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The Evolution of Homeothermy

Post of the Month: July 2005


Subject:    Re: Lizard engines and rat engines
Date:       5 July 2005

On Tue, 05 Jul 2005 12:22:50 -0600, dkomo wrote:

>Everybody knows reptiles are cold blooded and mammals are warm blooded,
>but not too many people are aware just how exorbitant are the energy
>demands of the heat engines that are the bodies of mammals.
>Typically, mammals require ten times the energy to run their bodies at
>their rated 37 degrees C (98.6 degrees F) as reptiles do, using basal
>metabolic rate as a comparison.
>Chris Lavers in his Why Elephants Have Big Ears provides an
>interesting illustration of this difference. Imagine a lizard and a
>rat of the same size sitting under a tree. If the lizard's body engine
>is idling at, say, 100 RPM, the rat's will be running at 1000 RPM.
>Engine speed equates to the rate of heat-producing metabolic reactions
>within cells.
>But when the engines are pushed, the rat gets much better performance.
>If they both decide to run to another tree a kilometer away, the lizard
>wll rev his engine to 1000 RPM, but the rat will push his to 10,000 RPM.
>If the outside temperature is 38 degrees C to give the lizard a fair
>chance (the lizard's body will be at the same temperature as the
>surroundings, and its muscular efficiency improves as temperature rises),
>the rat will cover the distance at about 88 meters per minute, while the
>lizard will manage only 13 meters per minute.
>However, if the two animals were cars and the lizard got, say, 30 miles
>to the gallon, the rat would only get 3 miles to the gallon.
>Given the severe energy demands of warm blooded animals, it is a wonder
>that mammals and birds (who are also warm blooded) ever evolved from
>their last common ancestor, which was a cold blooded reptile of some
>kind. And this full-blown warm bloodedness must have developed
>independently in the evolutionary lines leading to each.

Yes, it is true that poikilotherms (the "proper" term for animals commonly called "cold-blooded") have a metabolic rate an order of magnitude lower than those of homeotherms ("warm-blooded"). Note: one of the reasons for abandoning the warm vs cold names is that at 38 C, in the example given, the lizard is just as warm blooded as the mammal; when the temperature goes even higher, the lizard will be warmer! And a hibernating mammal may have very cold blood, indeed. So cold-blooded and warm-blooded are not used in technical discussions. A poikilotherm has a body temperature that varies; a homeotherm regulates its temperature relatively constant. Other words you see are endotherm, an animal whose body temperature is pretty much determined by internal heat sources, and ectotherm, an animal whose body temperature is determined primarily by external factors.

The most important advantage of homeothermy, well worth the enormous cost, is not in hot conditions but in cold. In the cold, the poikilotherm cools down and its metabolic rate drops drastically to the point where it may be virtually inert. Many large insects can't fly if it gets too chilly; reptiles (and insects and worms and the like) become very sluggish. As long as everybody is a poikilotherm, predator and prey alike, it isn't too bad. The prey can't get out of the way very fast, but then the predator can't run very fast to catch it. However a bird or mammal that keeps its body warm and can maintain a high and active metabolism even when it is cold will have an enormous advantage.

There are other, secondary advantages. The thermal sensitivity of proteins, hence the thermal sensitivity of biochemical reaction rates and other physiological processes, varies drastically from reaction to reaction and process to process. Although the common rule of thumb says that reactions double or triple in rate for every 10 C increase in temperature, the actual factor of increase can be well below 2 or well above 3. Cells, organs, and organisms require that all the internal biochemical reactions and biophysical processes be well matched in magnitude. If your muscles work 15 times faster in the heat but your biochemistry can only release the required ATP 10 times faster, things break down. Similarly if the first and fourth steps in a reaction chain increase drastically but not the second and third, or if your respiration increases more than your circulation, then things break down quite differently So an organism that does not regulate its body temperature has a great deal of difficulty adapting to wide swings in environmental temperature. Often that takes many days -- good enough to tolerate seasonal changes but not good enough for a sudden heat wave or cold spell, or even the daily changes from night to day. Keeping your body temperature constant allows all the different biochemical and biophysical systems to stay finely tuned and in harmony.

