In comparison with the reptiles of the Mesozoic Era, modern reptiles are mostly small and bradymetabolic. Their large surface-to-volume ratio would render tachymetabolism uneconomical. Furthermore, they are able to evade inclement weather by aestivating in summer or hibernating in sheltered retreats. Even so, in addition to behavioural temperature regulation, they are known to utilise a number of physiological thermoregulatory processes. These include the emergency cooling of the body of tortoises through salivation and urination, and the flexing of the body muscles of pythons - which increases metabolic heat production when brooding their eggs (see Cloudsley-Thompson 1999). There seems little doubt that the dinosaurs and other large Mesozoic reptiles must have possessed some physiological thermoregulatory processes. In view of their large size and small surface-to-volume ratio, these would have been much more effective than those of small modern reptiles. This argument applies equally, whether they were tachymetabolic or bradymetabolic. They were still probably homeothermic, maintaining fairly constant body temperatures.
In warm climates, giant reptiles would have some of the characteristics of existing homeotherms, even if they had a low metabolic rate. The behaviour of these reptiles could therefore have been relatively independent of diurnal changes in radiation and air temperature - so long as the average values of these parameters resulted in a body temperature that was within the limits of tolerance. Large size, and the consequent high capacity for storing heat, would have dampened out environmental changes over periods of several days and maintained body temperatures at relatively constant levels. Changes in the thickness of the subdermal fat layer would have been of minor importance in contributing to the overall stability of the body temperature, according to the calculations of Spolita et al. (1973) who concluded that gigantism was, no doubt, a very useful strategy for large reptiles, providing a constant, equable in ternal temperature in the absence of a high metabolic rate when living in a stable warm climate. Dennis Claussen has recently calculated that if an animal as large as Argentinosaurus (Sect. 10.4) were tachymetabolic, to pump blood up to its head would have required a heart with walls some 0.5 m thick and weighing about seven tonnes. If, however, it was bradymetabolic, a much smaller heart would have sufficed (cited in Natl Geogr Mag March 2003: 2-33).
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