Homeotherms, Poikilotherms and Body Temperature

[tanstaafl]

Question inspired by Staff Report on What makes some animals cold-blooded and others warm-blooded?

Reading the above Staff Report reminded me of a question I had thought of before. Why is it so important that my body maintain a temperature of 98.6^o F?

I know that we don't feel good when our body temperatures go even a little bit off (fevers, for example) and if it varys more than about five degrees or so then we are in extreme danger and beyond that we die. (I once ran a temperature of over 105^o F; lying in a bathtub filled with ice to keep your body temp down is no fun, believe me!)

What is it that makes normal body temperature so critical? I can see that some processes may be more efficient at certain temperatures but this seems to be something other than that. Cold blooded... excuse me, poikilothermic creatures seem to be able to survive much wider variations in body temperature, why can't we?

(I'll just add that I realize that becoming torpid at low or high temperatures isn't a survival advantage, but the danger to us at temperature extremes is of actual physiological danger, not the risk of being attacked by a predator and being unable to defend ourselves. Or, possibly, did the one lead to the other?)

[Karen Too]

What do panting birds look like? For example, do their tongues loll out the sides of their beaks?

[sunbear]

I read the article and wondered about the circulation systems. I seem to remember something from school about different types of heart cahmbers, pipes etc for the blood. The oxygenated blood mixed with the nonoxygenated in cold blooded animals.

[John W. Kennedy]

Being warm-blooded is a high-risk, high-payoff strategy. If your body is specialized to run at one particular temperature, it can be tuned for considerably greater efficiency. But, unless a desert-lizard bask/shade lifestyle is available, it means you have to eat a lot more, or die. Like a high-performance engine, you gotta get the octane, or you won't run.

That's why cold-blooded land vertebrates tend to be ambush hunters. Being warm-blooded wouldn't help them, and would cost a lot. So they haven't evolved homeothermy, while homeothermic creatures don't compete in that niche, because ambush hunters have to be prepared to go without meals for long periods.

[sunbear]

There's a good book on mammals and the cold blooded animals, called Why Elephants Have Big Ears.

[C K Dexter Haven]

Homeothermy is a life-style choice, and all you homeo-phobic people out there better learn to be more tolerant.

Huh? Oh. Wait. Sorry.

[Richard.Stridde]

Like tanstaafl I'm interested in an Answer to the question why does a poikilotherm just get a little slower or faster in his movement and metabolism when its body temperature varies and I bite the dust. With my little anatomic knowledge I can't tell what makes the difference and I really like to know.

By the way, what kind of definition is that for poikilotherm/homeotherm.
Q: Is this critter poikilotherm or homeotherm?
A: What's the time?
And you wonder why the old fashioned warm-/cold-blooded terms are still so popular.

greetings
Richard

[Colibri]

To answer the OP, the advantage of being a homeotherm is that it allows a much tighter integration and greater efficiency of the enzymes and other components of metabolic pathways. Any enzyme that controls a specific part of a metabolic pathway (a series of chemical reactions involved in metabolism) has a specific temperature at which it works best. Slightly different forms of an enzyme may work optimally at different temperatures. Having a stable body temperature allows an organism to develop a suite of enzyme forms that work together optimally at that temperature. If body temperature varies, enzymes can get out of sync, disrupting the metabolic pathway and making it less efficient.

Chemical reactions generally run fastest at higher temperatures. The homethermic strategy is based on having a high-activity life-style, so that homeotherms tend to regulate their temperatures to be close to the maximum feasible. Proteins, such as enzymes, start to denature at 45 C (113 F), so this represents a lethal temperature for most organisms, with the exception of some bacteria. Of course, it's best not to have your usual body temperature too close to the lethal, or else you might keel over dead with any excess exertion. Mammals tend to regulate their body tempertures in the 90s (F), birds as high as 110. Some ectothemic desert lizards may be active close to the theoretical maximum of 113.

