Physiological acclimatization of fowls to a hot humid environment

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Abstract
1. The resting heart rate of cocks and hens was measured in varying degrees of heat stress, and during acclimatization to heat.2. When the rectal temperature was below 110°F. (43·3° C), the heart rate varied inversely with the severity of the climate up to an air temperature around 99° F. (37·2° C).3. At air temperatures of 99° F. (37·2° C.) and above the heart rate was nearly constant over a wide range of rectal temperatures below 110° F. (43·3° C). It averaged about 50 beats/min. less than in an equable climate.4. Above 110° F. (43·3° C.) there was an increase of about 30 beats/min./° F. (17 beats/min./° C.) rise in rectal temperature. The rate equalled that found in an equable climate, when the rectal temperature reached 111–112° F. (43·9–44·4° C). Over this zone of body temperature the fowls were often agitated.5. In severe heat stress there was a slight fall in the heart rate on acclimatization, but this was small compared with the effect of climate.6. In severe heat stress the heart rate during the day was slightly higher than that at night.7. The relation of these findings to the mechanism of temperature regulation is discussed.