A Phylogenetic Perspective of Evaporative Water Loss in Birds

ABSTR•C?.--Allometry, the study of the consequences of body size on form and function, has been a powerful investigative tool in avian biology. Comparison of phenotypic data with allometric reference equations permits the identification of possible adaptations and the formulation of hypotheses for testing. The standard allometric equation that relates total evaporative water loss (TEWL) to body mass in birds, published more than two decades ago, was based on a relatively small sample size, and was constructed using procedures which may have biased parameter estimation. In this report, I have analyzed data for TEWL for 102 species of birds ranging in size from hummingbirds to Ostriches (Struthio camelus) using both least-squares regression and phylogenetically independent contrasts. Both approaches uggest that: (1) the slope of the relationship between TEWL and body mass is higher than the value originally proposed; (2) birds from arid environments have a statistically lower TEWL than do birds from more mesic environments; and (3) small birds have similar ratios of TEWL to oxygen consumed compared to larger species. The latter finding negates the idea that small desert birds replenish proportionately less of their TEWL with metabolic water than do larger species. Received 19 June 1995, accepted 22 September 1995.