Introduction

Birds and mammals have independently evolved substantial capacities for metabolic heat production which they use to maintain a high body temperature (approximately 37 °C for eutherian mammals, approximately 40 °C for neognathous birds). These high body temperatures result from their high rates of cellular metabolism (Else & Hulbert, 1987). Birds and mammals can further augment heat production as needed to offset heat loss. Skeletal muscle is the primary site of supplementary thermogenesis in birds (Hohtola, 1982; Duchamp & Barre, 1993). The uniquely mammalian tissue, brown fat, is a major site of thermogenesis (Hayward & Lisson, 1992) in some circumstances, such as during the neonatal period, hibernation and cold-acclimation. Birds and mammals living in extreme thermal habitats have adjustments in metabolic rate and insulation to compensate for the thermal extremes (Marsh & Dawson, 1989). The degree to which these phenotypic differences are genetically fixed or are environmentally determined (examples include Lynch et al., 1976; James, 1983) are not well characterised (Garland & Adolph, 1991). Metabolic rate and insulation show some phenotypic plasticity and may be adjusted during thermal acclimation (Marsh & Dawson, 1989). Birds and mammals normally maintain their body temperature within narrow limits, whether they live in temperate or thermally extreme habitats. Notable exceptions are when species employ torpor to avoid energetically unfavourable conditions and abandon thermoregulation, or regulate at a considerably lower set point.