THERMAL ADJUSTMENTS IN CARDIAC AND SKELETAL MUSCLES OF LIZARDS

1. Isometric twitch tension development was measured over a wide range of temperatures in skeletal and cardiac (ventricular) muscles from diverse Australian lizards representing several families and including both diurnal and nocturnal species. 2. The temperatures at which maximal twitch tension develops (optimal temperatures) in the skeletal muscles is higher, by about 10-15° C, than in the ventricular muscles from the same species. Resistance to heat damage (thermostability) is similar for the two tissues. 3. Interspecific divergence is evident in the optimal temperatures and thermostabilities of both muscles, and these tissue differences parallel the preferred body temperatures of the species from which they are taken. In several nocturnal lizards (geckos) that have high preferred body temperatures, the thermal adjustments of the muscles resemble those of diurnal species, whereas muscles from geckos with low preferenda have relatively low optimal temperatures. 4. Variations in the heat resistance of the tissues correlate with variations in organismal heat resistance and in general, lethal temperatures for excised muscles and organism are similar. 5. These data are interpreted as indicating that divergence in the thermal adjustments for contractility of muscular tissues are most directly associated with the temperatures characteristically maintained through the behavior of the species rather than to the general geographical (climatic) distribution or phylogenetic position of the species. © 1969 Springer-Verlag.