Oxygen transport by the circulatory system of the green iguana (Iguana iguana) at different body temperatures.

Since the early investigations of Cowles & Bogert (1944), it has become established that many lizards when active under natural conditions have mean body temperatures that are several degrees higher than air temperature in the shade and are relatively constant (Brattstrom, 1965). This difference between the mean (or eccritic) body temperature and air temperature is maintained primarily by absorption of heat from the environment since Lizards cannot heat themselves more than a few degrees by metabolic heat production alone (Bartholomew & Tucker, 1963, 1964). Both behavioural and physiological mechanisms influence the absorption of environmental heat. When they are below their eccritic temperature, lizards tend to move into the sun or flatten on warm substrates (Bogert, 1949; Norris, 1953; Pearson, 1954; Saint Girons & Saint-Girons, 1956). Changes in skin colour can alter the rate at which solar radiation is absorbed (Atsatt, 1939; Norris, 1958), and heat transport from the skin to the core of the body can be altered by changes in the circulatory system (Bartholomew & Tucker, 1963, 1964). However, lizards are not always able to attain their eccritic temperatures during cool seasons, at night or during periods of overcast skies and confinement in burrows. It is interesting to investigate how an animal that is found active and abroad at only a narrow range of body temperatures maintains physiological co-ordination over the wide range of body temperatures it may experience. The relation of the cardiovascular system to temperature is particularly interesting, for this system transports all of the oxygen required to meet the energy demands of lizards. Oxygen transport by the cardiovascular system is described by the equation