Thermoregulation 102

(Image from here.)

Before we spoke of the physical requirements of thermoregulation. Now we're going to have fun: we're going to see how animals solve these physics problems.

But it's a big question and the possible scope is vast. So we're just going to stay inside Kingdom Animalia and endothermy and work from there.

We'll leave poikilotherms and plants for another day. (Plants thermoregulate as well but have a completely different set of constraints.)

Okay. First we have to go back to that table from last time that shows the relationship between surface area and volume. Surface area is a square function and volume is a cube function. Consequently, surface area dominates for small animals where volume dominates as animals get larger. This means that the challenges animals face changes qualitatively as the animal size changes quantitatively.

This is interesting. Often when considering engineering problems the issue of scale comes up. A solution that works on the small scale can fail magnificently when the mere size of the problem is increased.

Many animals have significant size ranges. Birds range from hummingbirds to extinct moas. Reptiles range from tiny lizards to komodos and great crocodiles. Mammals have a huge size range, from shrews to whales. As said before, the problems at the ends of the scale are different. The problems of the very small are not the same as the problems of the very large.

Let's stay with endotherms. Remember, endotherms maintain a constant body temperature metabolically.

Down at the small end we have the three champions: the Etruscan shrew (Suncus etruscus), Kitti's hog-nosed bat (Craseonycteris thonglongyai) and the bee hummingbird (Mellisuga helenae). All of these animals range from 1.6 to 2 grams. A gram is about the weight of a paper clip-- for those of you who can remember those. Or about the weight of a single non-glossy business card. I expect these are the bottom end possible with Terran endotherms.

Etruscan shrews inhabit a relatively warm climate between 10 and 40 degrees latitude-- or from about Colombia in South America to the boundary between Kansas and Nebraska in the US. That's a fairly broad range. It's found across most of that range from North Africa up through France-- from places that have mild to nearly absent winters to places where winter would be a threat but does not last terribly long. It goes into torpor when it's cold but mostly prefers warm habitats.

Kitti's bat has a very narrow range in parts of Thailand and Burma. It roosts most of the time with brief feeding activity periods. These periods can be interrupted by wet or cold weather.

The bee hummingbird is restricted to Cuba. Like all hummingbirds, it's dedicated to a life of eating the sugary goodness of nectar. It can visit upwards a thousand or more flowers a day.

How would we analyze these little guys?

Well, bats and birds have dedicated their physiology to flight. This has huge energetics requirements over and above thermoregulation. However, while the cost of flight is high it has a benefit of generating heat. In the case of birds, feathers are wonderful insulators. Even so, hummingbirds are in the same boat as most small animals: they're dominated by heat loss. Hummingbirds make up for it by eating from a lot of flowers and using a very easily metabolized food.

Kitti's bat has taken a different strategy. Instead of eating a lot and staying active, the bat has instead taken the opposite tack and lowered its requirements. If it's only active an hour a day that's a major energy requirement that has been lowered. In addition, many bats can lower their body temperature when roosting.

It's interesting that many animals follow this same paradigm: work when you have to and let the body temperature drop when you don't. Hummingbirds are notorious for dropping their body temperature at night.

One wonders how penguins manage.

Let's think about hair, feathers and insulating fat for a moment. We always have to keep in mind that evolution is inherently an economic system. Every adaptation has a cost and that cost must be born in order to reap the benefits. Hair is not as good as feathers (especially down) at insulation. It traps less still air. But it's not as expensive as feathers. That said, there's a weight penalty to hair.

So consider our tiny shrew. How much hair can the shrew put on to insulate itself? What are the cost benefits? Hair has a metabolic and weight cost. It can interfere with the quick speed of hunting. It must be groomed-- one of the features of our shrew is that it grooms constantly. More hair might be good at keeping our shrew warm but the cost of maintenance might exceed its benefit. Ditto fat. Fat is much more expensive flesh than protein or sugars. And it doesn't serve any active purpose: it just insulates.(Except for brown fat. More later.)

This has got to be a problem with bats. They don't have the luxury of feathers. They have to make do with plain old hair with all its attendant cost. Especially with that living flight membrane that has to be kept alive. At least exposed feathers are metabolically inexpensive once they're out there. The flight surface of birds requires little metabolic upkeep. Birds have fat but, again, the needs of flight make the cost of fat a premium.

It's also important to remember that birds and mammals evolved endothermy separately. Both evolved from ectothermic ancestors. Consequently, it is unreasonable to expect that the endothermy mechanism would be different. They are, in fact, different and similar.

Energy capture in the cell mostly happens in the mitochondria. Sugar and fat is burned and that energy generates ATP-- adenosine triphosphate--as a sort of energy packet. ATP is distributed across the cell and used for processes that require energy. The process by which ATP is produced can be tightly coupled or loosely coupled. "Tightly coupled" means that the process is maximized towards ATP. It's efficient and doesn't produce a lot of heat. "Loosely coupled" means that not as much ATP is produced but more of the burned sugar escapes as heat.

The mitochondria is not only where the energy is produced, it is also the primary location where heat is produced. Again, it's a trade off. How much heat is actually necessary to keep the body going?

Both mammals and birds generate heat by decoupling but they use different enzymes to do so. (See here.) Not a surprise since the two groups developed endothermy separately.

Mammals have something extra called brown fat. This is fatty tissue that is riddled with capillaries and mitochondria. It serves to generate heat in animals that can't shiver: infants and hibernating animals. It's brown because it has so many capillary beds to get oxygen and distribute heat and so many mitochondria to burn, baby, burn. It's seen often in animals that need to get out of torpor or hibernation since in such states shivering not only doesn't work; it can't happen. Mammalian muscles don't do well when they're cold.

Which brings us to the three different ways animals can avoid spending energy maintaining their body temperature: torpor, hibernation and aestivation. Torpor is state where activity lessens and the body temperature drops on a temporary basis-- daily, when the temperature drops. Hibernation is a similar process where the animal prepares for the state, enters it and remains for an extended period of time. They look similar. In both cases, body temperature drops to ambient, heartbeat slows, brain activity reduces, metabolism is a bare thread of itself. The difference between an active bat metabolism and a torpid bat metabolism is about a factor of forty.

There's a lot of discussion as to whether mechanisms of the two are the same. Many authorities think that hibernation is just pre-planned torpor.The literature goes back and forth on this one. Until I get a good understanding of the underlying mechanisms involved, I tend to think of them as separate but related things. There is so much planning that goes into hibernating: finding a place, building up a fat load, determining optimal time from calendar and sunlight cues, determination optimal time to wake up. It's my intuition on these things that when there is an opportunity for selection and evolution to take place, it does. These pre-hibernation and post-hibernation operations are just such an opportunity. So all of the evolution goes into staging the event and the event itself remains unchanged? Dubious. But I've been wrong before. Twice.

Aestivation is also similar but for when dormancy is required for high temperatures. In all cases, maintenance of the body temperature and general metabolism is reduced to save energy, retain water, etc. But in the case of aestivation, the body temperature is much higher. Aestivating animals are much more easily aroused.

But these are fairly gross mechanisms. Are there any subtle and elegant mechanisms?

Certainly. Many of them centered on controlled blood flow and countercurrent exchange. But to do that we need to expand our horizons and start looking at large animals. Which we'll do in Tthermoregulation 103.

(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.)

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.