*(This is not an old transcript)
ZOO 138, Wednesday, January 12, 1997, 12:00 p.m.
Today's lecture is not in your lecture transcripts. So aren'tyou glad you are here?
I think that by Thursday afternoon I will have a transcript ofthis lecture available on a web page. So people can surf the netcan get at it, and I'll probably try to get a copy put on reservein the library as well. Your midterm is a week from Friday, sothat will give you a little bit of time.
Today I'm going to talk about thermoregulation and metabolicrate. One of the differences between the aquatic and terrestrialenvironments is the daily and seasonal changes in temperature.
And the problem that creates for animals making the transitionfrom water to land is that there is a potential for changes intemperature of the animal's body. Those body temperature'schanges can be so great as to be lethal for the animal. In otherwords, anytime an animal's body temperature drops below thefreezing point of the body fluids, which is just a little bitbelow zero Celsius, when the temperature drops below the freezingpoint of the body fluids ice crystals form in the cells.
And because ice is less dense than water, that means when thewater molecules go into the solid state. They actually expand asyou probably know if you have ever put a bottle of fluid in therefrigerator and had it freeze and break. It ruptures the cellmembranes of the cell when ice crystals from.
That would be one cause of a low temperature a way in which alow temperature would kill an animal. When temperatures get to beto high proteins become denatured. That's what happens when wecook something, we are raising the temperature and we are causingthe proteins to lose that critical tertiary structure.
However, even temperature changes that are not that extreme,not so high as to cook the animal or not so low as to freeze theanimal, even those changes in temperature can create problems foran animal's metabolism, because like any chemical reaction therates of the reactions are changed by the temperature.
So the way in which animals respond to this potential for thisproblem of changes in body temperature is by regulating theirbody temperature. Now, a physiologist uses the term"regulate" in a different way from the term"control."
Now, those 2 terms -- in everyday language those 2 terms areyou know, sort of similar or at least the differences betweenthem seem to be a little bit hazy. But a for physiologist theymean very different things. Regulate is a term that physiologistsuses for something that is held relatively constant. And controlis used for some other physiological parameter that is varied ina methodical conscious way, not necessarily conscious in themind, but is varied in a way usually so that something else canbe held constant.
So, for example, the temperature in this room is regulated. Itis held at a relatively constant value by a controller, thatlittle gray box back there on the wall. And that little gray boxback there on the wall is controlling the heat output of theheater in this building. So if air temperature in here starts todrop, the thermostat, which is what we call that controller,increases the heat output of a heater.
If it's very, very hot outside, the temperature of the roomstarts to rise, the thermostat activates an air-conditioningsystem that takes heat out of the air. So output of the andheater and the air conditioner are changed or controlled in orderto keep air temperature constant.
That's what we see when we look at body temperature ofanimals. Some things are controlled. So that body temperature canbe held constant. There are 2 basically different strategies thathave evolved among vertebrate animals for body temperatureregulation. And those strategies involve control of differentthings.
And to some extent I'm making a false dichotomy. In otherwords, I'm saying it's either/or, but, in fact, there are somegray areas ways. There are ways in which these things overlapwith one another. But in terms of how we think about it to begin,with we consider that there are 2 forms of body temperatureregulation.
Let me quickly describe these for you. The one that youpersonally experience as a mammal, as do all mammals and allbirds, involves increasing the heat output of your body. If yourbody temperature is starting to drop below your set point, whichis 98.6 Fahrenheit, which is about 37 Celsius, if your bodytemperature starts to drop below that, what happens? What are theexperiences that you have? You shiver.
And what is shivering? It's just high amplitude, lowfrequency, muscle contractions. Heat is produced when the ATP issplit in the cross bridge cycle. So shivering is a mechanism ofheat production. What else happens when it's cold? Do you haveany other experiences when it's cold?
STUDENT: You turn blue.
