(Picture from here.)
Last time we talked about small endotherms-- specifically small mammals.
I have nothing against small endotherms. But there are significant limitations to the adaptations small animals can achieve in the realm of thermoregulation. They are small so they're dominated by the surface area. If it's too cold they have manage maintaining body heat. There are only a few ways they can manage this: burn more food which means eat more food. (Think shrews.) Drop their body temperature as in hibernation. Or grow some sort of insulation. Since they're small, there are distinct limitations on this. A two ounce mouse is not going to benefit from an additional three inches of blubber.
On the other end, if it's too warm they have different responses. They can sweat. They can find a cool spot. They can estivate (the high temperature approach to hibernation.) They can lose their hair or other insulation.
But with small animals the major thermoregulatory adaptation is behavioral. They go underground. They go in the water. They hide from the sun. They migrate.
Not to say that large animals don't have behavioral adaptations. But it's unlikely to see an elephant burrowing to escape the heat. They'll seek the shade of trees and vegetation but digging out tunnels is beyond them.
It's also true that since large animals are no longer dominated by surface area we start to see issues of heat loss come into play.
So it's in big animals that we start seeing some very interesting physiological adaptations.
Let's start with large animals in a cold world.
Endotherms in the cold have to generate heat. There are a few things that can happen here.
One is the regular old metabolic mechanisms I mentioned last time. Another, in mammals, is brown fat. Both of these are fully expressed in large animals. In addition, because in large animals we have reduced surface to get rid of heat-- this can actually help in the cold. Then, we have the nature of many of the animals themselves.
Most really large terrestrial animals are herbivores: from cattle to elephants, we're talking animals that eat plants. By eating plants I mean eating cellulose. Which means they have a fermentation vat in their bodies because only select bacteria can digest the stuff. Herbivores don't convert plants to nutrition. They consume plants for microbes which digest the plants. The herbivore then digests the microbes and their off products. Most herbivores are hindgut fermenters which use a cecum off the entry point of the large intestine. Horses, marine iguanas, Galapagos tortoises and gorillas are hind gut fermenters A select few are foregut fermenters which use an adaptation in the stomach. Cattle are foregut fermenters.
The point here is that endothermic herbivores not only have the ability to heat via metabolism and brown fat, they have heater buried deep in their core tissue. It's not a coincidence that the largest terrestrial animals are herbivores and many of them, such as mammoths, live-- or lived-- in cooler climes.
The increasing size of the animal reduces the cost of carrying insulation. Heavy hair and a good layer of fat become cost effective. As long as the herbivore can feed the furnace it will do all right. Neglecting predation, the biggest problem cold weather herbivores face is getting enough food.
The cycle of big animals in the north is often large animals live of the land and smaller animals prey on the large animals. This is the wolf/elk relationship. The predators are usually quite a bit smaller herbivores-- they have additional constraints. Predators and prey are always in an arms race. Your fat and hairy-- but warm-- wolf is going to have a hard time catching up to that graceful elk laughing at him.
Predators, and other smaller mammals, have a different approach. They have no internal heater other than metabolism and brown fat and can't afford. There are considerable innovations in insulation. Consider the wolf skin and the elk skin. The wolf skin tends to be a bit thinner with a very think covering of fur. The fur is complex, consisting of a light, soft layer overlaid by a course layer. The wolf has bet on light insulation and an increased food consumption as its strategy for the cold. (Good discussion here.)
Now, if you look at the elk skin it is quite a bit thicker. Some fur but not as much as the wolf. The deer outweighs the wolf and, in addition, has that hindgut heater. It's bet on a larger body with decreasing surface area and increased dependence on its internal heater.
Beyond insulation animals in a cold world adapt by selectively altering their circulation. It resembles prioritization. What's important? Brain. Lungs. Liver. Intestines. Who needs the finger tips? The palms of the feet? The circulatory system shunts the blood back to the interior before it ever reaches the extremities. The extremities, feet, hands, paws, tail, become more tolerant of lack of oxygen and CO2 buildup. Not completely, of course. The shunts open periodically to circulate that cold, stale blood back to the nice warm interior-- which presents other problems. More on that later.
That's when everything works well. Anyone who's had frostbite has a good knowledge of what happens when things go wrong.
But animals often have to endure both cold and hot environments. Some large animals (elephants, for example) live almost exclusively in tropical areas. Now we have the problem of getting rid of heat. How do we do that?
