ZOO 138, Wednesday, January 12, 1997, 12:00 p.m.
Today's lecture is not in your lecture transcripts. So aren't you glad you are here?
I think that by Thursday afternoon I will have a transcript of this lecture available on aweb page. So people can surf the net can get at it, and I'll probably try to get a copy put onreserve in the library as well. Your midterm is a week from Friday, so that will give you a little bitof time.
Today I'm going to talk about thermoregulation and metabolic rate. One of thedifferences between the aquatic and terrestrial environments is the daily and seasonal changes intemperature.
And the problem that creates for animals making the transition from water to land is thatthere is a potential for changes in temperature of the animal's body. Those body temperature'schanges can be so great as to be lethal for the animal. In other words, anytime an animal's bodytemperature drops below the freezing point of the body fluids, which is just a little bit below zeroCelsius, when the temperature drops below the freezing point of the body fluids ice crystals formin the cells.
And because ice is less dense than water, that means when the water molecules go intothe solid state. They actually expand as you probably know if you have ever put a bottle of fluid inthe refrigerator and had it freeze and break. It ruptures the cell membranes of the cell when icecrystals from.
That would be one cause of a low temperature a way in which a low temperature wouldkill an animal. When temperatures get to be to high proteins become denatured. That's whathappens when we cook something, we are raising the temperature and we are causing the proteinsto lose that critical tertiary structure.
However, even temperature changes that are not that extreme, not so high as to cook theanimal or not so low as to freeze the animal, even those changes in temperature can createproblems for an animal's metabolism, because like any chemical reaction the rates of the reactionsare changed by the temperature.
So the way in which animals respond to this potential for this problem of changes in bodytemperature is by regulating their body 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 are you know, sort of similaror at least the differences between them seem to be a little bit hazy. But a for physiologist theymean very different things. Regulate is a term that physiologists uses for something that is heldrelatively constant. And control is used for some other physiological parameter that is varied in amethodical conscious way, not necessarily conscious in the mind, but is varied in a way usually sothat something else can be held constant.
So, for example, the temperature in this room is regulated. It is held at a relativelyconstant value by a controller, that little gray box back there on the wall. And that little gray boxback there on the wall is controlling the heat output of the heater in this building. So if airtemperature in here starts to drop, the thermostat, which is what we call that controller, increasesthe heat output of a heater.
If it's very, very hot outside, the temperature of the room starts to rise, the thermostatactivates an air-conditioning system that takes heat out of the air. So output of the and heater andthe air conditioner are changed or controlled in order to keep air temperature constant.
That's what we see when we look at body temperature of animals. Some things arecontrolled. So that body temperature can be held constant. There are 2 basically differentstrategies that have evolved among vertebrate animals for body temperature regulation. Andthose strategies involve control of different things.
And to some extent I'm making a false dichotomy. In other words, I'm saying it'seither/or, but, in fact, there are some gray areas ways. There are ways in which these thingsoverlap with one another. But in terms of how we think about it to begin, with we consider thatthere are 2 forms of body temperature regulation.
Let me quickly describe these for you. The one that you personally experience as amammal, as do all mammals and all birds, involves increasing the heat output of your body. Ifyour body temperature is starting to drop below your set point, which is 98.6 Fahrenheit, which isabout 37 Celsius, if your body temperature starts to drop below that, what happens? What are theexperiences that you have? You shiver.
And what is shivering? It's just high amplitude, low frequency, muscle contractions. Heatis produced when the ATP is split in the cross bridge cycle. So shivering is a mechanism of heatproduction. What else happens when it's cold? Do you have any other experiences when it's cold?
STUDENT: You turn blue.
INSTRUCTOR: That's true. And what is that about? That's about decreased blood flowto your skin. And so that if your blood is nice and warm, it's just come from heart down in thecore of your body, and when you are trying to conserve heat you don't send as much of it outthrough the skin. And that helps to reduce your heat loss. And that's a second mechanism of away in which your body responds.