INSTRUCTOR: That's true. And what is that about? That's aboutdecreased blood flow to your skin. And so that if your blood isnice and warm, it's just come from heart down in the core of yourbody, and when you are trying to conserve heat you don't send asmuch of it out through the skin. And that helps to reduce yourheat loss. And that's a second mechanism of a way in which yourbody responds.
There is another one besides putting on a sweater, thatdoesn't count, most mammals don't have that as an option. Whatelse is your experience with being cold? You have goose bumps orthese little bumps on your skin. Well, the interesting thingabout that is that human beings don't have all the necessaryequipment for that, but that's a common mammalian response.
What is happening there is that the hair follicles, eachindividual hair goes down into the skin and there is little tinymuscle that is attached to the base of the that hair, and whatyour body is trying to do is increase the thickness of your furcoat of your hair, of your fur by making the fur stand up on end.Although, most of us don't have very much fur to stand up on end.
In a mammal that does have a good fur coat, those same musclescontracting cause the fur to stand up on end, makes it thicker,so that what's really insulating a mammal or a bird when itstands its fur or its feathers up on end is the air that'strapped in there. You have this stabilized layer of air. Air isvery poor conductor of heat.
So that's another one the responses that's involved,increasing your metabolism, decreasing blood flow to the skin,and increasing the thickness of your insulation.
Now, let me describe to you an alternate mechanism, which is akind of thing that you would see if you were to study a lizardout in the desert.
In fact, let me describe to you an experiment that's really avery fun experiment. You go out to the desert, and you have,like, a short piece -- like a fishing rod, piece of bamboo, orsomething like that.
On the end of it, you have a little tiny length of nylon likefishing line tied into a knot so that makes like a noose. And youwalk up to this lizard, and if you are three or four feet awayfrom the lizard he will freeze because he thinks if he sits verystill you can't see him. Then you dangle this little noose overhis head and you pick him up.
He thrashes around. You are not doing it fast, it's not likeyou are strangling the guy. But you pick him up and he thrashesaround like a fish on a fishing line. And then you take a littletiny thermometer, it's a fast acting thermometer, and you stickit into his cloaca and you measure his body temperature.
There are more sophisticatedwa----********************************************-ys to do itnow. But back in the 40s when these experiments were first done,they were affectionately referred to as noose'em and goose'emexperiments.
If you do a noose'em and goose'em experiment, and you go outthere, and you get up really early in the morning, you go out toPalm Springs really early in the morning, and you start catchingthese lizards, you can get some data that might look like this.
We're going to make a graph made of body temperature obtainedthrough this noose'em and goose'em experiment as a function ofthe time of day. So there is going to be 12 noon. Here is 6o'clock in the morning, when we first get up. Let's say here is6:00 o'clock at night, which is 1800 hours on a 24 hour clock.
And if go out there and we are studying, let's say we studythe desert Iguana, which is really neat big lizard. They mightshow up out there, say, at around 6:00 o'clock in the morning.And what we would see if we caught animal or we're going to catchan animal and we're going to plot the time of day, thetemperature that we observed, and maybe that's somewhere around40 degrees. 42, 44 degrees Celsius.
And every time we catch an animal we are going to put a dot onthe graph here. We might get a set of data like that prettyeasily in a day.
Now, we could be also studying another lizard, same sizelizard, same environment. We catch one lizard, and another. Sowe're catching data. What we might find is that in this lizard weget data that looks like this.
And we might also at the same time decide, well, we need somekind of comparison. Let's measure air temperature. And airtemperature, if we were out in Palm Springs in the daytime in thesummer it might start down here like this, reach a maximum uplike this, and be back something like that in the course of aday.
Now, you are a scientist -- at least, you want to be ascientist or are forced to be a scientist. Tell me how are yougoing to interpret these data? What do these data tell you? If Isay I have a hypothesis that lizards actively regulate their bodytemperature, what evidence is there in that graph that would beconsistent with my hypothesis?
