So, in order to start our physiological discussion we have to understand what we are talking about, and that is fairly complicated. Walking dinosaur costume Most animals, including amphibians and reptiles, are “cold-blooded.” Animatronic dinosaur They have low metabolic rates that at any given size are about Seven to ten times less those of “warm-blooded” mammals and birds. I say “at any glven size” because as animals of similar ptrysiology get larger, their metabolic rates decline relative to each unit of mass. So a 1000-kg eland has a metabolic rate only 5.6 times greater than a 100-kg wildebeest (see Appendix A). Having low metabolic rates, reptiles are dependent on the environment for most of their body heat. Hence, they are called ectotherms. However, reptiles are not at the merqy of the environment; instead, they oploir it. Most reptiles operate best at high body temperatures, from 80′ to 100-plus”F (so the label coldblooded is a partial misnomer). They get there by sunning themselves, and can raise their body temperatures well above that of cool air. Of course, this method only goes so far. Mechanical dinosaur There has to be enough sunlight available, it does not work at night, and the air cannot be too cold. Because reptile body temperatures fluctuate, thev are heterotherms, or ectothermic heterotherms in sum.
It is important that bodSr temperatures fluctuate less in bigger reptiles. T-rex costume This is because their very bulk effectively insulates them against heat loss and gain. The effect is modest in the big monitor lizards and crocs, but in theory grant ectotherms could have fairly constant temperatures.2 This is called bulk homeothermy. Another idea suggests that as reptiles get bigger, their metabolic rates converge with those of big mammals.3 If true, an elephant-sized reptile would have a metabolic rate about as high as an elephant’s. Since with a metabolic rate this high the reptile is no longer dependent on the sun to keep warm, this is known as bulk endothermy. Remember the meanings of bulk homeothermy and bulk endothermy, because these are the two modern models for big, reptile-like dinosaur physiologies. Both envision large dinosaurs as continuously active animals. This is a key point, for the recent debate on dinosaur energetics has caused the old concept of dinosaurs as sluggish lower vertebrates to be universally rejected.