ZOO 138, Wednesday, January 29, 1997, 12:00 p.m. INVASION OF LAND
I'm going to start today with a lecture, and I will continue after I return your midterms onFriday, dealing with the invasion of land.
And there are several different purposes for this lecture.
One of them is to give you kind of a context, sort of an overview for a number of lecturesthat I will give you during the remainder of the quarter to show how these things all sort of relateto one another.
And, for example, I'll talk a little bit about the kind of egg that reptiles, birds andmammals produce, called an amniotic egg.
I'll talk about the physiology of mammalian kidney, how animals regulate bodytemperature. And all of those things fit together in a broader context of vertebrate zoology.
Another function of the lecture is that I will take little diversions on a series of shorttopics, none of which is substantial enough to warrant an entire lecture, but which I think is afundamental idea or set of related ideas that will help you understand the biology of vertebrateanimals, biology of all animals in some cases.
And these little vignettes do form the context of this question which is the issue of theinvasion of land.
The way that I look at it, we can identify the differences between the terrestrial andaquatic environments. The differences between freshwater because that is the environment thatwas inhabited by the Rhipidistian and Crossopterygiian fish, those animals that first invaded land.
What are the differences between freshwater and land, the terrestrial environment? Andthose differences, some of them pose problems for the animals that were making the transition.
For example, if you live in freshwater, if you are a vertebrate animal living in fresh water,you are continually inundated with water by osmosis. Your body fluid concentration is around300 millivolts of solute per liter of fluid. And freshwater is three or four millivolts of solute perliter. There is a substantial difference in total osmotic concentration that results in the osmoticmakeup of water.
A freshwater fish spends its entire life peeing, and does other things too. They produce avolume of urine that is equal to about one-third of their body weight every day just getting rid ofthis water that is continually flowing in across their gills.
Whereas, as you probably know, an animal in a terrestrial environment is continaullysubject to desiccation, to drying out. You put a wedge of orange on the counter and come backtomorrow it's dried up. The same thing happens to animals as well.
So the problem -- one of the problems that results from the difference is in really whateffectively the availability of water, is that animals tend to be subject to desiccation.
Then we can look at the solutions to those problems.
So that's the way I'm going to organize this lecture is, differences, and we will go throughand figure what are the differences. Then I will work with you in deciding what are some of theproblems that might be posed by those differences, some are more obvious than others, And I'llspend some time going over the solutions to those problems.
But there is an important relationship there. When I'm talking about a solution, youshould remember what problem that is a solution to, and what is the difference in the environmentthat produces the problem.
So what would you identify, other than the first difference, is the availability of water? Inother words, it sounds sort of silly to say, but when you live in water there is a lot more wateraround you than when you live on land, and there is air around you.
What are some other differences between freshwater and a terrestrial environment? Thesedifferences are physical differences, they would exist even if life had never evolved on the planet.
STUDENT: Why is an animal more buoyant in water than on land?
INSTRUCTOR: That's is a difference in fundamentally, when you get down to physics ofit all. That difference results from the difference in the density of the medium that surrounds theanimal. What do I mean by this word "medium"? What is the medium surrounding you right now?Air. Anybody know what the density of air is?
It's like one gram per cubic meter or something like that. It's really really low. And themedium surround something an animal in freshwater is water, and what's the density of that? 1.0grams per cubic centimeters.
So air is around one thousandths as dense as water. There are buoyant forces operatingon your body in air. There are buoyant forces exerted on anything in air. How do we know that?Where have you seen an example of something where the buoyant forces were really obvious?The problem is that they are not obvious to us.
Under what circumstance can you see a clear sign that there are buoyant forces operatingin the air?
STUDENT: Balloons.
INSTRUCTOR: A helium balloon, a hot-air balloon, it rises, it floats just like a piece ofwood floats in water. And the only reason for it is the density of the balloon is less than thedensity of the air. So the buoyant forces result simply from the differences in the density of theobject and the density of the medium which is displaced by the object.