STUDENT: Each specie of lizard has roughly the sametemperature no matter what time of day.
INSTRUCTOR: So basically the fact of the matter is that youcould calculate an average temperature here for that specieswhich is pretty flat. But "pretty flat," is kind of arelative term. Relative to what? What would you use as yourcomparison? Air temperature. The body temperature is a lot morestable than the air temperature is.
If you measured temperature of the soil, it would be evengreater in soil temperature. That's one piece of evidence is thatthe body temperature of the animal is more stable than thetemperature of the air.
Now, there was a guy in the 50s who really made a name forhimself, because he's like a lot of zoologists, when he's out hespent a lot of time drinking beer. And so he had all these emptybeer bottles around. And he filled them up with water, and hestuck them out underneath the bushes and out in the open, all thesame kind of places where -- the places where previous people hadbeen catching lizards.
And didn't have to use the lizard stick anymore, he just tookhis little thermometer around and he measured the temperature ofthe little bottles of water. And essentially, what he was askingwas -- you know, air temperature is going to change more than abottle of water sitting in the shade. So how can you allege thata lizard is actively regulating his body temperature? You needsome comparison like the one we just made.
He was comparing the temperature as a function of time in alizard-sized bottle of water, which animals are mostly water, soit has about the same kind thermal inertia as an animal as afunction of time.
And he found that it was also varied less than hairtemperature did. But it wasn't as stable as this. But he wasmaking the point, which is a good point, and that is that youneed to have some kind of comparison if you "say, It'sstable." Well, it's relative to what? It's not absolutelyflat.
What about the significance of the red points on that curve?That's another species of lizard. And we're going to make this bea fair comparison, a valid comparison, which means that we haveeliminated things that could cause this difference. In otherwords, if I just put those data up there and, say, "Well,look, there's another species and its temperature isdifferent."
Well, if it was in a different habitat, then it wouldn't be afair comparison. If it was a tremendously different sized lizard,that might mean it isn't a fair comparison. So I said this is afair comparison. This is same size lizard, different species, inthe same habitat on the same day. Because, obviously, if youmeasure one in summer and one in winter that could be the causeof the difference.
So we're going to try to make a fair comparison here. Andhaving made a fair comparison, we now have a second line ofevidence that really argues against the beer-can theory. Becausethis is the same size lizard in the same habitat on the same dayand it has a different temperature. It has a different averagetemperature.
And what the physiologist would call this temperature, eitherone of these, they would called this a "preferred bodytemperature." T sub B is the symbol we use for thetemperature of the body. This is a different preferred bodytemperature. We also sometimes call that an ecritic temperature.Ecritic can refer to the same thing as a preferred bodytemperature.
When we have 2 different species of lizards living in the samehabitat on the same day with the same body size but differentecritic temperatures, that also supports the idea that lizardsare actively regulating their body temperature.
Because if you study 2 different brands of empty beer bottlesof the same size in the same habitat on the same day, they wouldhave the same body temperature at any given time in one location.
So there is 2 lines of evidence that can be obtained from oneof these natural history kinds of studies, a noose'em andgoose'em study, that tells us that those lizards activelyregulate their body temperature. Now, if we observe the animal,we can see how it's doing that.
What we will see is in the morning the lizard, they live down-- they spend the nighttime when they are not active, they aredown in a burrow, buried in the soil someplace. They are going tocrawl out of the burrow, and they are going to find a nice hotrock or hot piece of sand, and they are going to lay down andflatten their body out on it, just like you did when you were alittle kid when you got out of the cold swimming pool and laid onthe hot cement.
They are laying there in the sun. Maybe they are up on a slantso that their body is exposed to the maximum amount of the sun'srays. And then after a while, after they get hot, then theymight, you know, go sit in the shade for a while. And after theycool up off they so back out in the sun again. Maybe they arerunning around in the sun looking for food and they start to gettoo hot and they go back into the shade again.
