Control of shivering and heart rate in incubating bantam hens upon sudden exposure to cold eggs

To distinguish shivering released by cooling of the brood patch from shivering released by low core temperature, incubating bantam hens were exposed to water perfused eggs. Responses to a period of egg cooling were compared to the recovery period after egg temperature had been returned to 40 degrees C, but cloacal temperature (Tb) still was low. At an ambient temperature (Ta) of 23 degrees C and exposure of the hens to between two and eight eggs cooled to 10-35 degrees C (series 1), electromyographic (EMG) activity of musculus iliotibialis increased rapidly with an occasional overshoot, and was higher during egg cooling than during recovery. This hysteresis in EMG activity and Tb was weakly correlated to egg temperature and clutch size. Heart rate (HR) showed an almost parallel increased to shivering except that a maximum HR was reached at high shivering intensities. These responses were also present at a Ta of 37 degrees C when the hens were slightly hyperthermic before exposure to eight eggs at 20 degrees C (Series 2). At the highest starting Tbs EMG activity increased linearly after a drop in Tb. Shivering in m. pectoralis showed a lower threshold Tb and lower activity than m. iliotibialis during egg cooling, and immediately ceased at the end of egg cooling. Total body thermosensitivity estimated from the recovery periods at low and high Ta was -9.7 and -6.4 W kg-1 degrees C-1, respectively. It is concluded that shivering in incubating birds warming cold eggs probably is stimulated both by peripheral and central thermoreceptors. The peripheral component shows phasic properties typical for skin receptors.