Thermal conductance in mammals and birds: Its dependence on body size and crcadian phase

It is demonstrated that neglecting the differences between α- and ρ-values in conductance can lead to a misinterpretation of the relationship between conductance and body size.

Abstract

1. Thermal conductance, expressed in units of weight-specific oxygen uptake (ml O2/g hr C) is often measured as minimal conductance, i.e. at ambient temperatures below the zone of metabolic thermoneutrality. A re-analysis of data published in the literature demonstrates that the minimal conductance of mammals and birds is about 50 % greater when measured during the activity time α of the animal than during the rest time ρ (Figs 1 and 3). 2. Minimal conductance decreases with increasing body weight in a and in ρ; the regression lines which describe this correlation have a slightly steeper slope in mammals than in birds. 3. On the average, mammals have a 35% higher conductance than birds. 4. It is demonstrated that neglecting the differences between α- and ρ-values in conductance can lead to a misinterpretation of the relationship between conductance and body size (Figs 1 and 2).