Re-evaluation of the allometry of wet thermal conductance for birds

Wet thermal conductance is an important thermoregulatory parameter for birds and mammals. It is generally calculated as C-wet (ml O-2 g(-1) h(-1) degreesC(-1))= V-O2/(T-b - T-a), where V-O2 is metabolic rate measured in ml O-2 g(-1) h(-1) T, is body and T, is ambient temperature measured in degreesC. Minimum C,,, is measured at T, at or below the lower critical temperature (T,,) of the thermoneutral zone, and is strongly influenced by time of day (rest or activity phase) and body mass [J. Aschoff, Comp. Biochem. Physiol. 69A (1981) 611]. Allometric analyses indicate differences in C-wet for passerine and non-passerine birds, in their rest and active phases (Aschoff, 1981). The allometric slope for non-passerine rest-phase (- 0.553) is lower than that for non-passerine active-phase (- 0.484), and passerine rest-phase (-0.461) and active-phase (-0.463), although none of these slopes are significantly different. This different-sloped relationship fur non-passerine rest-phase C-wet extrapolates to lower-than-expected values at high body mass, and so this allometric relationship may be inappropriate for predictive purposes. Consequently, we have reanalysed Aschoff's (1981) data, as well as more recent compilations, to determine a more useful allometric relationship for C-wet of non-passerine rest-phase birds. Re-analyses of minimum thermal conductance data from Drent and Stonehouse [Comp. Biochem. Physiol. 40A (1971) 689], Aschoff(1981) and Gavrilov and Dolnik [Acta XVIII Congressus Internationalis Ornithologici Moscow (1982) 421] indicate that the most appropriate regressions for predicting C-wet (ml O-2 g(-1) h(-1) degreesC(-1)) of birds from body mass (M; g) are the pooled regressions for non-passerine and passerine birds, in the active (alpha) and resting (rho) phases, using data tabulated by Aschoff (1981): alpha, C-wet = 0.994M(-0.509); rho, C-wet = 0.702M(-0.519). C-wet is approximately 40% higher in the active phase than the rest phase. Regressions of various data sets for C,,, of birds and mammals indicate a similar slope of approximately -0.5 for the allometric relationship, but significantly higher elevations for mammals compared to birds. The approximately 50% higher C-wet for mammals than birds indicates a better physical insulation for birds than mammals of the same body mass. The general scaling of C-wet with M-05 indicates that (T-b - T-1c) should scale with M-0.22, if mass-specific metabolic rate scales with M-0.28 [Reynolds and Lee, Am. Nat. 147 (1996) 735]. The observed scaling for (T-b - T-1c) of M-0.183 (calculated from Gavrilov and Dolnik, 1985) is consistent with this expectation. (C) 2001 Elsevier Science Inc. All rights reserved.