These animals are regulating their body temperature bybehavior by choosing to go to hot and cold places in theenvironment. There are places in the environment that are hotterand places in the environment that are colder. Those are called"micro environments."
In other words, the environment that a large animal like youout in the desert at Palm Springs experiences is just the airtemperature and the radiation coming up off of the soil andeverything. But a little lizard can find little tiny microenvironments, places that are cooler or hotter. And they exploitthose micro environments by going to them behaviorally.
And they might be darker. A lizard might turn its skin realblack in the morning when it's trying to warm up. And then whenit's getting to be too hot and wants to be running around lookingfor food, it contracts these little dark cells in the skin, andthen the animal is actually is whiter, physically looks whiterwhen it's hot than it is when it's cold. By making that skinblack it is trying to absorb more of the sun's energy.
Now, I said there were 2 strategies. I have just describedthem. One of them is the human being or any bird or mammalshivering, changing blood flow to the skin, changing thethickness of the insulation.
On the other hand, we have the lizard which is running intothese micro environments. If I was to tell that the 2 differentstrategies vary in terms of what is controlled, one of them wewould call "behavioral temperature regulation." And oneof them we would call "physiological temperatureregulation."
Which of those is the lizard, behavioral or physiological?It's behavior. Why don't we call the bird or mammal systemphysiological temperature regulation? Because changing bloodflow, contracting muscles, getting goose bumps, and standing yourfur up on end, changing your -- and shivering, those are allphysiological processes.
Now, if you're cold, have you ever thought of going andstanding in the sun? Sure. When I say that birds and mammalsutilize physiological temperature regulation, they can also usebehavioral temperature regulation.
And the lizard that contracts the little -- they're calledmelanophores, the little black things in their skin to changetheir color. Is that behavior or is that physiology? That's kindof physiology.
Let's say it's really hot. What happens to you when it'sreally hot and you have to be out the in heat? You sweat. What ishappening with sweat is that water is evaporating off your skinand so it cools your skin. Dogs pant. There aren't very manymammals that sweat, but there a lot of mammals that pant.
Not a lot of mammals have sweat glands, for example. Birds --no birds have sweat glands. No bird sweat. But panting is anothermeans of doing the same thing, evaporating water. And, in fact,lizards pant also. So if you consider panting to be aphysiological mechanism, then reptiles have some of that as well.
So it's really not an absolute difference between behavioraland physiological temperature regulation, but it's really amatter of the relative importance of one or the other.
When an animal is doing some components of physiologicaltemperature regulation, particularly when it's shivering. It isgenerating heat within its own body. So the source of heat, thereare 2 different sources of heat that animals can use forregulating the body temperature.
Those 2 sources of heat are energy that comes directly fromthe sun. That's what the lizard is doing when goes out and sitsin the sun and warms up. It's what it's doing when it goes andfinds a hot rock and lays on it, because that's energy that camefrom the sun recently. The other source of heat is the animal'smetabolism.
That is the heat produced through muscle contraction and allthe metabolic processes that take place in the cell whencarbohydrates and proteins and lipids are oxidized there is a lotof heat released. Not all the heat is trapped in ATP right away.
Only about 35% of the heat is trapped in ATP and the rest ofheat the is just lost, released in the animal's body. That'smetabolic heat production.
So those 2 sources of heat are really used -- are available toall different kinds of terrestrial vertebrates. And, in fact,they are even available to plants. Did you know that there areplants that generate enough heat to change their own temperature?Very interesting observation. There are many insects that dothat, too, by the way. But we're talking about vertebrate animalshere.
So if an animal has -- because both are available, but what istrue is that in some animals, one of these sources is much moreimportant. And in other animals, the other source is much moreimportant in terms of being a primary source of heat determiningan animal's body temperature.