There is another one besides putting on a sweater, that doesn't count, most mammalsdon't have that as an option. What else is your experience with being cold? You have goosebumps or these little bumps on your skin. Well, the interesting thing about that is that humanbeings don't have all the necessary equipment for that, but that's a common mammalian response.
What is happening there is that the hair follicles, each individual hair goes down into theskin and there is little tiny muscle that is attached to the base of the that hair, and what your bodyis trying to do is increase the thickness of your fur coat of your hair, of your fur by making the furstand 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 muscles contracting cause thefur to stand up on end, makes it thicker, so that what's really insulating a mammal or a bird whenit stands its fur or its feathers up on end is the air that's trapped in there. You have this stabilizedlayer of air. Air is very 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 a kind of thing that youwould see if you were to study a lizard out in the desert.
In fact, let me describe to you an experiment that's really a very fun experiment. You goout 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 like fishing line tied into a knot sothat makes like a noose. And you walk up to this lizard, and if you are three or four feet awayfrom the lizard he will freeze because he thinks if he sits very still you can't see him. Then youdangle this little noose over his head and you pick him up.
He thrashes around. You are not doing it fast, it's not like you are strangling the guy. Butyou pick him up and he thrashes around like a fish on a fishing line. And then you take a little tinythermometer, it's a fast acting thermometer, and you stick it into his cloaca and you measure hisbody temperature.
There are more sophisticatedwa----********************************************-ys to do it now. But back in the40s when these experiments were first done, they were affectionately referred to as noose'em andgoose'em experiments.
If you do a noose'em and goose'em experiment, and you go out there, and you get upreally early in the morning, you go out to Palm Springs really early in the morning, and you startcatching these lizards, you can get some data that might look like this.
We're going to make a graph made of body temperature obtained through this noose'emand goose'em experiment as a function of the time of day. So there is going to be 12 noon. Here is6 o'clock in the morning, when we first get up. Let's say here is 6:00 o'clock at night, which is1800 hours on a 24 hour clock.
And if go out there and we are studying, let's say we study the desert Iguana, which isreally neat big lizard. They might show 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 catch an animal and we're going toplot the time of day, the temperature that we observed, and maybe that's somewhere around 40degrees. 42, 44 degrees Celsius.
And every time we catch an animal we are going to put a dot on the graph here. Wemight get a set of data like that pretty easily in a day.
Now, we could be also studying another lizard, same size lizard, same environment. Wecatch one lizard, and another. So we'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 some kind of comparison. Let'smeasure air temperature. And air temperature, if we were out in Palm Springs in the daytime inthe summer it might start down here like this, reach a maximum up like this, and be backsomething like that in the course of a day.
Now, you are a scientist -- at least, you want to be a scientist or are forced to be ascientist. Tell me how are you going to interpret these data? What do these data tell you? If I say Ihave a hypothesis that lizards actively regulate their body temperature, what evidence is there inthat graph that would be consistent with my hypothesis?
STUDENT: Each specie of lizard has roughly the same temperature no matter what timeof day.
INSTRUCTOR: So basically the fact of the matter is that you could calculate an averagetemperature here for that species which is pretty flat. But "pretty flat," is kind of a relative term.Relative to what? What would you use as your comparison? Air temperature. The bodytemperature is a lot more stable than the air temperature is.
If you measured temperature of the soil, it would be even greater in soil temperature.That's one piece of evidence is that the 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 for himself, because he's like alot of zoologists, when he's out he spent a lot of time drinking beer. And so he had all these emptybeer bottles around. And he filled them up with water, and he stuck them out underneath thebushes and out in the open, all the same kind of places where -- the places where previous peoplehad been catching lizards.
And didn't have to use the lizard stick anymore, he just took his little thermometer aroundand he measured the temperature of the little bottles of water. And essentially, what he was askingwas -- you know, air temperature is going to change more than a bottle of water sitting in theshade. So how can you allege that a 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 a lizard-sized bottle of water,which animals are mostly water, so it has about the same kind thermal inertia as an animal as afunction of time.
