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.)