So the buoyant forces operating on our bodies are there, but they are insignificantbecause of the difference in the density of the air. So there are some problems that will result fromthat as well.
That's one difference.
Another difference -- there are 2 more differences.
STUDENT: How about ultraviolet radiation?
INSTRUCTOR: You know, I think that would be fair to say that in terms of radiationhitting an animal, ultraviolet doesn't penetrate through water very far. That may be a fifth one Inever even thought of it before.
I'm not sure it has a lot of the way of implication for living. I mean, certainly plants live in freshwater, but that's a given.
STUDENT: How are you going to produce --
INSTRUCTOR: Now, wait a minute, I said we're talking about physical differences in theenvironment that exist even if life had never evolved.
STUDENT: The amount of oxygen.
INSTRUCTOR: Amount of oxygen, there is a good difference. Amount of oxygen.
Let me put it this way: The oxygen -- the amount of gas you can dissolve if a fluid is afunction of temperature. If you saturate water with oxygen, you end up with something like about3 mls of oxygen per litter of water.
What is the concentration of oxygen in this room using those same units?
STUDENT: 210 milliliters.
INSTRUCTOR: 210 milliliters of oxygen per litter of air. 21%. So this is a thousand mls,21% of that is 210. Maybe this number is 7, it's not really important. The difference here howeveris a factor of 30. So there is tremendously more oxygen per unit of volume of air than there is ofwater, and that makes it more available.
Another thing that makes oxygen more available is the fact that oxygen diffuses throughair about a thousand times faster than it diffuses through water.
And diffusion is the sort of ultimate mechanism of transport of oxygen when you get rightdown to the immediate vicinity of the exchange surface. In other words, in your alveoli oxygen isdiffusing from the center of the alveolus into the respiratory epithelium and then into the blood,similarly in a fish's gills, where blood is passing over the gills, oxygen is actually diffusing into thetissue.
So the fact that the diffusion occurs in the gas phase in air means it's a thousand timesfaster.
Not only it is 30 times more concentrated, but it's a thousand times faster.
So do you suppose that this difference poses problems for animals that are -- if you are aRhipidistia and Crossopterygiian fish, and you are contemplating invading land, and you have acheck-off list of things you want to be sure you have before go ashore is the fact the oxygen is 30times more concentrated and diffuses a thousand more times faster, is that a problem you aregoing to worry? No.
Maybe that's one of the reasons you are going there. But it's definitely not going toproduce any problems. It's a difference, but it is not one that produces problems.
So there is one more.
STUDENT: Constancy of the temperature.
INSTRUCTOR: What do I mean by the "constancy of the temperature"? If you liveddown next to the ocean, say, you lived down in Venice, the changes in the air temperature on aseasonal and daily basis due to the presence of all that water in your immediate vicinity, thechanges in air temperature are going to be much less than, say, if you live in Lancaster, where itcan be 120 degrees in the summer and well below freezing in the wintertime.
And the simple fact of the matter is, that if you take one cubic centimeter of air and youtake one cubic centimeter of water, how much heat energy do you have to put into this one cubiccentimeter water to increase it by one degree centigrade? How much energy? What's the units?
STUDENT: Joules.
INSTRUCTOR: That would be good if we were good modern scientists, we would usejoules.
If we were an old-fashioned scientist we would use calories. It takes one calorie to raiseone cubic centimeter of water by one degree centigrade. That's why we old-fashioned people likesimple numbers like that. That same calorie is going to raise the temperature of the air by a lotmore than one degree Celsius.
A small amount of heat input will produce a much larger change in the temperature of airthan it does in the temperature of water.
Therefore air temperatures change tremendously more, both on a daily basis and on aseasonal basis than do water temperatures.
You know, the temperature of a swimming pool probably doesn't change by more than adegree or 2 on a daily basis.
But air temperature can change -- what would be a reasonable number, 20 or 30 degreesCelsius on a daily basis. And even on a seasonal basis around here, it can be below freezing forweeks on end. And certainly the temperature of your swimming pool will go down. But it's notgoing to go below freezing.
