OVERVIEW
The purpose of this lecture is to provide an overview ofthe materials that will be covered in the remainder of this courseand how these materials relate to one another. These thingsare really part of an overall theme of understanding the restof the evolution of the vertebrates and the differences betweenthe existing classes of vertebrates in the context of solvingthe problems that were faced by animals that were making the transitionfrom living in water to living on land. There are a series ofbasic biological ideas and facts that are going to be talked aboutin this lecture that will not be talked about in the future butare important in their own right. For example, the water-balancedequation are various ways in which animals gain and lose water. This is an important little piece of information in order tounderstand how animals maintain water balance.
INVASION OF LAND
In order to understand how animals adapt themselves to land,we have to first look at the physical differences between theterrestrial and aquatic environments. Secondly, what problemsdo these differences pose upon animals making the transition fromaquatic environment to the terrestrial environment. And finally,what are the solutions to those problems that vertebrate animalsevolve. This can be a quick transition or over the course of thenext couple of hundred million years of evolution.
DIFFERENCES IN THE TWO ENVIRONMENTS
The three factors faced by the animals in their transitionare:
1. Density of the medium
2. Availability of water
3. Constancy of temperature
Density of the medium (overview)
Changes occurred during the Devonian era when an overwhelmingmajority of vertebrate animals lived in fresh water environments. One of the differences is in the density of air and water. Water is more dense than air. The density of water is 1 gramper cubic centimeter or 1 gram per ml. So the difference hereis the density surrounding the animal's body.
Availability of water (overview)
Another important difference between the two environmentsis the availability of water. For a freshwater animal living in fresh water, it is constantly flooded with water that ittakes up osmotically. It has to get rid of the excess water by urination. The kidneys are continually removing this water fromthe blood. Freshwater fish will urinate a volume of water thatis equal to 1/3 of its body's weight every day. Whereas in theterrestrial environment, the water evaporates rather quickly. When you put a puddle of water on the sidewalk, it'll be dryin a couple of hours. Therefore, water is much more availablein the aquatic environment than it is in the terrestrial environment.
Constancy of temperature (overview)
The animal's body has to adapt to the temperature of itsenvironment. Temperature of water is much more constant than thetemperature of an equal volume of air. Air temperatures changeover a much wider range than do water temperatures. This hasto do with the number of calories that it takes to change thetemperature of a given volume of water versus the same volumeof air by a certain amount.
Oxygen
The availability of oxygen is another difference betweenthe two environments.
The availability of oxygen makes it easier for the animals inmaking their transition into the terrestrial environment. Itdoes not create any problems for the animals in their transition. The maximum amount of oxygen in water depends on the temperatureof the medium. The colder the water the more oxygen can be dissolvedin it. For example, a liter of cold water can be saturated with7 ml of oxygen (7ml O2/1L H2O). This is the maximum concentrationof oxygen in water. However, air is 21% oxygen per liter ofgas (210ml O2/1L Air). That means that a liter of air holds 30times more oxygen than a liter of water. Also, the diffusionof oxygen in air is about 1,000 times faster than water.
Solving The Density Problems
There are three main problems faced by the animals due tothe differences in the density of the medium.
1. Support of the body
2. Collapsing of the gill
3. Failure of the lateral line system
Supporting the body
A fish living in water is supported by buoyant forces actingequally over its entire body. The density of the fish's bodycan be considered to be equaled to the density of the water surroundingit. There are buoyant forces acting on our bodies by the air.This force is negligible. The reason that a helium balloon risesis because the density of a helium balloon is less than the densityof air. A rock sinks because its density is greater than thatof air. So an animal getting out of water has to develop a wayto support its body due to the negligence in the buoyant forceof the air. One way in dealing with this problem is the evolutionof bigger limbs. The bones and muscles associated with the pelvicand pectoral appendages get larger and more complex. There isalso an increase in size of the pelvic girdle. A limb girdleis defined as the bones that are within the main axis of the bodyto which the limb is attached. The limbs of the fish looked atin lab are the scapula and the coracoid .
The fish evolves a specialized vertebra over the courseof time called the sacral vertebra. It is a bony attachmentto the pelvic girdle. The amphibians have 1 sacral vertebra;reptiles have 2; and mammals have in most cases 3. Birds havesomething like 20 sacral vertebra.
As the animal walk on land, the force of gravity acts uponits body at different places. There are going to be torques placedupon this animal's body. There are twisting forces which operatealong the length of the animal's vertebral column as they walk. There are evolutionary changes in the vertebrate. The vertebrateof very primitive fish, the ones that first invaded the land,were fairly small. They were like rings of bone wrapped aroundthe notochord. During the evolution of the amphibians there isa great diversity of different kinds of vertebra that evolved.
