We've been talking about locomotion and how dinosaurs moved around. But, a lot of our ideas of dinosaur movement are closely related to another facet of dinosaur biology. Namely, their Metabolism. We've been comparing the locomotion of dinosaurs to a lot of living animals like mammals and birds. But two things that mammals and birds also have in common with each other is that they are both called warm-blooded. But what about dinosaurs? Well, let's start with what exactly warm-blooded and conversely, cold-blooded might mean. To start with, we're going to do away with the terms warm and cold-blooded. They're actually misleading terms and don't really describe reality. For example, lizards are often considered cold-blooded animals. But this doesn't mean that their blood is actually colder than that of a warm-blooded animal. In fact, many animals that are considered cold-blooded use different behaviors in order to keep their blood at a temperature relatively close to that of warm-blooded animals. And some desert dwelling lizards actually keep their body temperatures even higher than that of mammals or birds. In stead of the terms warm and cold-blooded, we're going to use Endotherm and Ectotherm. An Endotherm is an animal that can control its body temperature through metabolism and has a relatively constant internal body temperature. In contrast Ectotherms typically use behavior to control their internal temperature, which fluctuates between highs and lows. This isn't to say that Endotherms don't also have some variability in their body temperature. Humans are Endotherms. And when we go to sleep, our temperature typically drops by a degree or so. Another line of evidence that we can use to differentiate between Ectotherms and Endotherms comes from inside the bones themselves. Using a technique called Bone Histology where scientists slice up fossils into very thin sections, we can look at the internal structures found in fossils. Our bones are not solid things, but rather complex structures made up of many individual cells. Thanks to extensive studies on modern animals, we know that these bone cells or Osteons, are laid down in distinctive and different ways by endotherms and ectotherms. When we look through a thin section of dinosaur bone, the pattern of these osteons looks more like that found in endothermic animals rather than that found in ectothermic animals. Of course, there is still a lot of variation in these patterns, but in general, the bone structure of dinosaurs implies that they were more likely to be endothermic, like birds and mammals. Large reptiles are very rare on earth today. However one of these, the leatherback turtle, can grow to over two and a half meters in length and weigh close to a ton. Is this turtle an ectotherm, or an endotherm? It turns out that this turtle is more endotherm like than ectotherm like, and is able to maintain a relatively stable internal body temperature. A temperature that is much warmer than the surrounding waters it lives in. Although turtles as a group are considered ectothermic. The Leatherback has a large body size. This makes them unique. It turns out that Leatherbacks maintain a high consistent body temperature because they are so large. As a big animal, its surface area increases much more slowly than its internal volume and follows a rule called the Cube-square law. If you have a relatively large body volume and a relatively small surface area, your internal temperature is much less affected by your external environment. Leatherbacks, though not exactly endothermic, can be considered Gigantothermic. This is a term that describes an animal that is so large. Its internal temperature is relatively unaffected by the surrounding environment. Many of the largest dinosaurs could have had a similar temperature regulation scheme. Meaning that if they were not exactly endothermic, they would have been able to take advantage of many of the endothermic benefits. Like speed and continuous activity. However not all dinosaurs were large. So this theory doesn't tell us much about the state of those smaller forms. In the end the answer whether dinosaurs were ectotherms or endotherms is much more complicated than it seems. Dinosaurs most definitely came from an ectothermic ancestor, just like mammals did. Somewhere along the way, it seems that dinosaurs became full endotherms, like modern birds. The question of where to draw a line in the evolutionary record is difficult. And it's a question that won't go away anytime soon. Our lack of certainty about how warm or stable their body temperatures were, maybe less important than most people think. The behavior and the evolution of an animal is driven by far more than just it's body temperature. Our disagreement over the metabolism of dinosaurs should not stop our search for answers to many of the other questions we have about them.
