Total Evaporative Water Loss in Birds at Different Ambient Temperatures: Allometric and Stoichiometric Approaches

Valery M. Gavrilov (2017) Total evaporative water loss (TEWL) in Passeriformes and Non-Passeriformes was estimated by simultaneous measurements of energy expenditure and mass loss in resting birds. It was found that the percentage of heat dissipated by water evaporation depends on body size. Published data for 102 bird species were analyzed together with my own measurements for 157 bird species at thermally neutral temperatures (mostly 25°C) to establish the following relationship between TEWL and body mass: TEWL25°C Aves = 0.28 m^0.701, R ² = 0.92, where TEWL is in g H₂O/day and m is body mass (g). The scaling exponent 0.701 ± 0.007 is 0.05 greater than for the relationship of basal metabolic rate (BMR) to body mass. It was found that TEWL in passerines is higher than in non-passerines at all ambient temperatures by 50% at 25°C, 30% at 0°C, 39% at the lower critical temperature, and 59% at the upper critical temperature. The dependence of water loss on body mass at different ambient temperatures (T _A) was found to vary in the same manner as evaporative heat loss. TEWL in Passeriformes is approximately 25-60% higher than in Non-Passeriformes (particularly at high T _A), which is consistent with the ratio of their BMR levels. Within the thermoneutral zone, the proportion of heat dissipated by evaporation increases by approximately 2.6-fold in small passerines and by almost 4.1-fold in large passerines with the transition from the lower to upper critical temperature. In non-passerines, the proportion of evaporative heat losses increases by approximately 2.7 times within the thermoneutral zone in both large and small birds. The high basal metabolic rate in Passeriformes involves benefits like a higher maximum metabolic power and the ability to breed at lower ambient temperatures, but it comes with a cost: a significant expenditure of evaporative water. This cost is important because it is found to increase with body size in Passeriformes due to the forced evaporative heat loss, but it shows virtually no increase with body size in Non-Passeriformes. Thus, despite a high BMR significantly increasing ecological opportunities, this way of expanding the ecological niches is possible for the small size class only. These findings suggest that the high level of basal metabolic rate in Passeriformes in comparison to Non-Passeriformes determines the necessity for the former to utilize considerably larger amounts of water for evaporation to maintain the needed heat balance, especially at higher ambient temperatures and at larger body sizes.