Selection pressure for high body temperatures: implications for dinosaurs

Abstract

An understanding of the thermal strategies of extant animals is a prerequisite for any useful discussion on the possible thermal strategies of dinosaurs. Of cardinal importance is the identification of the selection pressures that have resulted in the evolution of high body temperatures. Although different selection pressures have probably been operational in different organisms, enhanced muscle power may have been a prime factor.Many dinosaurs, including the ornithomimids, carnosaurs and hadrosaurs possess cursorial features suggesting high activity levels, which in turn suggest the possession of high body temperatures. Even if it is denied that high body temperatures are a prerequisite for speed, it cannot be denied that muscular activity liberates large quantities of heat, and most dinosaurs, by virtue of their small area to volume ratio, must have maintained constant, and sometimes high, body temperatures. This thermal strategy, described as inertial homeothermy, does not require a high (mammalian/avian) rate of metabolism, and most dinosaurs were probably not endothermic.The smallest dinosaurs (e.g. Compsognathus) were probably too small to have been able to maintain constant body temperatures in the absence of external insulation. Perhaps they were feathered, like their close relative Archaeopteryx. Presence of feathers in Archaeopteryx is compelling evidence for endothermy.The largest dinosaurs, the sauropods, had an exceedingly small area to volume ratio, that probably imposed limitations on their rate of heat dissipation. This in turn may have restricted their rate of muscular activity; there are also sound mechanical reasons why they should have been relatively inactive. It is therefore concluded that sauropods probably maintained relatively low body temperatures.