So the environmental temperature is much more stable in water than is the airtemperature, even in the immediate vicinity.
And that produces some problems as well.
So of the 4 that I can think of that are differences: Availability of water, density of themedium, and constancy of the temperature, are differences that pose problems. And the amount ofoxygen and availability of oxygen -- I like the word "availability," this is kind of related to theamount, 30 times as much.
But the factor of diffusion is also important. And so the availability of oxygen may be abetter term than the amount of oxygen because it incorporates both the difference inconcentration and difference of the rate of diffusion.
That one doesn't pose problems; the other 3 do. And I'll have to stop and think aboutTom's UV. Maybe Tom will have to tell me what that does. That's a good one.
Now, what I want to do is look at these differences. And I want to talk about some of theproblems that result from these differences. And there is no particular significance to the order,except I'm just going to deal with the ones that create problems.
If we talk about availability of water, the problem that I told you about was desiccation.And that always looks like it's misspelled but it's spelled correctly.
Desiccation refers to drying out. And that's the problem that is posed for an animal that istrying to live in the terrestrial environment. And the solution to the problem of drying out is tomaintain water balance.
Water balance is just like any other balance. It means that there is an equality between thetotal amount of water gained and the total amount of water lost. If you are in water balance, theamount of water that you gain every day is equal to the amount of water that you
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lose every day.
And in order to understand -- this is one of these little vignettes that is an importantoverview of vertebrate biology or animal or biology in general, as well as something that relates tothe issue that I'm discussing right now. And that is what are the avenues of water gain and waterloss for an animal? In what forms can an animal obtain water?
And clearly the most obvious one of is just plain old liquid water. You just go over to thedrinking fountain and get a drink. So liquid water is one of these avenues of water gain. And it'sobviously something that's available to all animals.
You might call this drinking because the most common means by which vertebrateanimals obtain liquid water is by having it go in their mouth and go down their esophagus and intotheir stomach. And that's what we mean by drinking.
The reason I don't use drinking, is there are some vertebrate animals that absorb it directlyacross their skin.
A colleague of mine at the University of Nevada Las Vegas has done work with frogs.And he can take a piece of filter paper that's moist and sit a frog on the filter paper for tenminutes, and when he picks the
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frog up the paper is dry underneath the frog, because the frog can suck that water right out ofthe paper across its skin using osmotic forces. If it's distilled water, the body fluids have a higherosmotic concentration.
You can't do that, but some animals can.
In fairness to frogs, I call this liquid water and not drinking water. It just means water thatenters the animals body in the form of liquid water, H2O, molecular H20, and it doesn't haveanything around it because there is another gain and that's what's called "pre-formed water."
Now, that term will make more sense in a minute. But pre-formed water refers to liquidmolecular H2O that's in the food. Everything you eat has water in it. If you are eating celery andradishes, it's about 95 percent. If you are having steak and potatoes it's about 70 percent water.
But almost everything you eat has some liquid water as part of the molecular or cellularstructure of whatever kind of organism you are eating.
Now, you are not eating that food to get water. In most cases, you are eating that food toget the nutrients and energy sources that are in it.
When we look at water balance, we distinguish between water you can drink in liquidform and water that
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is -- because there are some animals never drink liquid water. Some of the best desert-adaptedanimals never see a drop of water in their life. And if you provide them with it, they don't knowwhat to do with it.
There are Kangaroo rats that live in our desert, if you catch them in a trap and put themin a cage at your house and provide them with a typical rodent water battle, they'll never drink it.They don't know what to do with it. There are animals for whom the most important source ofwater for maintaining water balance is this pre-formed water.
That's why we draw a distinction between the two, because they both refer to liquid H20.
There is third avenue of water gain, which is called "metabolic water."
And the thing that's important about metabolic water is that when it enters the animal'sbody, it is not in the form of molecular H20; it's formed by the animal's metabolism, that's why wecall it metabolic water.
This is water that the animal fabricates within the mitochondria of its body. And, youknow, you probably studied glycosis and cytochromes and electron transport, and you knowabout NADH and stuff like that, but you may have never have looked it quite the same waybefore.
