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Physiological Thermoregulation in Heat-Stressed Birds: Consequences of Body Size

1981; University of Chicago Press; Volume: 54; Issue: 3 Linguagem: Inglês

10.1086/physzool.54.3.30159949

1937-4267

Wesley W. Weathers,

Avian ecology and behavior

Metabolic heat production (Ḣm), evaporative heat loss (Ḣe), and body temperature (Tb) were measured simultaneously in six species ranging in size from 12 to 125 g. These data, together with those of previous studies, indicate that Tb is not maintained constant at ambient temperatures (Ta's) below the upper critical temperature (Tue), but increases linearly with Ta above about 30 C. Consequently, Tb increases within the thermoneutral zone while Ḣm remains unchanged. Plotting Ḣm within the thermoneutral zone as a function of Tb indicates that metabolism is independent of Tb (i.e., Q10 ≃ 1.0) in birds smaller than about 100 g. Calculations of rates of heat exchange show that hyperthermia contributes little to the cost of avian temperature regulation in hot environments, while it significantly enhances water conservation. In all species Ḣm increased linearly with increasing Ta above Tue. The slope of the line relating Ḣm to Ta above Tue is an integrated measure of the cost of thermoregulation in hot environments, here designated as the coefficient of heat strain (hs). In 26 species of birds, hs (mW gl⁻¹C⁻¹) varies with size according to the relation log hs = log 12.5 − 0.65 log m, where m is body mass in grams. At Tue the size of the Tb − Ta gradient was inversely related to body size, decreasing from 5.0 C in the 12-g blackthroated sparrow (Amphispiza bilineata) to 1.2 C in the 125-g Gambel's quail (Lophortyx gambelii). Consequently, small birds depend less on evaporative cooling at Tue than large birds, a circumstance favoring water economy in small birds. All species were able to increase Ḣe during heat exposure without increasing Ḣm. The extent to which Ḣe increased without a concomitant increase in Ḣm was, however, inversely related to body size. This may reflect a greater capacity to alter the plumage vapor gradient in larger birds.