body response immediately after the exercise. Body, skin and scrotal temperatures returned to normal within 2 hr. but respiratory rate remained elevated for a further 4 hr.2. There were breed differences in the initial and subsequent measurement of every response. At 7.0 a.m. when there was no thermal stress body temperatures and rates of respiration were: Merino 102·9° F.; 3·-6 cyc./min. Persian 101·4° F.; 23·8 cyc./min. Native 102·1° F.; 28·9 cyc./min. Merino rams had the greatest ability to combat rising ambient temperature, Native rams the least. Persian rams recuperated quickest after exercise, Native rams slowest. The different responses of wool and hair breeds were apparently due to their coats. Merino fleece stabilized body temperature, did not aggravate the effects of exercise unduly and did not greatly hamper elimination of heat from the body after exercise.3. In all breeds respiratory evaporation was the principal thermolytic process. Rate of respiration was a sensitive indicator of thermal stress but it could not be used as an index of heat tolerance because the associated changes in respiratory volume were not known.4. Age was seldom of significant importance to determine response to thermal burden. In general young rams were affected less adversely by exercise and more adversely by rising ambient temperature than were mature rams. These differential effects were probably due to leggy conformation, low live weight and physical fitness in young rams on one hand, and incomplete development of the thermoregulatory system on the other.5. Increase in scrotal pendulance was related inversely to ability to maintain normal body temperature.6. In view of the influence of age and body conformation on the response to exercise it was questioned whether comparable heat tolerance indices could be obtained by this method.