That is the overall equation for an animal's
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metabolism is some organic compounds composed primarily of carbon, hydrogen and oxygen,this can be sugar or lipid or whatever, combines with oxygen, which the animal breathes, in areaction that only goes in one direction.
This is not one of these little chemical reactions like that, because this one doesn't happenin animals. This is a reaction that only goes in one way. Producing -- what are the products ofthis? CO2, lot's of waste heat.
Why is the animal doing this? To get this energy. And in what form is that energy in?ATP. So this is not a good equation. My friends in the chemistry department would not approveof this equation, because I didn't put ADP and PI, these are over here too.
I mean, the animal doesn't make ATP out of oxygen.
But this is why the whole thing is going on. I mean, the reason the animal is doing this isto make ATP. But water is produced as a product of this reaction. Where does this come from?Where does the "H2" part of this metabolic water come from?
STUDENT: Food and oxygen.
INSTRUCTOR: Food that was consumed.
Which part of the food? There is only one place, the hydrogen in the food.
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Who can tell me something about that hydrogen that you learned about in cytochromeelectron transport? Where did that "H" come from?
STUDENT: Sugar?
INSTRUCTOR: I guess I'm not asking the question very well.
Does the term "NADH" mean anything to you? That's the "H." In other words, whenglycosis or the Kreb cycle take place, you produce lots of NADH and lots of FADH, those "Hs"came off the carbon chain and went over into mitochondria and got tacked onto the oxygen andproduced water.
This oxygen from the air goes to here. So oxygen is an electron receptor. It takes up thehydrogen in the electron, and that's is where we get the metabolic water.
These guys right here, the carbon and oxygen in the food go to here to form CO2 whichis excreted by the animal.
My point is, this water right here is brandnew water. When it came into the animal it waseither oxygen or food. And it was produced by the animal's metabolism, hence it's called"metabolic water." It is a product of the animal's metabolic machinery. And part of it came in asfood, right? That's where the H came in with the food. The Os came in with the air the animal was
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breathing.
Can you see why this is called pre-formed water now?
The second category, it's called pre-formed water, because when it crossed animal lips itwas molecular H2O. It was made into water before the animal got it. This is metabolic water thatthe animal produced.
Now, if the challenge to an animal invading a terrestrial environment is desiccation, thenthe problem is that there is not enough of this stuff around. That's where that problem reallyresides is in the shortage of liquid water.
Pre-formed water is water that's in the food. If that animal is going to live on land, it'sgoing to have to eat as much food as it did in the water, and there was probably just as muchpre-formed water in the food on land as there was in the water. That's not a problem. That's notbig difference between the terrestrial and aquatic environment. Metabolically, the animal will havethe same metabolic rate.
If you measure the metabolic rate of a frog and fish, they are going to have the same rateof metabolism, they are going to consume about the same amount of food and oxygen andproduce the same amount of CO2. This is
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not different. Animals are almost, in every case, always eating as much food as they can get.
The problem is that there is a limitation and availability of the liquid water.
So the solution to this problem of desiccation does not lie on this half of the waterbalance sheet. The solution must lie in reducing losses. If you are trying to balance a check bookor your savings account and not have it get smaller every month, you can either go out and earnmore money or you can try to cut back and not spend as much money on CDs and movies.
That's what animals have to do. They have to decrease their losses, decrease their rate ofwater loss, because they cannot gain as much as they used to be able to gain from theenvironment.
Now, the avenues of water loss, there are 4 avenues of water loss. One is in feces.
When an animal produces its waste products from its digestive tract there is water inthere. And that is something that animals can produce. Animals that are dehydrated produce dryerfeces than animals that are well hydrated.
Reduction in fecal water loss is one of the solutions, one of the ways in which animalsbalance the equation of balancing water gain and losses is by reducing
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fecal water loss.
The other means of balancing the equation of reducing loss so that loss now equals gainin the terrestrial environment, is reduced urinary water loss. Okay, the water contents of the urine.