Incidentally, body temperature regulation of one kind or another developed many times in the course of evolution. Most birds and mammals are at one extreme: very strict regulators using internally produced heat to keep warm (endothermic homeotherms). Most aquatic organisms, terrestrial invertebrates, and amphibians have body temperatures roughly equal to the environment (ectothermic poikilotherms). Many reptiles, some large fish, and even some larger insects partially regulate -- that is, keep their bodies different from the environment but still allow some variation, or else regulate just parts of their bodies, usually the core organs and brain, or else regulate just part of the time. These are sometimes called heterotherms. The mechanism might involve using external heat (basking in the sun, pressing against a sun-warmed rock, or otherwise selecting an appropriate microhabitat) or internal (chilly bumblebees "buzzing" to develop enough heat to allow flight). Several species of pythons actively regulate their body temperature using internal heat sources when brooding.

So birds and mammals certainly did NOT have to develop full-blown endothermic homeothermy all at once. It is generally quite well recognized that many of the large dinosaurs probably did regulate their temperature -- very large animals tend to heat up all too easily and the problem is usually how to stay cool, not how to get warm. Whether the synapsid reptiles did so is another story. However it is interesting to note that monotremes regulate at a relatively low body temperature (28 to 32C) and marsupials (plus a few placental groups like edentates and non-shrew insectivores) regulate at 33 to 36C. Most mammals regulate at 37 to 39C. "Primitive" birds like ratites regulate at 38 to 39C while passeriformes regulate at about 42C. That is, "warm-bloodedness" is most definitely NOT an "irreducibly complex" phenomenon where every part has to be in place before any part of it can be of advantage.

The price of thermoregulation by internal heat is, of course, paying the heating bill -- finding enough food to burn to produce all that heat. The problem is minimized by good insulation -- fur and feathers. The problem, of course, is most severe for temperate and arctic zone animals during winter, when food is scarcest. As a result, many small mammals simply give up maintaining their body temperature by hibernating. Large mammals have an easier time for several reasons. First, the relationship between body mass and surface area means it is easier for these to stay warm with less fuel. Second, the unusual relationship between metabolic rate and body size means that these animals can last for a much longer time on stored internal fat -- a small mammal can't lay down enough fat to last all winter. Birds usually just migrate rather than cope with a harsh winter.

This is a subject well treated by any text in Comparative or Environmental Animal Physiology. Schmidt-Nielsen, Prosser, Withers, Hill and Wyse are some authors in this area.

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Running the Scientific Gauntlet

Post of the Month Runner-Up: July 2005


Subject:    Re: The Danger of Becoming a Science Fanatic
Date:       21 July 2005

John M wrote:
>You were very courteous and I appreciate responses like yours. I can't
>say the same for others however.

Welcome to Usenet. Ignore the others, and respond to the ones you consider courteous.

>My thing is, I never presented any kind of theory to anybody. I thought
>this was a free discussion to engage each other in different
>possibilities. I had no idea the theory of evolution was frozen in
>concrete not to be touched or fiddled with by heathens such as myself.

Er, if you never presented anything, why did you take so many lines to not do so?

A possibility is something, however. Once you say you have a possibility, or suggest that it might be, then it comes under the usual fire that any suggested possibility does. That's nothing to do with scientists being arrogant, it is how science is done. We can all (scientist or not) come up with many ideas, dozens, hundreds, thousands of ideas. There has to be some way of thinning the herd, or we'll simply be overrun by the mass of just our own ideas, much less those of the thousands of other scientists or billions of other people.

Science is a method for weeding through the thousands of ideas to get to the few that are worth more serious work. As such, it is very much a selection or filtering process. Much of it requires knowing what is already known, which is a point where your posts have been falling down -- you don't appear (appearances can be deceiving) to know much about what biology knows prior to putting up ideas and getting upset when they're not well received.

Anyhow, I have a set, probably similar to what many scientists use, of questions I apply to my notions to decide if they're worth pursuing. If I (we) apply them to my (our) babies, do you really expect we humans are going to give yours a free pass?

Does the idea accord well with observations? Requires that you learn what the observations really are.

