Corap. Biochem. PhysioL, 1967, VoL 21, pp. 405 to 414. Pergamon Press Ltd. Printed in Great Britain THERMAL CONDUCTANCE IN BIRDS AND MAMMALS CLYDE F. HERREID II and BRINA KESSEL Department of Zoology, Duke University, Durham, North Carolina, U.S.A., and Department of Biological Sciences, University of Alaska, College, Alaska, U.S.A. (Received 14 December 1966) Abstract--1. Thermal conductance (Cmeasured in cal.g -1 hr -1 °C-0 in birds, mammals and lizards is exponentially related to body weight (W measured in g). The equation developed for thirty-one species of birds is log C = 0-662-0"536 log W while the relationship for twenty-four species of mammals is log C = 0"691 -0"505 log W. There is no significant difference between these curves. 2. Conductance of varanid lizards is ten times higher than for intact birds aad mammals and four times higher than defeathered birds. The high con- ductance of lizards is partly because measurements were made in an air stream up to 300 cm/sec and because of their large skin surface. 3. Heat loss in defeathered birds is two to three times higher than in intact birds. The greatest increase in conductance occurred in the larger birds, sug- gesting the greater insulation of the large birds partly resides in the feathers. INTRODUCTION HEAT lOSS from an inactive homeotherm below thermoneutrality is often closely described by the Newtonian model. Thus, heat loss (Qr~) is determined by the temperature difference between the animal (TB) and its environment (TA) and by a constant (C) for the particular animal: QL = C(TB- Ta). Conductance (C), measured in cal. g-1 hr-1 oc-1 ' is dictated by the body's shape, size and surface characteristics. The reciprocal of conductance, insulation, repre- sents the body's resistance to heat loss (Scholander et al., 1950). If C is at its minimum and constant for all T A below thermoneutrality, the slope of the regression line relating metabolism (M) with T~ may be taken as C. The assumptions under- lying this method have been discussed (e.g. King, 1964; Tucker, 1965). When C is not constant, minimum conductance must be determined from measurements of M, T B and T a. Insulation or conductance has been shown to be related to body weight in mammals (Scholander et al., 1950) and birds (Lasiewski, 1963). Yet a formal 405