The terms that we use to distinguish between these are"ectotherm" and "endotherm.""Therm" means heat. Endotherm and ectotherm. Now, oneof those terms applies to an animal in which the sun is theprimary source of heat. And one of those terms applies to ananimal in which its own internal metabolic heat production is theprimary source of heat. Now, which of those terms is which?
If the sun is your primary source of heat, then you are anectotherm, outside heat.
And if your own internal metabolism is your primary source ofheat then you are an endotherm.
That means, the primary source of heat. We already said thatif it's cold, you might go stand in the sun. There you are usingsome of the sun's energy direct.
Lizards have some internal metabolism as well. A lizard has ametabolic rate. It has to eat. It makes poop. It has to haveenergy. But the problem is that metabolic rate of a bird or amammal is a very, very much greater, it least 7 times greaterthan the metabolic rate of a comparable lizard. At least 7 timesgreater.
Therefore, because the metabolic rate or amount of caloriesliberated per minute is at least 7 times greater that allows thatto be the primary source of heat. So birds and mammals areendotherms because they have higher metabolic rates. And thatestablishes metabolism as the primary source of heat for thoseanimals.
Now, what is a comparable animal? Again, it's like making afair comparison. We want to have a fair comparison between a birdand a mammal. We have to find 2 different species that in whichthe only thing different between them is ones a reptile and theother is a bird.
In other words, the only thing that's important, only thingthat is likely to influence metabolic rate is just the class towhich the animal is assigned. Other things that are likely toinfluence metabolic rate need to be the same.
What are some of the things that are likely to influencemetabolic rate? Size. Do you have to feed a big dog more than youfeed a little dog? You bet.
If we want to compare the metabolic rate of a bird and lizardthen they better have the same body size.
And I already said that chemical reactions occur more rapidlyat higher temperatures, right? We know that. You heat up achemical something to make a reaction happen faster.
So we need to find a lizard and a bird with the same bodytemperature, or a lizard and a mammal if we want to make thatcomparison. That lizard, those species of lizards over there,they are out there in their natural environment where they areable to utilize all their normal behavioral repertoire and gofind micro-climates.
If you take a lizard and you put him in a laboratory, itdoesn't have the sun shining on it. It doesn't have all thiselaborate micro-climate stuff. You can take that lizard and putit in a constant temperature cabinet. It's like an incubator orsomething like that, where you mechanically regulate the airtemperature. And lizard's body temperature will be equal to theair temperature.
If there is no sun shining on that lizard, its bodytemperature will be the same as the air temperature. So you couldput that lizard in an environment where its body temperature wasthe same as the bird. And the bird's temperature is going to bearound 40 or 42.
So if you got a lizard like that, that would be his preferredbody temperature anyway. So we are going to find that acomparable animal is the same size and same body temperature, andboth of them need to be quite. Neither can be actively runningaround. It takes more energy to run around than it does to sitstill.
If we do that then we will find under those conditions thatthe metabolic rate, the rate of oxygen consumption, the rate ofCO2 production, the rate of energy utilization, the rate of heatproduction, any of those things can be used for metabolic ratewill be at least 7 times greater in the bird than it is in thereptile.
And, therefore, metabolism is the primary source of heat inthe bird or a mammal and we can refer to them both as endotherms.The way in which they became endotherms was by evolving highermetabolic rates. Now, they had to evolve a whole bunch of otherthings as well.
And that's what my lecture on the Synapsida will be about, howmammals made that transition.
But if we look at the relationship between body temperatureand air temperature in a mammal, as an example, you have a graphthat looks like this right at the end of your Illustrated Notes.This is a comparison of really 3 different variables on 2different graphs. And it's at the end of your Illustrated Notes.
It looks like one graph right above the other. And one of themgoes like this. So you can use that graph and you can fill in,you know, those little slanted dark lines, what I want you to dois fill in the labels on this graph.
And you really need to be able to reproduce this graph for theexam.
What I'm trying to represent here is the relationship between3 variables. Environmental temperature, body temperature, and therate of heat production or the metabolic rate of animal.