Humans sweat. Evaporative cooling is extremely efficient. It has a few problems. It uses up water where water might be quite precious. It's not always effective. Those of you out in Arizona or Southern California might not realize this but sweat doesn't always work. Anyone who's ever spent a summer in Mississippi can attest to this. And not many animals have it available to them. Human sweat is mostly water. Most sweat has a much larger percentage of oil-- think of the "lather" on a hot horse, for example. Some animals, like dogs, barely sweat at all. Since it has such limited applicability, we'll largely neglect-- though human thermoregulation, and the experiments on it, are so interesting it deserves its own post.
Remember the surface area/volume problem. One common solution is to increase the surface area of one part of the body. This is the reason for the elephant's ears.
Elephant ears are vast things, of many square feet of area and not much in thickness. There is evidence they play a role in elephant perception of infrasound-- sounds at very low frequencies. But they are filled with capillaries and act as huge radiators. And here we discover a truly elegant design. I'm referring to the countercurrent exchange mechanism.This is a simple concept that is used over and over again in the vertebrate body.
Of course it's not perfect. I have a friend of a friend who was making hamburger patties by squeezing very cold raw hamburger meat with his bare hands. His circulation was not the best and the cold blood pooled in his hands and forearms. Eventually, that cold blood made its way to the heart without being warmed much and it was enough to trigger cardiac arrest.
Counter current exchange is used over and over again. In the lungs for gas transport. In the kidneys for excretion. And it is one of the primary mechanisms by which mammals (and birds) handle extremes of temperatures. I am speaking, of course, of your friend and mine, the ocean endotherm.
The ocean endotherms we're talking about are the penguins, cetaceans (whales and dolphins) and pinnipeds (seals and sea lions.) We're not going to talk about otters and beavers this time though they['re quite interesting.
Here is where physiological adaptation tends to trump behavioral adaptation. Water transmits heat 25 times more efficiently than air-- to the point that many cases of insulation such as fur and feathers depend on the trapping of air within the insulation mechanisms.
Both pinnipeds and cetaceans derive from land animals that presumably had fur. However, neither has hair in any sort of insulative capacity so the cost of maintaining fur must have been too costly and not much benefit. I can hazard a few guesses on this.
We know that feathers on penguins and fur on otters serve as insulation. Both of these animals spend a significant amount of time on land. We know that fur and feathers require grooming. The adaptive direction seals and whales have taken preclude that. It's hard to groom with flippers.
Both furn and feathers depend on trapped air for insulation. Air compresses at depth so one would presume loss of insulation as the volume of the air is reduced. Otters forage down to about 100 meters. The emperor penguin dives down to 565 meters. And the sperm whale goes down nearly 1200 meters. (See here and here.) 100 meters is about 11 atmospheres so the volume of the air, as insulation, would be 1/11 of the same air at the surface. One would expect its insulative capacity to be close to nil. Deeper would be worse.
Fat, however, is not terribly compressible and so has similar insulative ability in water at depth than in air.
Marine mammals and birds exist in a three dimensional world rather than the boring terrestrial mammal two dimensions and those three dimensions range from quite warm at the surface to bloody cold at depth.
To manage this problem the marine mammal has a number of mechanisms available to it. Interestingly enough, penguins, toothed whales like dolphins, baleen whales like humpbacks and pennipeds like sea lions have evolved very similar mechanism when dealing with heat and gain. These fall into three general approaches:
- Mass. Marine mammals tend towards the large end of mammals. They start bigger at the small end and get way bigger at the big end. Emperor penguins are quite large though not all penguins are that big.
- Fat insulation. This is blubber in whales and its equivalent in seal, sea lions and penguins
- Controlled radiators to get rid of excess heat or to retain heat depending on circumstances.
Whales have such a significant problem that mother whales that swim south to birth and nurse stop eating. One reason is that the additional heat generated by feeding can't be removed easily in tropical waters.
Seals and sea lions have a different problem. The gray seal, for example, is tiny next to any whale. It does generate heat but nowhere near as much. Another reason for the seal to shutdown blood flow to the periphery is to reduce oxygen requirement during the dive. So the seal dives to the deep Arctic water, feeds, and returns to the surface. The cells in the flippers and periphery are still there during the dive, making CO2 and requiring oxygen, even though now they are cold.
So when the seal comes to the surface, it has to give some blood to its flippers. But, in so doing, it gets some mighty cold blood on the return. Another example of the challenges these animals face.