And he found that it was also varied less than hair temperature did. But it wasn't as stableas this. But he was making the point, which is a good point, and that is that you need to havesome kind of comparison if you "say, It's stable." 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 oflizard. And we're going to make this be a fair comparison, a valid comparison, which means thatwe have eliminated things that could cause this difference. In other words, if I just put those dataup there and, say, "Well, look, there's another species and its temperature is different."
Well, if it was in a different habitat, then it wouldn't be a fair comparison. If it was atremendously 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, in the same habitat on the same day.Because, obviously, if you measure one in summer and one in winter that could be the cause ofthe difference.
So we're going to try to make a fair comparison here. And having made a faircomparison, we now have a second line of evidence that really argues against the beer-cantheory. Because this is the same size lizard in the same habitat on the same day and it has adifferent temperature. It has a different average temperature.
And what the physiologist would call this temperature, either one of these, they wouldcalled this a "preferred body temperature." T sub B is the symbol we use for the temperature ofthe body. This is a different preferred body temperature. We also sometimes call that an ecritictemperature. Ecritic can refer to the same thing as a preferred body temperature.
When we have 2 different species of lizards living in the same habitat on the same daywith the same body size but different ecritic temperatures, that also supports the idea that lizardsare actively regulating their body temperature.
Because if you study 2 different brands of empty beer bottles of the same size in the samehabitat on the same day, they would have the same body temperature at any given time in onelocation.
So there is 2 lines of evidence that can be obtained from one of these natural history kindsof studies, a noose'em and goose'em study, that tells us that those lizards actively regulate theirbody 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 nighttimewhen they are not active, they are down in a burrow, buried in the soil someplace. They are goingto crawl out of the burrow, and they are going to find a nice hot rock or hot piece of sand, andthey are going to lay down and flatten their body out on it, just like you did when you were a littlekid when you got out of the cold swimming pool and laid on the hot cement.
They are laying there in the sun. Maybe they are up on a slant so that their body isexposed to the maximum amount of the sun's rays. And then after a while, after they get hot, thenthey might, you know, go sit in the shade for a while. And after they cool up off they so back outin the sun again. Maybe they are running 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 by behavior by choosing to go to hotand cold places in the environment. There are places in the environment that are hotter and placesin the environment that are colder. Those are called "micro environments."
In other words, the environment that a large animal like you out in the desert at PalmSprings experiences is just the air temperature and the radiation coming up off of the soil andeverything. But a little lizard can find little tiny micro environments, places that are cooler orhotter. And they exploit those micro environments by going to them behaviorally.
And they might be darker. A lizard might turn its skin real black in the morning when it'strying to warm up. And then when it's getting to be too hot and wants to be running aroundlooking for food, it contracts these little dark cells in the skin, and then the animal is actually iswhiter, physically looks whiter when it's hot than it is when it's cold. By making that skin black itis trying to absorb more of the sun's energy.
Now, I said there were 2 strategies. I have just described them. One of them is the humanbeing or any bird or mammal shivering, changing blood flow to the skin, changing the thickness ofthe insulation.
On the other hand, we have the lizard which is running into these micro environments. If Iwas to tell that the 2 different strategies vary in terms of what is controlled, one of them wewould call "behavioral temperature regulation." And one of them we would call "physiologicaltemperature regulation."
Which of those is the lizard, behavioral or physiological? It's behavior. Why don't we callthe bird or mammal system physiological temperature regulation? Because changing blood flow,contracting muscles, getting goose bumps, and standing your fur up on end, changing your -- andshivering, those are all physiological processes.
Now, if you're cold, have you ever thought of going and standing in the sun? Sure. WhenI say that birds and mammals utilize physiological temperature regulation, they can also usebehavioral temperature regulation.
And the lizard that contracts the little -- they're called melanophores, the little black thingsin their skin to change their color. Is that behavior or is that physiology? That's kind ofphysiology.
Let's say it's really hot. What happens to you when it's really hot and you have to be outthe in heat? You sweat. What is happening with sweat is that water is evaporating off your skinand so it cools your skin. Dogs pant. There aren't very many mammals that sweat, but there a lotof mammals that pant.