It's difficult to produce a single graph that represents 3different variables. You can only do 2 at a time in a standard XY plot. What we're going to do is, on the upper graph we're goingto look at the relationship between body temperature, and "Tsub B" means body temperature.
One of the things you need to get out of this lecture is thesedefinitions of these various abbreviations.
T sub B equals body temperature.
And T sub A is ambient temperature. That means air temperatureor environmental temperature.
And so let's just direct our attention to that upper graph fora second. What we can see is that we have one part of this graph,which is a straight line, but it has ends on it. This is asegment of a line. This does not go on to infinity.
This is a graph of body temperature versus ambient temperaturefor a bird or a mammal. And what we can see is that this animal'sbody temperature is pretty constant. But that there are limits tothe range of ambient temperatures. That's why there are ends tothis line. The points that represent the ends of the line areenvironmental temperatures, they are ambient temperatures.
And they are the temperatures beyond which the animal dies. Sothey are called "lethal temperatures." This is "Tsub UL." Upper lethal temperature.
Temperatures higher than that, you have a cooked animal. Theother one is "T sub LL." Lower lethal temperature. Thisis T sub is LL, here. Temperatures lower than that you have afrozen animal.
Now, the slanted line here, which is dashed in yours, shouldnot be as long as it in your diagram. You should shorten it up sothat is considerably shorter than the flat line.
And that is the relationship between body temperature andenvironmental temperature for a lizard in a physiologylaboratory. That is not a lizard that exploit its microenvironments. This is a lizard that is trapped in an experimentaldesign -- in a can, in a constant temperature cabinet in alaboratory some place.
And the point is that this lizard, when he cannot exploit themicro climates, his body temperature is going to equal to the airtemperature. He's going to be like a hot-dog or piece of meat.
And obviously this point right here is pretty close to thezero. When the air temperature gets down to freezing you have afrozen lizard Popsicle. And this temperature up here is probablyaround 50 degrees Celsius something like that. That's where youhave lizard on a stick.
But the point is that the range of temperatures that the birdor the mammal can handle is much broader because they havephysiological capacities. And even within that range their bodytemperature is relatively constant.
So those 2 patterns, the relationship between body temperatureand environmental are described by another pair of terms. One ofthese is "Homeotherm." And the other one is"Poikilotherm."
Now, I don't expect you to have a clue what poikilo means, butwhich of those lines, the flat line or slanted line, would yousay most likely is the line for Homeotherm? The flat line. Homeomeans what? Same or constant.
So the Homeotherm line is this one. And Homeotherm meansconstant or same body temperature. Poikilotherm means varying orchanging body temperature. And a lizard or an amphibian or a fishis a Poikilotherm, it's body temperature changes when it'senvironmental temperature changes.
A bird or a mammal is a Homeotherm. They are able to have afairly stable body temperature over a fairly wide range ofambient temperatures.
Well, what is it that the mammal or the bird does -- what doesthe bird or the mammal control so that it can regulate its bodytemperature? That's the question. Part of the answer is rightdown here in the lower graph.
In order to keep body temperature constant, heat productionmust be changed. It's the same thing that's going on in this roomright now. In this room air temperature is held constant bychange in heat production. Well, part of the answer for a bird ora mammal involves changes in metabolism.
And so we need to take a look at this lower relationship whichis the relationship between metabolic rate, and we can measurethis in units of, say, calories per hour or kilo-calories perhour. That's the amount of heat produced per unit of time. That'sthe metabolic rate of the animal as a function of ambienttemperature.
That's why these 2 graphs are drawn one right below the otheris we have exactly the same X axis on both graphs. So that meansthat you can relate a particular metabolic rate to a particularbody temperature, because they are all measured at exactly thesame ambient temperature.