(Side note: when I was in college, Sam Ridgeway, who worked on the Navy Marine Mammal Program, came to speak. As I recall, he mentioned a gray seal that they had trained to take a breath from an monitored opening and then go to the other side of the sealed tank and hold down a switch. Then, it would return, exhale into the opening and get a reward. They were measuring CO2 and oxygen respiration. They were not even close to stressing the seal in their little five minute tasks. So they decided to try something. While the seal was holding down the switch, they closed the door on the opening. The seal returned and found there was no oxygen to be had. Instantly, the seals heartbeat dropped from about twenty or so per minute to three. It then exhaled against the surface of the sealed tank making a large bubble. After a moment, it snarfed that air back up. CO2 is more soluble in water than oxygen so the seal was, effectively, getting rid of the CO2. After a moment, they opened up the tank again and the seal began inhaling air and exhaling it against the back of the tank.)This is all fine and good for endotherms. But many fish and reptiles are quite large. Is there anything interesting there?
Animals do all sorts of interesting things. Really large fish such as the Great White Shark, bluefin tuna, marlin and swordfish, are so large and active the volume/surface area ratio begins to work for them.
Bluefin tuna travel up and down the water column regularly from fairly warm surface water to the cold of 3,000 feet. This is a huge change in temperature for an animal as intimately involved with its medium as a fish. Surface temperature might be as high as 50F depending on where the tuna was and at depth be as low as 36F.
The tuna has-- wait for it-- countercurrent exchange flow in its gills that conserves heat in its deep swimming muscles-- called red muscles for the amount of blood in them. Heat from the motion of swimming is conserved in these muscles along with the core such as brain and organs. It's not an endotherm but one could call it a limited homeotherm. (See here.)
The Great White Shark goes one step further. It actually has a strip of red muscle near the center of the body that actually releases metabolic heat. It also has similar mechanisms to the tuna. (See here.)
Blue marlin and swordfish have less ability to maintain body temperature but conserve waste heat of movement to help.
Okay, you ask. What about really big animals? You know what I'm saying. What about dinosaurs?
Oooooooooooooookay. This has been a long and heated debate in the scientific community.
We know a few things. The sauropods were huge. Big enough that they might well be able to conserve body heat by sheer thermal mass alone. Certainly tuna do it. There's no reason the laws of thermodynamics would make an exception of apatosaurus.They might not have been endotherms but they certainly could have been homeotherms.
Birds are definitely endothermic and they evolved from dinosaurs. Many of the late dinosaurs that were branching into birds have similar internal construction and had feathers. Feathers came long before flight and probably evolved to retain heat. It's a reasonable assumption that the heat retained originated within the body rather than coming in from outside.
Robert Bakker devoted an entire book gathering the evidence for endothermy neatly summarized here. The evidence is good but not, to my mind, completely conclusive.
For one thing, the bigger therapods such as T. Rex or even allosaurus were large enough that there might not be a real need for endothermy as we know it. Homeothermy and conservation of heat by behavioral means might have been sufficient to keep a quite high body temperature.
We have a tendency to think of the ancient world such as the Jurassic as just like ours but with differently shaped animals. That's not surprising. Behaviors don't fossilize easily and we don't have a lot of dissectible lying around. Like none.
But we do know that animals were different before dinosaurs. At some point endothermy and homeothermy evolved. There were amphibians, then reptiles, then dinosaurs, then birds and mammals. Thermoregulation evolved just like anything else. There's absolutely no reason the mechanisms we see in the modern world were merely replicated a hundred million years ago.
And the scale of dinosaurs is something we have absolutely no experience with. The largest land mammals we've shared the earth with have been no more than a few tons. How does that compare to a forty ton sauropod? The largest terrestrial predators we've seen are less than a ton. How does that compare to a six ton T. rex?
I do think they thermoregulated for one good reason. Antarctica had no ice cap when there were dinosaurs but it had six months of dark cold and six months of sunlight. It had a thriving dinosaur population. Some of those animals look to me to be too small to migrate. Did they hibernate and warm up in the spring? Maybe-- though we have no land reptiles that big now that do that though there are large salamanders that do. Maybe they warmed up over the polar summer and then cooled down in the winter purely by external means. Or, maybe, they were like the Great White Shark and had a little strip of muscle that warmed them up to the point they could conserve the heat of movement. Or maybe they were like bears and were true endotherms when it suited them.
They did something. But it may not have been anything like what animals do now.
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For further reading, I strong and enthusiastically recommend Knut Schmidt-Nielsen's How Animals Work. You can get a sample of it here or buy it used.
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