And there is sort of a series of terms here that are similar and related and it's importantthat you learn to distinguish between these terms, because these things they relate to areimportant.
"Urine" is a word that we use for the liquid product of the kidneys. Pee, for those thatdon't recognize it already.
That's urine. Now, urine contains a lot of different things. But obviously one of the thingsthat it contains is water. It also contains various kinds of metabolic break down products.
Metabolic waste products.
The reason the body has kidneys and produces urine is to get rid of various kinds oftoxins and things it can't metabolize anymore.
I mean, the reason that drug tests work, is because the metabolic breakdown products ofdrugs are excreted in urine. The reason that pregnancy tests work is because the metabolicbreakdown of products from hormones are excreted in the urine.
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So those are a couple of kinds of metabolic waste products that appear in the urine.
From the standpoint of water balance, the most important kind of metabolic wasteproduct is found in the urine are products of nitrogen metabolism. Nitrogenous waste products.
Now, that word "nitrogenous" means nitrogen producing. And that's an old-fashionedchemical term. If you collect the urine from a mammal and you get rid of all the water, you cantreat the solid product that's left in such as to evolve the gas nitrogen. So that's a nitrogenous basewaste product.
There are 3 different nitrogenous waste products that are produced by vertebrate animals.
All vertebrate animals produce the same nitrogenous waste products. These are ammonia,urea and uric acid. All three of these are nitrogenous waste products produced by all vertebrateanimals.
Different groups of vertebrates will produce more of one than of the other 2 for variousreasons.
Fish, for example, produce a lot more ammonia than anything else.
So it's what we would call, the primary waste product of fish is ammonia. They alsoproduce the other 2, but most of the nitrogen they excrete is going to be in
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the form of ammonia.
Birds and reptiles produce primarily uric acid.
That's the white stuff, you know, the pigeons use to paint statues with. That's uric acid. Itis fairly insoluble, it forms and precipitates out a solution. It's a good nitrogenous waste productbecause it precipitates out and freezes up water.
Mammals primarily produce urea. And that really relates to some fundamental differencesin the function of the mammalian kidney. The mammalian kidney is one of the major adaptationsof mammals. Because their metabolism produces urea, it allows mammals to minimize theirurinary water loss.
And they are doing it in a different way from birds and reptiles, which are also reducingusing their urinary water loss by producing uric acid.
All of these things are produced to some extent or another by all animals. You produceuric acid -- even though uric acid is the primary metabolic waste product of birds and reptiles, youproduce uric acid when you are metabolizing some of nucleic acids.
A couple of the nucleic acids, your body cannot convert into urea, it makes it into uricacid. That's why you get gout, which results from the deposition of uric acid crystals in yourcartilage, or some kinds of kidney
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stones that human beings get are formed from uric acid if their metabolism is producing uricacid. That's why if you have gout or kidney stones, they tell you to cut down on the consumptionof meat.
But the bottom line is that all vertebrates produce all three. And both urea and uric acidare water conserving adaptations, so they allow different groups of vertebrate animals to reducetheir urinary water loss.
STUDENT: What do amphibians produce?
INSTRUCTOR: They usually produce urea.
Now, there are 2 other avenues of water loss. And I'm going to mention them because Iwant to be complete in this coverage of this little vignette of water balance.
One of these is cutaneous water loss, and other one is respiratory water loss.
What do you suppose cutaneous water loss refers to? It refers to skin, but it does notmean sweating because only a few types of mammals have sweat glands and can produce sweat.
But all animals have cutaneous water loss. The cells that are in the somewhat deeperlayers of your skin are living cells. Their cytoplasm is 70 percent water. And that water diffusesacross the dead outer layers of your skin and is lost to the air. And, you know, when a dry SantaAna condition hits like today and sits around
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for a while, your skin begins to feel dry and scratchy, and that is due to cutaneous water loss.
That is something that occurs in all animals, cutaneous water loss. In the mammals thatsweat we can consider sweat to be special adaptation of an increase cutaneous water. Butcutaneous is not the same thing as sweat, it's something that occurs in animals that cannot sweat.