So when we look at this metabolic rate down here, which is thelowest metabolic rate, the body temperature of the animal is 37degrees. Over here, when the metabolic rate is higher. Bodytemperature is still 37. This is Homeotherm all after. All theway down here, this is below freezing. The metabolic rate of thisanimal is as high as it's ever going to be. The body temperaturethe still 37 degrees.
The animal is controlling its heat production in order toregulate its body temperature. It is keeping its body temperatureby changing its metabolic rate.
Now, physiologists are very interested in, or used to be, it'sa sort of historic area of physiological study of metabolism andhow it varies in different sized animals and how it varies as afunction of temperature.
What they found was that there is a range of temperatures,which I'm going to direct your attention to right now. These areanother pair of environmental temperatures. And between thosetemperatures, the animal's metabolic rate does not change.
Look at your graph that you have in front of you, the twolittle slants in the bottom picture depict these points. Thedetermination of these temperatures was very interesting tophysiologists because they really wanted to know the whatmetabolic rate was between those temperatures, because that's thelowest metabolic rate you can get in this animal.
You can only get that if the animal is resting and hasn'teaten recently and a number of other things as well. But theywanted to know what that minimum metabolic rate was called orwhat that minimum metabolic rate was. And in order to make surethey were measuring, they had to be sure that they were betweenthese 2 temperatures. And so they called those the "criticaltemperatures."
So that we have a "T" with subscripted"UC" upper critical. And we have t subscripted"LC." The lower one, the one on the left is lowercritical. "TLC." Lower critical. "T sub UC"is upper. Upper critical temperature.
Again, those are environmental temperatures. They aredifferent. What is the animal's body temperature at uppercritical temperature if this is a mammal? What is the animal'sbody temperature if this is a mammal? 37 Celsius.
What is the animal's body temperature at the lower criticaltemperature? 37. What is it 15 degrees below lower criticaltemperature? What is animal's body temperature.
STUDENT: 37.
INSTRUCTOR: What is it five degrees upper criticaltemperature?
STUDENT: 37.
INSTRUCTOR: 37, 37, 37, 37. If you find yourself talking aboutthis, thinking about it, or writing it on an exam, if you everhear yourself saying "body temperature changes," youneed to slap yourself on the hand. This is a Homeotherm. What ischanging is the animal's metabolic rate. It is changing itsmetabolic rate in order to keep its body temperature constant. Itis turning up the heat production of its heater in order to keepit constant.
So those critical temperatures were very interesting to thephysiologists who were trying to measure that lowest metabolicrate; but they are not very important to an animal.
Most animals probably live a good portion of their lives attemperatures that are lower than lower critical temperature orhigher than higher critical temperature. The critical thing is tophysiologists; not to the animals.
Now, the range of temperatures that these physiologists wereinterested in, the range of temperatures that are between lowercritical and upper critical temperatures, that's a veryinteresting range it's called the "TNZ." The"thermal neutral zone."
Because changes in temperature in that range have no effect onmetabolic rate. Metabolic rate is constant over that entire rangefrom upper critical, all the way down to lower critical.
The thermo neutral zone is the zone of ambient temperatureswhere there is no change in metabolic rate. That's what theneutral part refers to. There is a neutral effect or no affect onmetabolic rate.
That's the thermal neutral zone between the upper and lowercritical temperatures. The metabolic rate that we measure in thethermal neutral zone is the "basal metabolic rate" orthe "BMR." That's the metabolic rate. That's the valueon the Y axis that corresponds to the points within that zone.
Basal metabolic rate or BMR.
That's what the physiologists were interested in measuring,the basal metabolic rate of these animals.
It's "basal" because it's the "bottom."It's the lowest metabolic rate we can get in this animal.
If we study this animal at temperatures below the lowercritical temperature what we find is a higher metabolic rate.That is the range that's called the "range ofThermoregulatory Thermogenesis." That's definitely a 75-centword.
