The role of hypothalamic temperature in the control of panting in the chicken exposed to heat.

1. In unanaesthetized chickens, the temperatures of the hypothalamus, colon and skin have been recorded in relation to the onset and cessation of thermally induced panting.2. During control conditions, hypothalamic temperature showed fluctuations associated with arousal and movement. It was lower than colonic temperature by about 0.9 degrees C but this difference generally decreased during exposure to heat.3. When the birds were exposed abruptly to 40 degrees C, or to ambient temperatures increasing gradually from 20 to 52 degrees C, there was a delay of 15-65 min and marked increases in both peripheral and central body temperatures before panting commenced.4. Infra-red irradiation of the thorax and abdomen caused vasodilatation in the comb and toe before detectable increases in the deep body temperatures. Increasing the colonic temperature by up to 2 degrees C did not cause panting until hypothalamic temperature was also raised. This inhibitory effect of normal hypothalamic temperature was enhanced by low ambient temperature.5. Infra-red irradiation of the head increased hypothalamic temperature by up to 3.5 degrees C and caused vasodilatation in the toe without changes in colonic temperature. Panting, however, did not occur so long as colonic temperature was within the normal range. The inhibitory effect of normal colonic temperature was again enhanced by low ambient temperature.6. In anaesthetized chickens, selective heating of the head and body caused panting only after increases in both the hypothalamic and colonic temperatures.7. Repeated exposure of birds to 40 degrees C did not reduce the time delay before panting started.8. It is concluded that panting in the fowl requires an increase in some extracranial deep body temperature as well as in that of the hypothalamus.