Respiratory water loss, what that does refer to? Breathing. And what happens when youbreathe is that you bring in air and you warm that air up to your body temperature, and it becomessaturated with water vapor. In order to not dry out the very thin respiratory epithelium of yourlungs, it has to be saturated with water vapor before it gets to your lungs.
It becomes saturated as it passes down the nasal passage and down your trachea. Now,when you breath out, that air is near your body temperature, and it's still saturated with watervapor. There is more water in the air you breathe out than there is in the air you breathe in undermost natural circumstances.
And that represents an avenue of water loss as well. If it's a cold morning, and you go outand go like that, a big gray cloud appears, that's water vapor that you just lost and which is beingcondensed by being
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lowered in the temperature of the air surrounding you. So that's respiratory water loss. Both ofthose are things that animals couldn't really do much about at all.
When we look at rates of cutaneous and respiratory water loss, we find that respiratorywater loss is mostly determined by the metabolic rate of the animal.
If you go to a job interview and elevate your metabolic rate, your respiratory water lossgoes up. Cutaneous is very passive, it just happens. It just due to difference -- it's basically justdue to the vapor pressure of the air surrounding the animal.
So there are 3 avenues of water gain. There are 4 avenues of water loss.
The problem of desiccation is reduced the availability of liquid water. And these are the 2which animals evolved solutions to. Reducing fecal water loss just means that the large intestinepulls more water out of fecal material.
Urinary water loss is a more complicated one. Reptiles and birds evolved to emphasizeuric acid instead of ammonia. Mammals evolved to emphasize urea instead of ammonia. And themammalian kidney, which I will spend a lecture and a half talking about after the second midterm,is I will be talking about the mechanism of this urinary -- reduction of urinary water loss inmammals.
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There is one more thing, a problem that results from the problem -- from the difference ofdesiccation. And I want to get that one out.
So what is the solution of desiccation? How would you articulate the solution with theproblem of desiccation?
STUDENT: Not enough water --
INSTRUCTOR: Well, not enough water gained, that's not the solution to the problem.
STUDENT: Minimize your water support.
INSTRUCTOR: And how do vertebrates minimize their water loss? Urinary fecal waterloss is reduced. And the mammalian kidney is part of that equation.
The other problem, and I'm just going to get it out here, because I'll actually come back toit is the absence of a free-living larva or larval stage. When a fish egg hatches you have little tiny,maybe, three or four millimeter, maybe a half a centimeter long little organism that swims aroundin the ocean and finds food and grows up to bigger and bigger.
That is free living-larval stage or a free-living juvenile stage. It's able to survive in itsenvironment and find food and grow up into a bigger organism.
When we look at vertebrate animals we do not see that happening in terrestrialvertebrates. Terrestrial
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vertebrates produce substantially larger young. You seem to have to be a bigger organism to bevertebrate animal in order to survive in a terrestrial environment. They way in which they did thatwas they evolved a much more elaborate egg called the "amniotic egg."
Reptiles, birds and mammals all produce an amniotic egg. And I'll talk about it later on interms of what it does. But basically it allows these animals to produce a much larger new bornyoung, which is able to make a living in the terrestrial environment. It is much larger than youcould get out of even the largest known fish eggs.
And so that adaptation among the vertebrates can be seen as solving this problem ofabsence of a free-living larva stages.
I will have your exams ready to return to you on Friday.
Let's talk about the invasion of the terrestrial environment. We identified 4 differencesbetween the environments. Somebody tell me what one of those differences was?
STUDENT: Density of the median.
STUDENT: Available of water.
STUDENT: Temperature constancy.
INSTRUCTOR: Temperature constancy and availability of oxygen. Which one of thosewas the good news?
STUDENT: Oxygen.
INSTRUCTOR: Oxygen, it made it easier to invade land. We talked about the availabilityof water, it causes desiccation. The solution is to balance the water balance equation by reducingwater loss by means of urinary and fecal water loss. The other problem that I'll talk about later isno free-living larval stage and the amniotic egg as a solution to that problem, and I'll talk aboutthat more.