Evolution ultimately gives rise to a new structure called a zygapophyses,a bony connection between the vertebra. The vertebrate of thefish look like a simple spool shaped structure. There are neuralarches and neural spines and so forth, but they are designed toresist compressional forces that are exerted along the long axis. In other words, when this animal's muscles contract, they exertcompressional forces on the vertebra that cause the body to spinto the side and then the muscles on the other side contract andthe body spins the other way. But these vertebra are flattenedlike a stack of poker chips designed to resist compression onthe end. The vertebra of these animals have these special structurescalled zygapophyses that are found upon the neural arches. Theyare there to resist the twisting forces acting on the animalsbody. This is the last big change in the skeleton of the animalsdue to the density problem.
Collapsing of the gill
The gills of the fish have are complex structures with avery large number of secondary lamellae. These lamellae producea large surface area for respiratory gas exchange as water flowover them. When the fish is out of water, the gills tend tocollapse due to the lack of water and the density of air. Thesurface area becomes a small fraction of what it was in water. The fish can no longer be able to efficiently exchange respiratorygas through the gills even when the amount of oxygen is abundantin air due to the reduction in surface area.
The solution to the collapsing of the gills is by using the lungs as the primary respiratory organ. The fishes did notevolve the lungs as a solution to living on land. They alreadyhad lungs back in the Devonian Era due to the stagnant conditionin some fresh water habitats.
Associated with the changes in the respiratory structuresis the evolution of the three-chambered heart, called a doublecirculation. The circulatory system of the fish in the lab iscalled a single loop circulation. The blood from the atrium drainsinto the ventricle. It then goes through the gill lamellae, upthe dorsal aorta, and into the body. The blood from the bodygoes back to the atrium by passing through the sinus venosus andthe common cardinal veins. This is a diagrammatic representationof the fish's circulatory system.
In amphibians and some reptiles, a new atrium evolve aspart of the system. Blood from the right atrium flows into theventricle which is then pump into the lungs. This oxygenated-poorblood receives fresh oxygen from the lungs. It then drains backinto the new atrium, the left atrium. This blood then enters theventricle and is pump into the body. Well oxygenated blood returningto the left atrium and going through the ventricle is crossedwith blood relatively low in oxygen content going to the rightatrium. Both of these streams of blood flow through the ventriclewith little amount of mixing. Special valves and complex structuresinside the ventricle reduce the amount of blood mixing in livingamphibians and reptiles. This is called a double circulation. One circuit, the systemic, directs blood through the body. Theother circuit, the pulmonary, drains blood into the lungs. Thesenew structures evolved as the animals depend on the lungs as thenew respiratory organs.
Lost use of the lateral line
As the animals make a transition onto land, their laterallines, the sensory system, fail to function. This is a systemof tiny tubes filled with gelatinous kind of materials. Littlecilia, hair cells, stick up into the gelatin along the wall ofthese tubes. When sound waves and pressure waves in the waterhit the side of the fish's body, the waves enter the tubes andthe vibrations are then transmitted along the lateral tubes. Thehair cells sticking to the gelatin can detect the vibrations. This system allows the fish to sense sound waves in the waterfrom considerable distances. The lateral line serves the samepurpose as the ear. The sound waves in air are pressure waves. Since the density of air is less than water, the sound wavesjust reflect off the pores. No significant amount of vibrationsget transmitted into the lateral line.
The solution to the problem is the evolution of the tympanicmembrane, also known as the eardrum. The eardrum is a large livingtissue membrane located inside the skull of the animal and isexposed to sound waves. It is connected to the inner ear structureswhich functions like the lateral line of the fish. The innerear structures are fluid-filled tubes that have hair cells hookingup to nerves which can sense vibrations. The tympanic membraneis connected to the inner ear by a bony connection called thestapes or middle ear ossicle. This area is called the middleear cavity.
The middle ear is connected to the pharynx by a structurecalled the Eustachian tube. Located inside of the Eustachiantube is the inner ear. The inner ear opens up into the middleear cavity by very small openings that have a small membrane. The total cross-sectional area of the tympanic membrane is verylarge compares to the small area of the inner ear. This amountsto be a mechanic ampification system. The pressure waves goingthrough the air and striking the tympanic membrane exert a forceonto the large area which then gets transmitted to the smallerarea of the inner ear. Therefore, the pressure waves in the lessdense air are mechanically amplified and produced in the innerear. This is the solution to the problem due to the disfunctionof the lateral line system.
In an evolutionary sense, the middle ear cavity evolves fromthe spiracle of the fish's ancestors. All of the terrestrialvertebrates, except for the mammals, have only one ear ossiclecalled the stapes. The mammals have three ossicles called theincus, stapes, and malleus.
