In a hot and dry environment birds must master the two major physiological problems of heat dissipation and water conservation. These problems are related, as water is used when animals are forced to resort to evaporative cooling. Successful adaptation to the desert therefore not only involves water conservation by the kidney and other organs, but also the control of evaporative cooling (see p. 51). This involves setting of critical temperature, level of heat loss by radiation, convection and conduction, and adjustment of motor activity. Some species of mammals increase body temperature during day-time, whereby heat is stored and evaporative cooling is reduced or avoided. The excess heat is then released largely as radiation during the night. By a hypothalamic mechanism panting is suppressed in such animals when they are dehydrated. Respiratory water loss may also be a regulated parameter through an adjustment of H2O lost per unit uptake of O2.This effect may be caused either by more efficient respiration, or by a counter-current exchange in the airways that lowers the temperature of the expired air.