Let's talk about the density of the medium. That's the issue of the fact that air is so muchless dense than the animals body that there are no significant buoyant forces acting on the animalsbody, any part of the animals body.
And, therefore, one of the problems -- so if the difference is in the density of the medium,one of the problems is support of the body. And the solution to that is the evolution of largerlimbs and a stronger back bone.
Specifically, you know, if you look at a fish in water -- do you ever see pictures of fishwith bubbles coming out of their mouth? How many have seen pictures of fish with bubblescoming out of their mouth? What is that? What is it? Why are fish blowing bubbles?
STUDENT: It regulates their swim bladder.
INSTRUCTOR: That's the only condition in which the fish blows bubbles is to regulate aswim bladder. But they don't normally do that, they can take it in and out of their blood, theydon't have to burp it out. Most fish cannot burp air out of their swim bladder; they can onlyabsorb it through their blood.
People always associate that with breathing air. Fish don't breathe air; fish breathe water.
This animal's body is supported by forces that are acting uniformly all over the surface ofthe guy's body. He has a vertebral column that runs along the axis of the body. But the forces thatare exerted on that are exerted by the muscles along the side, which are forces that tend tocompress the vertebral column.
And because the column is not compressible, the body bends, and fish's little body wigglesfrom side to side and he propels himself through the water. Now, when we come out on land andwe have some kind of an animal on land, now we have -- this is animal is a representative of aterrestrial vertebrate, and it has some serious problems in terms of mass.
It has a lot of mass suspended here in the middle of this long vertebral column. It has nottoo much mass suspended up here at this end. And the support for this animal's body is a nowthrough the hind limbs, right here, and the front limbs.
And it has this vertebral column that goes along like this that connects the masses to thesupport.
And so what we see when we look at the solution is to the problem of the support of thebody are a number of things. We find that they have larger limb bones and muscles associatedwith those, just the arms and the leg muscles are bigger and stronger and the bones associatedwith them.
Another thing we find is that they have a larger scapula. Actually, the whole pectoralgirdle which includes the scapula and as you will see in lab some other bones associated with that.
They have evolved an attachment at the back end. The hip bones are really basically newstructures and there attachment to the vertebral column is a totally new arrangement from anevolutionary standpoint if we look at a Rhipidistia and Crossopterygiian fish. So they haveevolved what is called the sacral vertebrae.
Those are the not religious objects; those are vertebra that are involved in the sacrum,that is the attachment of the pelvic girdle and vertebral column. And they have really evolved apelvic girdle. The pectoral girdle is the one associated with the front limbs, and even a fish has apectoral girdle, but it doesn't have a pelvic girdle.
So this structure back here, the pelvic girdle, and its attachment to the vertebrae are newstructures. And up here we have the pectoral. And these are simply enlarged and have largermuscles associated with them.
The other thing are some changes in the vertebrae. The vertebrae of a fish are basicallysort of fairly simple spindle-shaped structures. Looking at it from the side, they look like that.And if you look at them from end on, they are just round. They are just round plates that stack up.
And that's good because all they have to do is resist these compressional forces operatingon the ends of them. But the vertebrae of a terrestrial vertebrate, not only has this kind of roundbasal part, but it also has some connections between the neural arches. Remember a vertebrae hasa basal part like this. And then you have the nerve cord that goes through like that. And you havethese neural arches that stick up like that.
And on a fish the neural arches are not connected to one another. But on a terrestrialvertebrate there are new extensions like this that stick off of both the front and the back sides ofeach of the vertebrae. And these extensions are called zygapophyses. They are extensions off theneural arch of the vertebrae. And they are there to resist the sagging forces.
In other words, if you have support at the front and the back, and the major hunk of massis in the middle here, you are going to tend to have a sagging effect. So the zygapophyses helpresist that sagging effect.