Not a lot of mammals have sweat glands, for example. Birds -- no birds have sweatglands. No bird sweat. But panting is another means of doing the same thing, evaporating water.And, in fact, lizards pant also. So if you consider panting to be a physiological mechanism, thenreptiles have some of that as well.
So it's really not an absolute difference between behavioral and physiological temperatureregulation, but it's really a matter of the relative importance of one or the other.
When an animal is doing some components of physiological temperature regulation,particularly when it's shivering. It is generating heat within its own body. So the source of heat,there are 2 different sources of heat that animals can use for regulating the body temperature.
Those 2 sources of heat are energy that comes directly from the sun. That's what thelizard is doing when goes out and sits in the sun and warms up. It's what it's doing when it goesand finds a hot rock and lays on it, because that's energy that came from the sun recently. Theother source of heat is the animal's metabolism.
That is the heat produced through muscle contraction and all the metabolic processes thattake place in the cell when carbohydrates and proteins and lipids are oxidized there is a lot of heatreleased. Not all the heat is trapped in ATP right away.
Only about 35% of the heat is trapped in ATP and the rest of heat the is just lost, releasedin the animal's body. That's metabolic heat production.
So those 2 sources of heat are really used -- are available to all different kinds ofterrestrial 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 do that, too, by the way. But we're talking about vertebrate animalshere.
So if an animal has -- because both are available, but what is true is that in some animals,one of these sources is much more important. And in other animals, the other source is muchmore important in terms of being a primary source of heat determining an animal's bodytemperature.
The terms that we use to distinguish between these are "ectotherm" and "endotherm.""Therm" means heat. Endotherm and ectotherm. Now, one of those terms applies to an animal inwhich the sun is the primary source of heat. And one of those terms applies to an animal in whichits own internal metabolic heat production is the primary source of heat. Now, which of thoseterms is which?
If the sun is your primary source of heat, then you are an ectotherm, outside heat.
And if your own internal metabolism is your primary source of heat then you are anendotherm.
That means, the primary source of heat. We already said that if it's cold, you might gostand in the sun. There you are using some of the sun's energy direct.
Lizards have some internal metabolism as well. A lizard has a metabolic rate. It has to eat.It makes poop. It has to have energy. But the problem is that metabolic rate of a bird or amammal is a very, very much greater, it least 7 times greater than the metabolic rate of acomparable lizard. At least 7 times greater.
Therefore, because the metabolic rate or amount of calories liberated per minute is atleast 7 times greater that allows that to be the primary source of heat. So birds and mammals areendotherms because they have higher metabolic rates. And that establishes metabolism as theprimary source of heat for those animals.
Now, what is a comparable animal? Again, it's like making a fair comparison. We want tohave a fair comparison between a bird and a mammal. We have to find 2 different species that inwhich the only thing different between them is ones a reptile and the other is a bird.
In other words, the only thing that's important, only thing that is likely to influencemetabolic rate is just the class to which 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 influence metabolic rate? Size. Do you haveto feed a big dog more than you feed a little dog? You bet.
If we want to compare the metabolic rate of a bird and lizard then they better have thesame body size.
And I already said that chemical reactions occur more rapidly at higher temperatures,right? We know that. You heat up a chemical something to make a reaction happen faster.
So we need to find a lizard and a bird with the same body temperature, or a lizard and amammal if we want to make that comparison. That lizard, those species of lizards over there, theyare out there in their natural environment where they are able to utilize all their normal behavioralrepertoire and go find micro-climates.
If you take a lizard and you put him in a laboratory, it doesn't have the sun shining on it.It doesn't have all this elaborate micro-climate stuff. You can take that lizard and put it in aconstant temperature cabinet. It's like an incubator or something like that, where you mechanicallyregulate the air temperature. And lizard's body temperature will be equal to the air temperature.
If there is no sun shining on that lizard, its body temperature will be the same as the airtemperature. So you could put that lizard in an environment where its body temperature was thesame as the bird. And the bird's temperature is going to be around 40 or 42.
