Meeting Abstract
Speakman and Król (2010a,b) have hypothesized that heat dissipation limits are responsible for constraints on activity rates and energy expenditure in endotherms and developed a model for heat loss in mammals and birds. Interestingly, the clearly stated assumptions of the heat dissipation model require that animals are not heat stressed: that air temperatures (Tair) remain below upper critical temperature (Tuc), solar radiation is negligible, and internal surfaces do not participate in heat exchange with the environment as “heat is normally conserved in the process of conserving water” (Speakman and Król, 2010a, p. 730). As diurnal endotherms, desert birds are often exposed to intense solar radiation and high Tair that leads them to experience environmental temperatures above Tuc and normothermic Tb. Although birds typically reduce activity levels and seek shaded microsites during the heat of the day, operative environmental temperatures exceeding Tb result in the evaporation of large quantities of water for heat dissipation. As a consequence, for desert endotherms, during a large part of their annual cycle, the assumption of the HDL model that evaporative water loss is unimportant to heat balance is not valid. Our data on birds indicate that they may dissipate more than two times their metabolic heat loads evaporatively and thus water loss rates may in fact constrain performance and behavior. Therefore, we propose that the HDL model be modified to account for evaporative heat loss in endotherms and in desert birds specifically.