And the other thing is that when the animal lifts up its legs to walk, then there are twistingforces that are exerted along the length of vertebral column as well. So when the front leg getslifted up there is a tendency for the vertebrae to twist relative to one another, and thezygapophyses resist those twisting forces as well.
So those are all the components of the solution to the problem of support of the body.Evolution of limbs and pectoral girdles, and enlargement of pectoral girdles and changes in thevertebral column.
When you get a chance to look at those things in lab, when you look at a skeleton of thecat, you look at zygapohyses and you'll learn to identify those structures at that time.
Another problem which evolves due to the density of the medium is that the gills collapse.
The gills are composed -- if you take a look at it when you are dissecting your fish, youcan see that the gills are supported by a bony arch, and then they have a whole series of little soft,not even caritlaginious, just flaps of tissue that stick out, and that is where the exchange surface islocated.
Well, those little flaps of tissue normally have water flowing between them and have avery large surface area available for the exchange of respiratory gases. But when the fish comesout on land and those gill filaments are no longer supported by water, they all collapse together.
Look what happens -- if you think about my fingers as being an analog of these gills. Allof this exposed area of my fingers is surface area available for exchange of respiratory gases.When my fingers are not supported by water anymore, they collapse, you loose almost all thatsurface area.
So a fish out of water is going to asphyxiate just as surely as you would asphyxiate if youwere having to breathe water. And the reason for that is that gills collapse. But, fortunately,remember that the animals that made this transaction were the Rhipidistian and Crossopterygiianfish, had already evolved a lung as a mechanism of obtaining oxygen when there wasn't enoughoxygen in the water.
They had evolved solutions that had a pre-adaptation to land. And when they startedcrawling around on land and their gills collapsed, they just simply used their lungs. And they veryquickly lost the gills altogether.
And so what we can say is that the solution to this problem of the gills collapsing is reallythere are several components to it, but the clearest statement we can make is that the lungsbecome the primary respiratory organ.
In other words, the lungs were kind of a back up or secondary organ when the fish hadgills and was living in water. Now, they come out on land, the gills are not functional and so thelungs become the primary organ.
We can really say that it's the only organ, except there are some vertebrates like frogs thatuse skin as a secondary one. So we'll just say that the lungs become the primary respiratoryorgan.
Now, associated with that are some other changes in the circulatory system. Thecirculatory system of a fish is composed of an atrium, that's one chamber of the heart, and aventricle. Fish have a 2-chambered heart. Then it has a an aorta. It has some gills. And then it hasanother section of the circulatory system, and the aorta, and then it goes to little arterioles andsome capillary beds, and then it returns back.
This is a single loop. A single circuit. What happens with the evolution of lungs is that theanimals that have lungs have a double circulatory system. They have another entire loop. Andwhat happens is that the lung gets output from the ventricle, and then it goes back from the lungsand they evolve a new atrium, a left atrium, evolutionary a new structure.
And so they have a double circulation. They have one loop that goes through the lungsand back, which is called the pulmonary circuit. That's evolutionarily a new one in a sense,although it's not completely new.
And then the other loop is the one that goes -- they know longer have gills here, but all ofthe rest of plumbing is the same and it's going through the capillaries. It's out of the rest of thebody. This is called the systemic circuit.
You have the evolution of a 3-chambered heart by the addition of second atrium and youhave the evolution of a double circulation. And that's really part and parcel of this shift to lungs,the shift to complete independence on lungs is what requires and produces circumstances wherethis double circulation would exist.
Lung fish do not have a 3-chambered heart or a double circulation, because they are stillusing the gills. It's only when you get the gills out of the loop here that you really have a --although, the condition in lung fish is very interesting.
Well, that's as much time as I have for that today to finish it off.
Again, I'm not going to be on campus this afternoon. But I will be back on Monday.Please feel free to come by and knock on the door, and we'll look at your exam anytime. Youdon't have apologize when you come in. But do come in. Bring in your lecture notes and bring inyour study aides, so we can analyze this problem and figure what the your whimpy link is.
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ZOO 138 Lecture Transcripts (W'97) Page 7