Availability of water
The availability of water creates another problem for terrestrialanimals. One main problem is desiccation or drying out. Theprimary solution to this problem is the reduction of water lossin urine. In order to understand why this was the particularsolution evolved by the animals you must understand the waterbalance equation. The fractional water content of an animal'sbody has to remain very constant. That means that the rate ofwater gain must equal the rate of water loss.
WATER BALANCE EQUATION
Three Mechanisms of water gain available to all animals
1. Liquid water-This is molecular water encounterd by animalsin streams, rivers, etc. Most animals would acquire this formof water by drinking. The term liquid water is used because thefrog has specialized patches of skin on their hind legs that cansoak up water directly.
2. Preformed water-Water that exists as molecular water inthe animal's food.
3. Metabolic water-Water that is produced by the animal'smetabolism.
The general equation for animal metabolism is;
Organic food + Oxygen ---> Carbon dioxide +water + ATP + Heat
Four avenues for water loss
1. Cutaneous- This is water that diffuses through the animal'sskin. This is not sweat. Most animals do not have sweat glands.
2. Respiratory- This is water that is in the exhaled gasthat wasn't in the inhaled gas. Every exhalation is saturatedwith water vapor. So there is a net loss in the amount of watervapor.
3. Fecal- This is the water contained in the animal's feces. This amount of water is determined by the large intestine becausethe large intestine is able to reabsorb water from the fecal matter.
4. Urinary- This is the amount of water loss in the animal'surine. The urine is composed of water and metabolic waste products. It also contains salts such as potassium, magnesium, and sulfate.
There are a number of different kinds of metabolic wasteproducts that are in the urine of an animal. Metabolic break-downproducts of drugs for example, is why drug testing works becausethe drugs are eliminated in the urine. Pregnancy test works becausethere are hormones that are produced during pregnancy that appearin the urine. But from the standpoint of water loss, the mostimportant ones are the ones that are called Nitrogenous WasteProducts, wastes that contain nitrogen compounds. When an animalmetabolizes protein, it will produce these nitrogenous waste productswhich compose of long strings of amino acids. Amino acids arecomplex molecules that have a variety of structures but they allhave the same amino group as the side chain. These amino groups,containing the nitrogen molecule, will be eliminated in the urine.
The Three Forms In Which Amino Groups Are Eliminated Are;
1. Ammonia
2. Uric acid
3. Urea
Ammonia
Ammonnia is the primary waste product in fishes because itis highly soluble in water. Ammonia molecules floating aroundin the blood of the fish diffuse rapidly across the respiratoryepithelium and are carried away in the blood. In fact, the urineof a fish does not contain very much nitrogenous waste productsbecause the water that flows through the gills carries it away. Uric acid is the primary waste product in reptiles and birds. It is highly insoluble so it precipitates out. Urea is the primarywaste product in amphibians and mammals.
Each of these nitrogenous waste product works well for theanimals that have them but their kidneys work in different ways. Birds and reptiles are able to greatly reduce their urinary waterloss by producing uric acid but they have a kidney that differsfundamentally from that of mammals. Mammals are able to greatlyreduce their urinary water loss by primarily producing urea astheir major nitrogen containing waste product. The reason isthat mammalian kidneys contain a special structure called theLoop of Henle which enables mammals to produce urine with a highertotal solute concentration than their body fluids. Only mammalscan do that. And that reduces the amount of water that's in theurine.
Liquid water is the limiting factor in terrestrial environment. The amount of food that an animal eats pretty much is going tobe determined by it's metabolism. An animal cannot go out andeat extra amount of food in order to get water. Metabolic wateris a very small fraction of water in the total water gain formost terrestrial animals. And the primary means of reducing waterloss and maintaining water balance is by reducing urinary waterloss.
One more problem that results from the reduced availabilityof water is the absence of a free-living larval stage. Fisheggs are very small and they produce very small baby fishes. Baby fish can make a living in the aquatic environment. Theyswim around and eat algae and other microscopic or very smallplants. Tadpoles can do the same thing. But there are no examplesof terrestrial vertebrates that lay very tiny eggs and producelittle microscopic young that can make a living as a terrestrialvertebrate. Insects do but vertebrates do not. The vertebrateshave abandoned the idea of producing tiny babies that go outand make a living for themselves. They produce a big egg thatproduces a much larger newborn young that is capable of makinga living as a terrestrial vertebrate. In order for vertebratesto produce a larger egg, they have to develop a very differentkind of egg which we call the amniotic egg. This is an inventionof the reptiles. The two major descendent group, the mammalsand the birds, inherited this idea from the reptiles. So thereptiles, birds and mammals can be collectively referred to asthe Amniotes because they possess this complex structure of theamniotic egg.