"Thermogenesis" means heat production, to generate.Heat production. "Thermoregulatory" means heatproduction done for the purpose of regulating body temperature.This animal in this zone down here, the zone of ThermoregulatoryThermogenesis, this animal is increasing its heat production forthe specific purpose of keeping its body temperature constant.Shivering is one of things the animal is doing down there.
This zone up here, the zone that is above upper criticaltemperature, that's also temperatures that are higher than theanimal's body temperature, that's the zone of "augmentedevaporative water loss." That's where the animal is sweatingor panting. That's why an animal sweats or pants is to augment orincrease evaporative water loss.
Because it's only by evaporating water than an animal cancool, they don't come equipped with a compressors and airconditioners. All they have is swamp coolers. And that's how ananimal cools itself by increasing its water loss. Panting andsweating. And that allows the animal cool.
It takes energy to pant or sweat. The body must dophysiological work. To pant it has to pull air in and out of thelungs. To sweat it takes water out of the blood, activelytransports the ions out of it and puts it out into the skin.
So metabolic rate goes up in a circular linear fashion when ananimal is having to pant or sweat. It is not making that heatbecause it wants that heat. It is making that heat because that'sthe only way it can make sweat. It's different. Okay.
This heat below it in a zone of ThermoregulatoryThermogenesis, this animal is making that heat because it needsthat heat to make up that for the heat that it's losing to thatcold temperature. This animals doesn't really want the heat, butit doesn't have any choice but to make this heat, because that'sthe only way it can sweat and pant. It gets rid of more heat bypanting and sweating than it produces in the process of doingthat.
Now, why were the physiologists so interested in basalmetabolic rate. That's the one that is 7 times higher in the birdor mammal than in the reptile. If this is a plot for a bird or amammal right here, remember we were going to make thatcomparison, same body size, same body temperature, by putting thelizard in a constant temperature environment, we're going tomeasure the metabolic rate.
The metabolic rate of the lizard is going to be way down heresomewhere at one seventh of the value for the bird or mammal.
But what do you suppose happens to the metabolic rate of alizard when we put it in this same constant temperatureenvironment and lower the ambient temperature? What happens tothe body temperature in the lizard? It goes down. What happens tothe metabolic rate of the lizard when we cool its body? Whathappens to any chemical reaction when you lower the temperature?It goes down.
So if we just look at this little tiny part of the graph righthere and kind of blow it up so we can see it -- if we look thethis graph and blow it up a whole lot, if this is the pattern --there is the lower critical temperature. If there is a patternfor the mammal or the bird, here is a reptile right here. This isalready one seventh. This guy is going to drop down like that.
So as we get to progressively lower temperatures that arestill tolerable by the lizard, this could be off by 40 fold. It'sonly off by 7 fold here because this is as close as they get. Atlower temperatures the lizard is going to have a lower bodytemperature and a lower metabolic rate.
The mammal is going to be involved in ThermoregulatoryThermogenesis. She is going to making heat in order to keep itsbody temperature constant. Therefore, the difference in theirmetabolism is going to be much greater and reach values as highas 40 fold at very low temperatures.
Where the lizard is still alive, but he's not very active.
The 2 sources of energy are available. The sun, which isutilized in behavioral thermo regulation by lizards andamphibians and reptiles and fish and internal metabolism, whichbirds and mammals have evolved the capacity to have tremendouslyhigher metabolic rates.
And because they have higher metabolic rates their ownmetabolism is the dominant source of heat. They are endotherms.And they are, therefore, able to be Homeotherm. They can have aconstant body temperature in the face of a changing environmentaltemperature. They can survive over a much wider range oftemperatures.
When we talked about the distribution of lizards, we saidprimarily in the temperate and tropical regions. You don't worryabout lizards getting chased by polar bears, do you? You don'tworry about penguins chasing snakes. Why? Because they can'tsurvive in that cold of an environment, because they arepoikilotherms, they are not endotherms.
ZOO 138 Unedited Lecture Transcript (W97) Metabolic Rate andThermoregulation Page 2