So if you got a lizard like that, that would be his preferred body temperature anyway. Sowe are going to find that a comparable animal is the same size and same body temperature, andboth of them need to be quite. Neither can be actively running around. It takes more energy to runaround than it does to sit still.
If we do that then we will find under those conditions that the metabolic rate, the rate ofoxygen consumption, the rate of CO2 production, the rate of energy utilization, the rate of heatproduction, any of those things can be used for metabolic rate will be at least 7 times greater inthe bird than it is in the reptile.
And, therefore, metabolism is the primary source of heat in the bird or a mammal and wecan refer to them both as endotherms. The way in which they became endotherms was byevolving higher metabolic rates. Now, they had to evolve a whole bunch of other things as well.
And that's what my lecture on the Synapsida will be about, how mammals made thattransition.
But if we look at the relationship between body temperature and air temperature in amammal, as an example, you have a graph that looks like this right at the end of your IllustratedNotes. This is a comparison of really 3 different variables on 2 different graphs. And it's at the endof your Illustrated Notes.
It looks like one graph right above the other. And one of them goes like this. So you canuse 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 the exam.
What I'm trying to represent here is the relationship between 3 variables. Environmentaltemperature, body temperature, and the rate of heat production or the metabolic rate of animal.
It's difficult to produce a single graph that represents 3 different variables. You can onlydo 2 at a time in a standard X Y plot. What we're going to do is, on the upper graph we're goingto look at the relationship between body temperature, and "T sub B" means body temperature.
One of the things you need to get out of this lecture is these definitions of these variousabbreviations.
T sub B equals body temperature.
And T sub A is ambient temperature. That means air temperature or environmentaltemperature.
And so let's just direct our attention to that upper graph for a second. What we can see isthat we have one part of this graph, which is a straight line, but it has ends on it. This is a segmentof a line. This does not go on to infinity.
This is a graph of body temperature versus ambient temperature for a bird or a mammal.And what we can see is that this animal's body temperature is pretty constant. But that there arelimits to the range of ambient temperatures. That's why there are ends to this line. The points thatrepresent the ends of the line are environmental temperatures, they are ambient temperatures.
And they are the temperatures beyond which the animal dies. So they are called "lethaltemperatures." This is "T sub UL." Upper lethal temperature.
Temperatures higher than that, you have a cooked animal. The other one is "T sub LL."Lower lethal temperature. This is T sub is LL, here. Temperatures lower than that you have afrozen animal.
Now, the slanted line here, which is dashed in yours, should not be as long as it in yourdiagram. You should shorten it up so that is considerably shorter than the flat line.
And that is the relationship between body temperature and environmental temperature fora lizard in a physiology laboratory. That is not a lizard that exploit its micro environments. This isa lizard that is trapped in an experimental design -- in a can, in a constant temperature cabinet in alaboratory some place.
And the point is that this lizard, when he cannot exploit the micro climates, his bodytemperature is going to equal to the air temperature. He's going to be like a hot-dog or piece ofmeat.
And obviously this point right here is pretty close to the zero. When the air temperaturegets down to freezing you have a frozen lizard Popsicle. And this temperature up here is probablyaround 50 degrees Celsius something like that. That's where you have lizard on a stick.
But the point is that the range of temperatures that the bird or the mammal can handle ismuch broader because they have physiological capacities. And even within that range their bodytemperature is relatively constant.
So those 2 patterns, the relationship between body temperature and environmental aredescribed by another pair of terms. One of these is "Homeotherm." And the other one is"Poikilotherm."
Now, I don't expect you to have a clue what poikilo means, but which of those lines, theflat line or slanted line, would you say most likely is the line for Homeotherm? The flat line.Homeo means what? Same or constant.
So the Homeotherm line is this one. And Homeotherm means constant or same bodytemperature. Poikilotherm means varying or changing body temperature. And a lizard or anamphibian or a fish is a Poikilotherm, it's body temperature changes when it's environmentaltemperature changes.
