Thermoregulation: Today's Hot Topic

It's been hot this week so it seems appropriate to talk about it.

Humans are mammals and all mammals exercise some form of internal thermoregulation. The temperature setting might not be exact (marsupials have a somewhat wider temperature variation than placental mammals) but we all keep our temperature within relatively narrow ranges. Remember 98.6? Your dog and cat have a higher temperature.

Humans are vertebrates so we like to lump the animal world as vertebrates versus invertebrates. Sorry about the rest of all animal life on this planet. You're now all lumped together.

Similarly, we like to lump thermoregulatory groups together, too. Endothermy: animals like us that keep our body temperatures close to the same temperature by metabolic or behavioral means. Ectothermy: animals that are completely dependent on external temperature and regulate solely be behavioral means. Sort of a humans versus snake mentality.

Like everything else in biology, it's more complicated than that.

Endothermy at it's most narrow sense is demonstrated pretty well by killer whales, elephants and human beings. Temperature is kept within a very narrow range. An increase in an adult human of 1 degree Fahrenheit is a fever and is accompanied with fair distress. Children have a bit more variation and infants have to be protected from their own defective thermoregalatory systems until the get older.

Mammals have endless mechanisms to control their temperature. Elephant ears (radiators), human sweat (evaporative cooling loss), controlled blood flow to the extremities. Marmots and bears have the additional problem of two settings: one when they are in normal mode wandering around eating and reproducing and hibernation. They have what is called brown fat, a mitochondria rich adipose tissue that can generate a great deal of heat in a short time.

We also have behavioral mechanisms. Sam Ridgeway gave a talk on his work at the university I attended. He was talking about determining heat responses of sea lions. They put the seal lion subject in a room and heated it. When it got hot enough the animal needed to response it did not respond physiologically. It defecated and urinated on itself. Evaporative cooling loss is a wonderful thing.

Which brings us to the most important point of mammalian thermoregulation. It's not how you keep warm; it's how you keep cool. Certainly, retaining heat is a necessary thing. Humans have clothes. Whales have blubber. Rabbits have hair. But being able to shed heat is equally as important. Humans sweat. Heretofore elephant ears. The highly vascularized flippers of sea lions and seals to to shed heat when necessary.

Endothermy, at least as mammals practice it, is costly and complex.

Ectothermy, by contrast, is cheap. The metabolic needs of crocodiles, an ectothermic species, is so small that one big meal might be enough to keep them going for a year. It should come as no surprise that those heat filled environments are filled with ectotherms: amphibians and reptiles. Endotherms, birds and mammals, are there where the environment is rich enough. But as the environment shifts to the more marginal such as deserts the reptiles start gaining ground. Why? Because maintaining a reptile is cheap. Most of the energy consumed by a mammal or a bird goes straight to maintaining the body temperature. Reptiles get it for free from the sun.

But wait, you say. Birds and mammals both started (presumably) from exothermic animals. We know how diverse the animal kingdom is. Who's to say endothermy hasn't arisen from the ectothermic animals? Are there endothermic animals in the ectothermic classes?

Funny you should ask that question as I sit down to tuna maki.

Anyone who's ever ordered a tuna steak has noticed it doesn't taste like other fish meat. It is tougher. Almost like a real steak. That's because, like cattle, tuna muscle is richly filled with blood vessels, the muscles are highly oxygenated and operate at a relatively warm temperature.

Tuna are warm blooded. Literally.

Endothermy, of a sort, has been demonstrated in tuna for a while. A bluefin tuna can keep a muscle body temperature between 75-95 degrees in water that is as low as 46 degrees. They do this by creating heat via muscle exertions and then conserving that heat by a collection of mechanisms so they don't lose it to the outside water. (See here and here.) It is not endothermy as mammals practice it-- we generate heat regardless of muscle use. But it is not ectothermy as practiced by lizards and snakes.

Which, as all things inevitably must, brings us to dinosaurs. Birds are clear endotherms. Reptiles are clear ectotherms. Birds evolved from dinosaurs-- does that mean dinosaurs are endotherms? If they were endotherms of a sort, what kind were they? Dinosaurs were big, which means that if they were mammalian style endotherms they would have had problems shedding heat. But there's significant evidence they had high body temperatures.

Recently I read a paper in Nobel Intent on how to determine dinosaur temperature by analyzing their tooth structure. This is a very elegant technique based on the tendency of Carbon-13 and Oxygen-18 to preferentially clump together in the formation of the calcite in bones and teeth. This "clumping" is temperature dependent: lower temperature means more clumping. The researchers used two sauropods to derive a temperature of 96.8-100.4-- pretty much right on the dot around mammals. Low for birds but definitely higher than crocodiles.

However, there are a few limitations. 1) It says nothing of the style of temperature regulation practiced by the animals. Mammal style endothermy? Tuna style ectothermy? Lizard ectothermy? All it says is the temperature of the animals. 2) It only reflects the temperature of the head.

But we always knew dinosaurs were hot.