An investigation of some aspects of the transition from ectothermic to endothermic metabolism in vertebrates
The standard metabolic rates (SMRs) of 19 lizard and a single crocodilian species were measured at 30 "C, and 14 lizards at 37 "C. The basal metabolic rates (BMRs) of 8 mammal ian species were also determined using similar techniques. Although the inter-specific exponent relating BMR to body mass in rodents was close to the widely used value of 0.75, intra-specific exponents and that of the genus Sorex were lower. Intra-generic exponents of Lacerta and CordyZus are significantly higher than 0.75, and a general exponent of 0.85 is proposed for inter-specific comparisons of small lizards. Standard metabolic levels (SMLs) calculated using this exponent ranged from 0.096 to 0.230,at 30°C, demonstrating that lizards are not a metabolically homogeneous group. Mammalian BMLs varied from 2.108 to 11.096, and therefore the metabolic differences between this class and the reptiles cannot be described by a single factor. The in vitro oxygen consumption rates were measured of liver, kidney, cardiac and skeletal muscle from lizards and mammals coveri ng a range of body masses. Brain, lung and visceral smooth muscles were also compared in Mus muscuZus and CordyZus jonesi. All mammalian organs possessed higher metabolic rates than their reptilian equivalents. However, these differences, which varied considerably between tissues, were less than those of living animals. The reasons for the higher cellular and organismal metabolic rates of mammals are discussed. Lacerta liZfordi and CordyZus jonesi acclimated to 20°C displayed the same preferred body temperatures (PBTs) as lizards maintained at 30°C, despite experiencing partial compensation of their SMRs. Lizards allowed to behaviourally thermoregulate during their photophase possessed similar SMRs to those acclimated isothermally to the same background temperatures. The PBTs of 4 European and 8 African lizard species were determined in a thermal gradient . Possible adaptative differences in saurian PBTs and SMLs are discussed in relation to their thermal environment.