The Effect of Extended Exposure to a Hot Environment on the Response of the Chicken to Hyperthermia

Adult female White Leghorns lived for 12 weeks at either 32–35 °C and 50–60% R.H. (adapted) or 22–24 °C, R.H. uncontrolled (controls). Hyperthermia was induced by exposing the restricted animal to 40.5 °C and 60% R.H. Body temperature (TB), respiratory rate (RR), minute volume [Formula: see text], tidal volume (VT), heart rate (HR), and systolic, diastolic, and pulse pressure (SBP, DBP, and PP) were monitored at [Formula: see text] changes in TB. Adapted birds resisted hyperthermia better than controls, but the time–TBrelationship was biphasic. Between TBof 42.0 and 43.5 °C regression coefficients relating time to TBwere not different (adapted, 20.4 ± 0.6 and controls, 22.1 ± 0.8 minute/°C) but they did diverge significantly between TBof 44.0 and 45.5 °C (35.9 ± 0.8 and 25.6 ± 0.8 minute/°C). Body weight (BW), RR, [Formula: see text], HR, SBP, and DBP were significantly lower in adapted birds before hyperthermia. Covariance analysis indicated that, of these differences, the lower BW handicapped and the lower [Formula: see text], SBP, and DBP aided the adapted birds in resisting heat stress. The pattern of change in respiratory–cardiovascular variables during hyperthermia was similar for both groups. Covariance analysis of such quantitative differences as did exist indicated that higher values for RR and V either had no effect on or handicapped the adapted birds in their resistance to heat stress. The results suggest that adaptation depends on mechanisms other than increased respiratory elimination of heat.