Scaling of total evaporative water loss and evaporative heat loss in birds at different ambient temperatures and seasons

Endothermic animals exchange heat with the environment via four modes: conduction, convection, radiation and evaporation). Evaporation is of significant ecological interest. Evaporation from an animal always results in a decrease in the temperature of the surface from which the evaporation occurs. Therefore, evaporation is oneway transfer which causes heat loss from the organism. Biological evaporation always implies loss of water which is a vital resource for nearly all biochemical processes. Evaporation is the loss of heat via the loss of body mass.1 First guidance for the direct weighing method to determine evaporative water loss was obtained from the studies of Hutchinson and Sykes2 and Hutchinson3 on evaporation in domestic fowl. Robert С Lasiewski, Alfredo L Acosta and Marvin H Bernstein using new techniques and equipment, invented new methods different from the original method. A major disadvantage of the direct weighing method is that it does not permit the simultaneous determination of the energetic cost of evaporative cooling by monitoring oxygen consumption or carbon dioxide production, as is possible in the open flow method. As previous studies have shown,4–7 it is possible to determine evaporative heat loss from the ratio (q) between heat production (RM) (determined by the rate of oxygen consumption) and body mass loss (Dm) at various ambient temperatures (TA): q=RM/Dm. High water loss necessitates regular water ingestion through drinking, consuming water in food, and metabolic water production. The method is based on the law of conservation of mass where the total mass of the reactants equals the total mass of the products leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers (stoichiometry). The method uses the metabolic rate (oxygen consumption) of the bird at thermo neutral ambient temperatures (TA), respiratory quotient (RQ), the loss of body mass and the energetic equivalent of the loss of body mass (q) as the ratio between heat production and the loss of body mass. It is known that the loss of mass in bird exposed to controlled conditions is due to three major factors:8–10