Diurnal changes of thermoregulatory functions in pigeons. I. Effector mechanisms.

In pigeons, we studied the diurnal variation of body temperatures and its dependence on the diurnal variations of shivering, vasomotor activity and panting in light-dark (LSD) - conditions (12:12 h, 100:1 Lux) and in different, but for at least 24 h constant ambient temperatures. In low ambient temperatures (below thermoneutrality) shivering was reduced shortly after lights off (early dark phase), and it increased again some hours before lights on (late dark phase); the lower the ambient temperature, the larger the reduction of shivering and the shorter the time of reduction. Within thermoneutrality, the decrease of Tb in the early dark phase was associated with an increase of foot skin temperature, probably due to vasodilatation. Later-on foot skin temperature decreased again (vasoconstriction). The feet remained constricted even in the late dark phase, when Tb already increased. At higher ambient temperature (above thermoneutrality) vasodilatation persisted all over the 24 h, while in these conditions respiratory rate increased significantly after lights off. The described diurnal variations of thermoregulatory effector mechanisms are in agreement with the concept of an active adjustment of the body temperature rhythm.