The role of hyperthermia in the water economy of desert birds

A number of authors have suggested that hyperthermia, the elevation of body temperature (T-b) 2 degrees-4 degrees C above normal, contributes to a red uction in total evaporative water loss (TEWL) in birds. Information about t he role of hyperthermia in the water economy of birds is scattered througho ut the literature. We purposed to collate the available information on this subject, to reevaluate the benefits and costs of this process, and to asse ss its net effect on the water economy of birds, especially species living in deserts. In this review, we first consider the current model of heat bal ance in birds at high ambient temperatures (T-a), and show that, in most st udies performed at a high T-a, birds were increasing their T-b, a violation of the assumption of steady state conditions. Next, we incorporate the rat e of heat gain into calculations of the dry heat transfer coefficient (h), below and above temperature equality (T-a = T-b). We develop a method to ca lculate h at T-a = T-b, using l'Hopital's rule. The combined result of our approach suggests that birds increase their dry heat transfer even when T-a is above T-a = T-b, contrary to our prediction. Finally, we explore aspect s of hyperthermia that reduce water loss, such as an improved thermal gradi ent and heat storage, and aspects that may augment water loss, the latter a result of increased respiratory water loss when T-b is elevated. Our analy sis of the combination of these three factors suggests that, during acute e xposure to high T-a (1 h), birds over a size range of 10-1,000 g save about 50% of their TEWL by becoming hyperthermic. For chronic episodes of high T -a (5 h), small birds save water by hyperthermia but large birds do not.