A bird or a mammal is a Homeotherm. They are able to have a fairly stable bodytemperature over a fairly wide range of ambient temperatures.
Well, what is it that the mammal or the bird does -- what does the bird or the mammalcontrol so that it can regulate its body temperature? That's the question. Part of the answer isright down here in the lower graph.
In order to keep body temperature constant, heat production must be changed. It's thesame thing that's going on in this room right now. In this room air temperature is held constant bychange in heat production. Well, part of the answer for a bird or a mammal involves changes inmetabolism.
And so we need to take a look at this lower relationship which is the relationship betweenmetabolic rate, and we can measure this in units of, say, calories per hour or kilo-calories perhour. That's the amount of heat produced per unit of time. That's the metabolic rate of the animalas a function of ambient temperature.
That's why these 2 graphs are drawn one right below the other is we have exactly thesame X axis on both graphs. So that means that you can relate a particular metabolic rate to aparticular body temperature, because they are all measured at exactly the same ambienttemperature.
So when we look at this metabolic rate down here, which is the lowest metabolic rate, thebody temperature of the animal is 37 degrees. Over here, when the metabolic rate is higher. Bodytemperature is still 37. This is Homeotherm all after. All the way down here, this is belowfreezing. The metabolic rate of this animal is as high as it's ever going to be. The bodytemperature the still 37 degrees.
The animal is controlling its heat production in order to regulate its body temperature. Itis keeping its body temperature by changing its metabolic rate.
Now, physiologists are very interested in, or used to be, it's a sort of historic area ofphysiological study of metabolism and how it varies in different sized animals and how it varies asa function of temperature.
What they found was that there is a range of temperatures, which I'm going to direct yourattention to right now. These are another 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 two little slants in the bottompicture depict these points. The determination of these temperatures was very interesting tophysiologists because they really wanted to know the what metabolic rate was between thosetemperatures, because that's the lowest metabolic rate you can get in this animal.
You can only get that if the animal is resting and hasn't eaten recently and a number ofother things as well. But they wanted to know what that minimum metabolic rate was called orwhat that minimum metabolic rate was. And in order to make sure they were measuring, they hadto be sure that they were between these 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 lower critical. "TLC." Lower critical. "T sub UC" isupper. Upper critical temperature.
Again, those are environmental temperatures. They are different. What is the animal'sbody temperature at upper critical temperature if this is a mammal? What is the animal's bodytemperature if this is a mammal? 37 Celsius.
What is the animal's body temperature at the lower critical temperature? 37. What is it 15degrees below lower critical temperature? What is animal's body temperature.
STUDENT: 37.
INSTRUCTOR: What is it five degrees upper critical temperature?
STUDENT: 37.
INSTRUCTOR: 37, 37, 37, 37. If you find yourself talking about this, thinking about it,or writing it on an exam, if you ever hear yourself saying "body temperature changes," you needto slap yourself on the hand. This is a Homeotherm. What is changing is the animal's metabolicrate. It is changing its metabolic rate in order to keep its body temperature constant. It is turningup the heat production of its heater in order to keep it constant.
So those critical temperatures were very interesting to the physiologists who were tryingto measure that lowest metabolic rate; but they are not very important to an animal.
Most animals probably live a good portion of their lives at temperatures that are lowerthan lower critical temperature or higher than higher critical temperature. The critical thing is tophysiologists; not to the animals.
Now, the range of temperatures that these physiologists were interested in, the range oftemperatures that are between lower critical 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 on metabolic rate. Metabolicrate is constant over that entire range from upper critical, all the way down to lower critical.
The thermo neutral zone is the zone of ambient temperatures where there is no change inmetabolic rate. That's what the neutral part refers to. There is a neutral effect or no affect onmetabolic rate.
That's the thermal neutral zone between the upper and lower critical temperatures. Themetabolic rate that we measure in the thermal neutral zone is the "basal metabolic rate" or the"BMR." That's the metabolic rate. That's the value on the Y axis that corresponds to the pointswithin that zone.
Basal metabolic rate or BMR.
