[MUSIC] We are hot animals. We are able to produce body heat and maintain that body heat and keep it regulated at an incredibly narrow range. So if any of you woke up this morning not feeling too well and you developed a fever, that's one of the indications that your body is not doing well. So the fever is showing that you're moved off your tightly regulated temperature, because in that case your body is responding to bacterial infection and then using the heat to hopefully kill the bacteria that invaded your body. So we take for granted this absolutely miraculous machine called our body that can maintain a high temperature and keep it absolutely consistent. Now if someone asks you to create a machine that could maintain a high temperature and keep it consistent, we'd be really hard pressed. It's a hard achievement to do. So, mother nature and evolutionary biology approach this experiment with the rise of the three great lineages of the reptiles. And these early reptiles are known to have not had the ability to maintain the kind of body heat that we take for granted. So, heat regulation or another word for this is thermal regulation, was a key strategy that had to be achieved, at least to some level, eventually by these lineages of the reptiles. And the three great lineages of the reptiles, the anapsids, the diapsids and the synapsids. The synapsids rose eventually to evolve into mammals. And we are one of the products of that, and so we know that that lineage ended up being extremely good at thermal regulation. But a lot of the questions evolved around the diapsids, the lineage that led to the dinosaurs. And were dinosaurs cold-blooded or hot-blooded? So, the ability to generate your own body heat, we call that endothermic. You have chemical reactions within your body that then result, partially at least, in the production of heat. Another thing that these reactions do is they feed us and create energy. So, endothermic organisms, the other end member is this is where in this case a reptile is not able to generate its own body heat and has to rely on gaining heat from the environment, and we call that ectothermic. So the first reptiles were all ectothermic. They need to have some heat in order to move around use their muscles and hunt and do their every day lifestyle. So they had to do things by engaging with the environment to attain that heat. And the easiest way to do that is, and we've seen this before in many environments on modern day Earth, is that if a reptile needs to get warm, they go out and sit in the sun. So sun basking was one great way to warm up your body, and once your body warmed up, then you would go about your daily activities. Now that all sounds fine and good. Now one aspect of this is that if you are going to be in the business of making your own body heat, it's much better for you to a, be small. And b, be able to access primarily proteins in your diet instead of carbohydrates and plants and some of the other aspects of what can be available for you to digest and intake in the environment. So there are some trade-offs here. If you want to generate your own heat, you have to have a different kind of diet, and that diet has to be protein-rich. So the endothermic and the ectothermic. Now one of the questions is, we have good evidence that indicates that the first reptiles in the Pennsylvanian and into the Permian that they were ectothermic. However, as we moved into the Permian, and especially within lineage of the synapsids, as we went from pelycosaurs into therapsids. We have now entered the regime where we start finding the fossils of these reptiles distributed in relatively cold latitudes on the ancient Earth. So instead of being in tropical latitudes, the thinking is if something is ectothermic, they're probably going to live in the tropics because that's where you have the most heat all the time. We actually start finding at mid-range to relatively high latitudes the fossilized remains of some these synapsid reptiles. So we know that because these fossil friends were at high latitude to mid latitude that they had to, at least at some point, been able to generate their own body heat by the Permean time period. Now another aspect of this, and this all comes into play as we're going to see the great distribution and radiation of the different lineages of the reptiles through the Mesozoic to the Cenozoic. We have some trade-offs that we want to remember here. One, if you have a small body size, you have what we call a larger surface area to body mass ratio. So in other words, if you're relatively small, if you're the size of a cat or a dog or a mouse. If you're an organism that's small, a, you're going to have a diet that is going to allow you to generate more heat. And that's a higher protein type of diet. But b, the other part of this is that by definition if you are small the relative area of the surface of your body compared to your mass, it's relatively large. So the smaller you are, the larger it is. And what difference does that make? Well, the mass your body is the proportional driving force for how much heat you can either obtain or create or release. And so you have the body mass that encompassed in a certain given amount of surface area. So when you surface area to body mass ratio is really large, that means that you can exchange readily with the environment. It's easy for you to either gain heat from the environment or lose heat to the environment. It goes relatively quickly. How about the other end of the spectrum? What if you become a gigantic herbivorous plant-eating long neck Saurischian dinosaur, you know, the ones we all like to think about that weigh tons and tons. And they could actually just standing there, they can eat plants that are up on a 30 meter high tree. These are really large organisms. So, but they paid a penalty for being so big, because when you're that large, you have a small surface area to body ratio. That seems counter intuitive right, but your body mass is so large, that whatever area you have surrounding your body mass. If you have body mass to that scale then your surface area is relatively low compared to that. So your small surface area to body ratio means that you are really bad at either gaining heat from the environment or letting heat go. So the giant dinosaurs, they had the small surface area to body ratio, and they had to be very careful. When they went out to warm up in the sun, they had to be there a long time. Now one of the ways that they fought against predation is simply their size, right. Their bodies were so large that they were much bigger than the jaws of some of, even of the biggest predators like T-Rex. Now T-Rex would hunt in packs, and take down these organisms, eventually. But it was very hard for single allosaurs or T-Rex or any of the big Sauriscian dinosaurs to be able to take down some of these big herbivorous dinosaurs. So they had to stay out there a long time, which made them more vulnerable to some of this predation. But the other thing, actually more important about this small surface area to body mass ratio, is that it took them a lot of time to cool off. So if they were to start generating some kind of heat endothermically, it took them a long time for it to be released. So if there was some endothermic heat reaction that would build up, let's say, as those dinosaurs would try to move quickly or even run. Then there was the danger that they could actually start cooking themselves because they were so bad at releasing heat into the environment. So, this concept of endothermy and ectothermy is kind of an underpinning for the entire evolution of these two major lineages of the reptiles, the sinapses and diapses. And we'll see this then coming into play over and over again as we see the different waves of radiation and extinction of the diapses and sinapsids with time.