In the preceding chapter my main emphasis was on how difficult it is to lose heat and water from the respiratory tract, especially for small animals. Nevertheless, many animals, both mammals and birds, resort to evaporation from the respiratory tract to dissipate excess body heat. This is the phenomenon we know so well as panting in dogs, and for that matter in other domestic animals as well, such as sheep and cattle. Birds also respond to a heat load with a greatly increased respiratory activity, although there are differences from the mammalian type of panting to which I shall return later.

On cooling hot dogs

The panting of dogs is often described as a rapid in-and-out breathing through the open mouth with evaporation taking place from the moist oral surfaces and the large hanging tongue.* Air flowing close to the moist surface will rapidly become saturated with water vapor, but much of the air will not be in the immediate vicinity of the moist surface and it is easy to imagine that the entire volume of air does not readily become fully saturated. In that event correspondingly larger volumes of air would have to be moved, thus increasing the muscular effort of breathing, and the heat load due to the increased energy expenditure.