[Colibri]

To elaborate a bit more, various animals may have adaptations that permit them to survive changes in body temperature that would kill a human. Remember, we are basically tropical, non-desert mammals, and the temperature extremes we can tolerate reflect that. Evolutionarily, there has been no reason to develop greater tolerance, so when our core temperture drops too low or gets too high, our metabolism is disrupted enough to kill us. Camels, on the other hand, can tolerate a degree of hyperthermia that would kill a human. On the other hand, some ground squirrels can allow their body temperature to drop to near freezing during hibernation, far below the level that would be fatal to a human. It's all a matter of adaptation, within limits.

SD Staff George (Colibri)
Straight Dope Science Advisory Board

[JillGat]

Oohh, George and Doug do a staff report together. My secret sexual fantasy comes to life (of course George just gets excited talking even more about hummingbirds and Doug gets off on saying "honeybee drones").

Back to the topic here, isn't it true that leopard geckos' babies are hatched as males or females, depending on the heat of the eggs as they incubate? And aren't there some mammals with variations in their body temperatures, besides what occurs during hibernation?
Jill

[JillGat]

The naked mole rat Heterocephalus glaber from Africa for example, whose temperature matches that of its burrow for, among many other bizarre, unique biological and behavioural traits.

[JillGat]

If I were still a moderator, I would go back and delete that "for" in the previous post. But it's not worth it.

[Colibri]

Originally posted by JillGat
Oohh, George and Doug do a staff report together. My secret sexual fantasy comes to life.

Do tell. It ain't the first time the two of have gotten together. And when are you going to do one with me, hunh?

Back to the topic here, isn't it true that leopard geckos' babies are hatched as males or females, depending on the heat of the eggs as they incubate?

I don't know offhand about leopard geckos, but that's true of a variety of reptiles, including turtles and crocodilians. Seems kind of risky in case of a heat wave and you end up with 100% males.

And aren't there some mammals with variations in their body temperatures, besides what occurs during hibernation?

Yes, in addition to the repulsive Naked Mole Rat, sloths have exceptionally poor heat regulation for mammals, and have relatively variable body temperaturesquite

[C K Dexter Haven]

<< sloths have exceptionally poor heat regulation >>

... explaining why the cole sloth that comes with my ribs is usually warm.

[Colibri]

Originally posted by C K Dexter Haven
<< sloths have exceptionally poor heat regulation >>

... explaining why the cole sloth that comes with my ribs is usually warm.

Finely chopped sloth, with a mayonnaise dressing, does make a nice accompaniment for BBQ tapir ribs, but it should always be served chilled.

[Akatsukami]

Originally posted by Colibri
I don't know offhand about leopard geckos, but that's true of a variety of reptiles, including turtles and crocodilians. Seems kind of risky in case of a heat wave and you end up with 100% males.

A heat wave probably won't last for the entire reproductive lifetimes of the females hatched out the previous year.

I can't say either if it's the case that sex is determined by incubation temperature for all reptiles. Given that mammals and birds have opposite chromosomal schemes for determining sex (a female mammal is homozygous XX, but a male bird is homozygous WW), however, I'd bet that the reptilian sex-determination is external in all cases.

[Colibri]

Originally posted by Akatsukami
A heat wave probably won't last for the entire reproductive lifetimes of the females hatched out the previous year.

True, probably not an issue for crocs or turtles, but it could play hob with some shorter-lived lizards.

Actually, I have seen speculation that rising temperatures could have helped kill off the dinosaurs by screwing up their sex ratios. (This was before the Alvarez asteroid hypothesis had been proposed.) Extremely unlikely, in my book, but the suggestion has been made.

In any case, a system like that has a lot more potential for being thrown out of whack than a chromosomal system.

[JillGat]

Colibri: [[Do tell. It ain't the first time the two of have gotten together. And when are you going to do one with me, hunh?]]

I'm happy just to keep being the wind beneath your wings, George.