If (when) there are observations the idea doesn't accord well with, are they at least particularly difficult (large error bars, few replications, ...) observations to make? (Perhaps the notion is correct, and the discordant observations have sufficient weakness to not really rule out the idea.)

Most ideas don't make it past this point. Most of the time good observations are discordant with the idea, once you consider them past the couple you started the notion from.

Is the idea new? Many of mine get shot down on this ground -- the idea is often in a field I don't know a lot about prior to the idea. Scientists are very creative, and have been at it for a very long time. Chances are excellent that someone, somewhere and somewhen, has already had the notion and, worse, shown why it doesn't work. I give myself points for the recency of invention or how long it took to shoot down.

Why did nobody think of it before? The preceding questions are pretty common among people thinking about science; this one seems less common. I get good mileage out of it, however, because it can lead me to better answers to the previous question -- 'this is the kind of thing so-and-so might have come up with, are you really sure that he didn't?'. Answers like 'I'm such a sooper geenius that those morons can't even understand my brilliance' are not allowed. What is it that I knew about in coming up with the idea that people in the field didn't, or don't commonly? (new observations, ideas swiped from one field and applied to another, new mathematics, new computational abilities, ...)

I've recently had a particularly good notion, in that it actually survives all the preceding questions. So now I've taken the first step towards 'going public' -- talking to someone knowledgeable in a relevant area. But first I do my 'homework' to have some confidence that the idea is worth somebody else's time. He's actually more enthusiastic about it than I am. (!) If I can nail down the observational details, which I'm well along towards, I'll have solved a problem in his niche that has been outstanding for over 100 years. (In answer to my last question: a) because I'm swiping ideas from 3 fields that don't normally look at each other b) because some of the effects buttressing the notion have only become observable in the last few years.)

If you consider yourself to have presented an idea, you've failed to test it against any of the preceding questions. It isn't a sin, but it does mean that you'll get a barrage as people apply them for you. It's a rough process. But nobody is going to be much easier on your notions than they are on their own; and I'm pretty brutal with mine -- I have a lot of them.

The weeding out of ideas is also one of the great pleasures of doing science. Arguing about how good they are is a staple of lunchtime discussion and conference hallway meetings (say, did you see that talk by A? I thought it was pretty good. No, C says it ignores X, which kills it. But it answers X by Y, which seems a good response. ...)

From The Acoustical Foundations of Music 2nd edition, by John Backus, Norton, New York, 1977. pg xiii:

"[One] way of dealing with errors is to have friends who are willing to spend the time necessary to carry out a critical examination of the experimental design beforehand and the results after the experiments have been completed. An even better way is to have an ennemy. An enemy is willing to devote a vast amount of time and brain power to ferreting out errors both large and small, and this without any compensation. The trouble is that really capable enemies are scarce; most of them are only ordinary. Another trouble with enemies is that they sometimes develop into friends and lose a good deal of their zeal. It was in this way that the writer lost his three best enemies."
quoting Georg von Bekesy, Experiments in Hering, New York, McGraw-Hill, 1960 p8..

von Bekesy was a Nobel Laureate in physiology.

Robert Grumbine Science faqs and amateur activities notes and links.
Sagredo (Galileo Galilei) "You present these recondite matters with too much
evidence and ease; this great facility makes them less appreciated than they
would be had they been presented in a more abstruse manner." Two New Sciences

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Speciation and Chromosome Number

Post of the Month Honorable Mention: July 2005


Subject:    Re: Request
Date:       20 July 2005

"Zoe" wrote...

> Following is a portion of a post in response to Howard Hershey, which
> he has not answered. Can anyone else please provide an answer to the
> following evolutionary scenario?
> Can you trace the pathway of how chimps and humans manage to
> produce offspring as they first separate from their common ancestor?
> The common ancestor has, let's say, 23 pairs of chromosomes. A rare
> "beneficial mutation" causes an offspring to be born with an extra
> pair of chromosomes. He is the forerunner of the chimp. Where does
> the human ancestor come in? Does the common ancestor also produce an
> offspring with 23 pairs of chromosomes, but with some other rare
> "beneficial mutation" that takes the offspring, now ancestor to
> humans, in a different direction?

