THERMAL CONTROL OF METABOLIC COLD DEFENSE IN PIGEONS COLUMBA-LIVIA

Metabolic rate (MR) and various body temperatures were recorded in pig eons Columba livia during gradual lowering of the ambient temperature (T-a) and during cold exposure (-10 degrees C) for an entire circadian cycle. The aim was to study how changes in MR accorded with the obser ved body temperature displacements and our knowledge of regional cold sensitivity. When T-a was lowered from 28 to -10 degrees C, the threef old increase in MR was accompanied by a significant increase in deep-b ody and spinal cord temperatures. Cold exposure also resulted in a dis tinct rise in breast skin temperature, whereas the skin temperatures o f the neck, back and leg decreased significantly. Thus, during acute c old exposure of pigeons, peripheral thermosensors seem to have the pri mary effect in mediating metabolic cold defence, while internal thermo sensors are apparently of minor importance. By using differential weig htings of various skin areas, cold-sensitivity values of between -3.9 and -14.7 W kg(-1) degrees C-1 were estimated. These values indicate a much higher skin thermosensitivity of birds than has been previously suggested. When cold-exposed for an entire circadian cycle, the pigeon s usually maintained a low leg skin temperature. However, during these experiments, the pigeons showed regular distinct increases in leg ski n temperature, obviously due to cold-induced vasodilatations (CIVDs). The flushing of cold peripheral blood through the body in connection w ith these CIVDs resulted in a decrease in deep-body temperature (T-b), to which the pigeons responded with a distinct increase in MR. From t he concomitant changes in MR and T-b, a total body cold sensitivity of -8.0 W kg(-1) degrees C-1 was calculated, a value that is close to th e previously published cold-sensitivity value of pigeons. This shows t hat, at least under some naturally occurring conditions, internal ther mosensors will provide a significant error signal in the control of me tabolic cold defence. Furthermore, the results of our study show that the relative importance of various thermosensitive sites in the contro l of metabolic cold defence is contextual.