That's what the physiologists were interested in measuring, the basal metabolic rate ofthese animals.
It's "basal" because it's the "bottom." It's the lowest metabolic rate we can get in thisanimal.
If we study this animal at temperatures below the lower critical temperature what we findis a higher metabolic rate. That is the range that's called the "range of ThermoregulatoryThermogenesis." That's definitely a 75-cent word.
"Thermogenesis" means heat production, to generate. Heat production."Thermoregulatory" means heat production done for the purpose of regulating body temperature.This animal in this zone down here, the zone of Thermoregulatory Thermogenesis, this animal isincreasing its heat production for the 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 critical temperature, that's alsotemperatures that are higher than the animal's body temperature, that's the zone of "augmentedevaporative water loss." That's where the animal is sweating or panting. That's why an animal sweats or pants is to augment or increase evaporative water loss.
Because it's only by evaporating water than an animal can cool, they don't come equippedwith a compressors and air conditioners. All they have is swamp coolers. And that's how ananimal cools itself by increasing its water loss. Panting and sweating. And that allows the animalcool.
It takes energy to pant or sweat. The body must do physiological work. To pant it has topull air in and out of the lungs. To sweat it takes water out of the blood, actively transports theions out of it and puts it out into the skin.
So metabolic rate goes up in a circular linear fashion when an animal is having to pant orsweat. It is not making that heat because 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 Thermoregulatory Thermogenesis, this animal is makingthat heat because it needs that heat to make up that for the heat that it's losing to that coldtemperature. This animals doesn't really want the heat, but it doesn't have any choice but to makethis heat, because that's the only way it can sweat and pant. It gets rid of more heat by panting and sweating than it produces in the process of doing that.
Now, why were the physiologists so interested in basal metabolic rate. That's the one thatis 7 times higher in the bird or mammal than in the reptile. If this is a plot for a bird or a mammalright here, remember we were going to make that comparison, same body size, same bodytemperature, by putting the lizard in a constant temperature environment, we're going to measurethe metabolic rate.
The metabolic rate of the lizard is going to be way down here somewhere at one seventhof the value for the bird or mammal.
But what do you suppose happens to the metabolic rate of a lizard when we put it in thissame constant temperature environment and lower the ambient temperature? What happens to thebody temperature in the lizard? It goes down. What happens to the metabolic rate of the lizardwhen we cool its body? What happens 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 right here and kind of blow it up sowe can see it -- if we look the this graph and blow it up a whole lot, if this is the pattern -- there isthe lower critical temperature. If there is a pattern for the mammal or the bird, here is a reptileright here. This is already one seventh. This guy is going to drop down like that.
So as we get to progressively lower temperatures that are still tolerable by the lizard, thiscould be off by 40 fold. It's only off by 7 fold here because this is as close as they get. At lowertemperatures the lizard is going to have a lower body temperature and a lower metabolic rate.
The mammal is going to be involved in Thermoregulatory Thermogenesis. She is going tomaking heat in order to keep its body temperature constant. Therefore, the difference in theirmetabolism is going to be much greater and reach values as high as 40 fold at very lowtemperatures.
Where the lizard is still alive, but he's not very active.
The 2 sources of energy are available. The sun, which is utilized in behavioral thermoregulation by lizards and amphibians and reptiles and fish and internal metabolism, which birds andmammals have evolved the capacity to have tremendously higher metabolic rates.
And because they have higher metabolic rates their own metabolism is the dominantsource of heat. They are endotherms. And they are, therefore, able to be Homeotherm. They canhave a constant body temperature in the face of a changing environmental temperature. They cansurvive over a much wider range of temperatures.
When we talked about the distribution of lizards, we said primarily in the temperate andtropical regions. You don't worry about lizards getting chased by polar bears, do you? You don'tworry about penguins chasing snakes. Why? Because they can't survive in that cold of anenvironment, because they are poikilotherms, they are not endotherms.
ZOO 138 Unedited Lecture Transcript (W97) Metabolic Rate and Thermoregulation Page 2