For a couple of reasons, it seems more likely that the last common ancestor of humans and chimps had 24 pairs of chromosomes. First, chimps, gorillas, and orangutans all have 24 pairs of chromosomes (which makes it seem more likely that was the number of the last common ancestor of great apes, and continued until it was reduced in the human lineage). Second, while all chromosomes have a centromere in the center and telomeres at the end, human chromosome two has a vestigial centromere and telomere embedded in the chromosome itself, suggesting that it was formed by a fusion of two chromosomes. Indeed, there are two separate chromosomes in chimps that are very similar in sequence to two "halves" of human chromosome 2.

By the way, although you didn't explicitly ask, the okapi (a rare, short-necked giraffe) species has individuals with 22 pairs of chromosomes, 23 pairs, and even 22.5 pairs (for 45 chromosomes in all -- in which case two chromosomes from one parent must be paired with one -- fused -- chromosome from the other parent). So a mutation that produced the first human with a chromosome 2 (rather than the ancestral chromosomes 2a and 2b) would not have prevented that individual from mating successfully. Or, take the case of Przewalski's horse and the domestic horse: domestic horses, like their human breeders, have one fewer chromosome pairs than their wild ancestors, due, apparently, to a chromosomal fusion -- but domestic and Przewalski's horses can still interbreed to produce fertile offspring. In other cases (e.g. "chromosomal races" of mice), having different chromosome numbers reduces interfertility.

There is no reason to suppose that the difference in chromosome numbers started with the last common ancestor; it may well have been much more recent, long after the human line had separated from the chimp line. There's no particular reason, for that matter, to suppose that the human line started out with some particular beneficial mutation, rather than our ancestors simply moving into a different part of Africa from Cheetah's ancestors, so that they could no longer interbreed (geographical separation prevented any new beneficial -- or neutral or harmful -- mutations in the hominin line from entering the chimp gene pool).

> Here's your budding tree.

> common ancestor       Chimp ancestor (single individual)
> _____________________/
>                      \
>                       Human ancestor (single individual)

No, surely the branching involved entire breeding populations -- one band, or a few bands, of apes moving into a new territory far from the lands where other members of their species lived. As noted, at the branch point, both populations would have been apes of the same species; they wouldn't become different species until after the branch point, after geographical separation left them free to evolve in two different directions. Remember that, just as there was no "first French speaker" struggling to make himself understood in a nation of classical Latin speakers, so there was no "first human" or "first chimpanzee," but only a gradual change over many generations from the same ancestral species.

> Can you take it from there? What's the pathway? At this point, can
> the chimp ancestor still interbreed with either the common ancestor or
> with the human ancestor? It has to interbreed with something in order
> to produce more offspring after its own kind, so where does the
> partner come from?

Most evolutionists hold that most speciation events are "allopatric," meaning that they occur after the ancestral population has split into two groups that could interbreed if they met, but which no longer meet up. Afterwards, mutation, genetic drift, and selection to different environments gradually change the populations into different species. No particular mutation (unless you count polyploidy) is likely to produce a new species. A better (though still oversimplified) approach would be to think of a whole series of mutations, some beneficial, most neutral (but they still made us different from chimps), that each made the bearer a tiny bit more "human" (or, in the other lineage, a tiny bit more "chimp"). No single gene would have made its bearer much different from other members of his species, or unable to interbreed with them.

It doesn't seem likely that a modern human could (or at least would) interbreed with a modern chimp, but presumably five million years ago, our ancestors were just a tiny bit more "human" than the ancestors of modern chimps. They probably could have produced fertile offspring with the chimp ancestors, but as noted, they lived in different parts of Africa and no longer met.

By the way, polyploidy is duplication of the entire genome; plants speciate this way all the time, but it's rarer for animals (though there are strongly supported examples for frogs, rodents, and other vertebrates; presumably, they can't form a new species unless they can either reproduce parthenogenically, or unless polyploidy happens often enough that eventually it produces two members of the same species at the same time and place). But this has nothing to do with how humans split off from apes.

-- Steven J.

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