Thermoregulation 101

I write science fiction and I like writing science fiction about aliens. To do this I look at how animals handle challenges the world throws at them. Since it's winter, we'll talk about a particular issue all life faces on this planet: temperature.

We're mammals. One of the characteristics of mammals is that they keep their own temperature constant in the face of an inconstant world. This is called  homeotherms. "Homeo" for similar and "therm" indicating heat.

The opposite of homeothermy is poikilothermy  where the internal body temperature is variable.

Homeotherms maintain their internal body temperature by some mechanism. Poikilotherms tend to be at the mercy of the elements. The activity of that mosquito buzzing the light is driven largely by temperature: it is a poikilotherm.

But wait, you say. Where are endotherms and ectotherms in this? Isn't that what I learned in high school biology?

Well, yes. But they more describe how an animal manages its temperature rather than the process of maintaining that temperature.

In our case, we burn energy to maintain a close temperature tolerance. That defines us to be endotherms-- also called being warm blooded.

But consider a tortoise basking in the sun. It likes a particular temperature. If it thinks it's about to get too hot it moves into the shade. If it thinks its going to be too cold it moves back into the sun. It is maintaining homeothermy but not metabolically. It uses energy external to its own metabolism. It is an ectotherm.

Of course, it's a bit more complex than that. Our friend, the tortoise, might maintain a fairly constant temperature over a portion of the day when it's grazing, moving in and out of the shade. But when the sun sets, it seeks as warm a place as it can and waits for the next day. During that time its temperature can drop.

In this example, the tortoise is practicing homeothermy during the day and then allowing poikilothermy during the night.

Why do that? you might ask. Surely it must be better to be warm.

Certainly, I would agree with you but there are grave costs to endothermy. It's expensive. My resting metabolism requires about 2025 calories today. (Calculated here.) This is what is called the Basal Metabolic Rate.

Here is a site that discusses the calorie requirement for various forms of exercise. According to that table, one hour of extremely hard stationary bicycle work would require about 1386 calories. It would probably be less since I'd likely die part way through.

Still, it's quite a bit less than the metabolic requirement of merely breathing. What could we possibly be doing with all that energy?

Well, thinking for one. About 20% of human metabolic energy goes to run the brain. That's 400 calories/day-- the equivalent of running two and a half miles at about 8 mph. Which one would do on top of those 400 calories/day.

A small portion of it is overhead just running the system: energy for heart beat and lungs, kidneys and the like. But most of it is just to keep us warm. If the temperature drops, we burn more. If the temperature rises, we burn less. After a point the energy cost of trying to cool down can actually generate more heat than the cooling can remove resulting in a positive feedback loop. Typically, animals die if that's not rectified.

Interestingly, this is one of the reasons the traditional desert garb in societies like the Bedoun involves so much clothing. Sure it's hot but the heat load generated by the body is nothing compared to the heat load being received from the sun.

Maintaining a constant body temperature metabolically is good for body processes but it comes at a high cost. Endotherm efficiency for translating food energy into usable biomass, minus overhead, is about  1.4%. Ectotherm efficiency for the same process is about 50%. (See here.) Most of the world is either poikilotherm or ectotherm largely for energy reasons.

Why have a specific temperature at all?

This has to do with chemistry.

Almost all chemical reactions operate optimally at a particular temperature. Many metabolic reactions happen in the cell at much lower temperatures than they might in a laboratory. This is because of enzymes: particular proteins that promote specific chemical reactions by lowering the required energy. A given chemical reaction when mediated by an enzyme might take place at a much lower temperature (say, like that within a cell) than without that enzyme.

This is great but many enzymes are themselves sensitive to temperature. Some work best at mammalian body temperature. Some higher. Some lower. And when those enzymes aren't at the right temperature, they don't function as well as they could. Too hot or too cold and they might not function at all. More extreme temperatures can kill the protein entirely-- as anyone can see when transparent egg white becomes white. The protein is permanently broken.

Mammals range in body temperature around 97F to 103F, depending on species and size. Birds about 104F-108F. This appears to be more or less optimum. It's interesting how close those ranges are for birds and mammals. Mammal and bird lineages separated long before endothermy evolved in them. It would be interesting to know if the thermoregulatory mechanisms in the two groups were the same-- a topic for another post.

Temperature is one of the driving forces behind many of the interesting adaptations in mammals.

All animals exist in the tension between surface area and volume. Surface area increases as square function while volume increases as a cube function. Take a cube of edge length s and the surface area is:

sa = 6 * (s*s)

However, volume increases to the cube. That same cube's volume is described by:

v = s*s*s

Consider the following table:

S	SA	V	SA/V
1	6	1	6.00
2	24	8	3.00
3	54	27	2.00
4	96	64	1.50
5	150	125	1.20
6	216	216	1.00
7	294	343	0.86
8	384	512	0.75
9	486	729	0.67
10	600	1000	0.60
11	726	1331	0.55

Where S = the size of the cube edge, SA is the calculated surface are, V is the calculated volume and SA/V is the ratio between surface area and volume. Notice how initially the ratio between the amount of surface area and the volume hugely favors surface area. But then, for a cube of s=1, there's a cross over and from then on volume dominates surface area.

This relationship factors into a lot of things like respiratory gas exchange, blood flow patterns and, of course, heat exchange. Heat traverses into and outside of the body via the surface. So if the volume scales up more quickly than the surface area, heat exchange slows down as the animal gets bigger and speeds up as the animal gets smaller. It's one of the reasons children are more likely to get hypothermia than adults. Not wearing a coat is another.

It means that large animals, like elephants, whales and sauropods have a problem getting rid of heat. While tiny animals, like shrews mice and chickadees, have a problem retaining it.

Mass factors in here, too. The more mass in an object the more heat can be stored in it. We're essentially water. The laboratory definition of calorie is the amount of heat required to heat a cubic centimeter of water one degree C. But the calories used in nutrition are actually kilocalories-- 1000 calories (known as Calories as opposed to calories, if that wasn't confusing enough.) Two thousand kilocalories is two million calories: the amount of heat required to raise 1000 liters 1 degree C. Or the amount of heat to raise 100 liters of water (100 kg) 10 C.

This means that 2025 Calories can maintain my imperfect body 20C over current body temperature if there were no overhead or brain. (Insert joke here.)

This is the physical reality animals have to live with. In my next post we'll talk about how they manage.