Temperature Changes
The third difference between the environments is in constancyor stability of the temperature. Air temperatures change overa much wider range than do water. The problem that this createsfor the animals is a greater daily and seasonal changes in theirbody temperatures. The temperature of the animal's body cannotmove outside of the range of the temperatures which are compatiblewith existence. If an animal's body temperature is lowered more than about a degree or so below 0, ice crystals will form inthe cells. When liquid changes from a liquid state to a solidstate, it actually increases in size. The expansion of the icecrystals will rupture cell membranes and result in destructionof tissues.
At the other end of this spectrum, where the temperaturegets too high, something in the mid to high 40's, the proteinswill become permanently denatured. The animal is essentiallycooked and all of it's enzyme systems will stop working. So thereis that possibility of body temperature going beyond the rangethat's compatible with life. Changes in temperature create problemsbecause temperature influences the rate of chemical reactionsthat are taking place in the cells of the body.
The solution to this problem is in body temperature regulation. There are two different types of body temperature regulationamong reptiles, birds, and mammals.
1. Behavioral body temperature regulation - Reptiles, lizards,snakes, crocodiles, and turtles utilize this type of temperatureregulation. What they do is when they want to warm up, they goand find a warm place to stay. If they want to cool down, theygo and find a cool place. It's been shown that they are ableto maintain a fairly stable body temperature by taking advantageof a wide variety of what is called a "micro-climates",that is places within the animal's habitat where it's a littlebit warmer or a little bit cooler.
2. Physiological body temperature regulation - This hasapparently evolved independently in both the mammals and in thebirds. Now all animals have available to them two sources ofheat which can be used to regulate the body temp.
a. Environment - energy coming from the sun
b. The other source of heat is the animal's metabolism
Animal's metabolism
The basic equation for animal metabolism is that some organiccompound composed of carbon, hydrogen, and probably nitrogen arecombined with oxygen to produce carbon dioxide, water (metabolic),ATP (fuel for muscle contraction & membrane bound proteins)and waste heat. Waste heat is something in the neighborhood ofmaybe 20% to 35% of all of the chemical energy in the bonds ofthe food which is trapped in the form of ATP. The other 65% isimmediately lost. And when this ATP is used, most of the chemicalenergy in that ATP is also ultimately leads to the productionof waste heat. In some cases the ATP is actually used to makemore complex molecules. But in most cases, most of that ATP energyleads to the production of heat.
CHO + 02 ----------- CO2 + H20 + ATP + Heat
(35%) (65%)
That's why when you go for a run, you get all sweaty. It's because of all the heat are being generated through musclecontractions. Metabolic heat is the heat that is available toan animal that constitutes this second source of heat that canbe used to regulate body temperature.
Animals can be grouped according to which one is the most importantsource of heat:
1) If the sun is the most important source of heat for an animal,then the animal is called an ectotherm. The sun or heat fromthe environment is the primary source of heat.
2) If the animal has it's own metabolism as the most importantsource of heat, then they are called an endotherm. It's own metabolismis the primary source of heat.
This aspect of primary source of heat is being emphasizedbecause reptiles which are ectotherms do have metabolism and theydo produce some metabolic heat. But they have a much lower rateof metabolism in # of calories that they produce per minute. And so it isn't enough to significantly elevate their body temperature. But they do have metabolism and they do have some metabolic heatproduction. Birds and mammals which are endotherms have at leastanywhere from 7 to 20 times higher rate of metabolic heat production. They consume at least 7 times as much oxygen. They have to eatat least 7 times as much food. They have to digest 7 times asmuch food. They produce 7 times as much waste products when theyget through. And they produce 7 times as much heat. Becausethey have a higher metabolism, they are endotherms. And the energycoming from the sun can also be used as a means of controllingtheir body temperature.
There are a number of avenues of heat exchange available to allanimals by the sun.
Avenues of Heat Exchange:
1. Visible radiation (+) = This is sunlight that comes directlyfrom the sun or bounces off of something in the environment. When it hits the skin of the animal, it is in a range of wavelengthswhich we can see. One of the important features of visible radiationis that the amount of visible radiation absorbed (# of caloriesabsorbed) is influenced by the color of the animal. Objects lookwhite because they are reflecting all of the wavelengths fairlyevenly. Objects look black because they are absorbing all ofthe wavelengths of visible radiation.
If an animal wants to influence the amount of visible radiationthat it absorbs, it can change the color of its body. Lizardscan do this. The lizard has little pigment cells in its skincalled melanophores. If the lizard is cold, it can simply expandsthose cells and makes its skin look darker. And because blackabsorbs the visible radiation, it will warm up more rapidly. After it gets warmed up and its body temperature is up to whereit wants it to be, it then contracts those melanophores and makesits skin whiter to reflect the same visible radiation. Now about50% of the incident solar energy is in the visible range. Abouthalf of all the calories coming directly from the sun to the surfaceof the earth are in the range of wavelengths that our eyes canrespond to and which are therefore the wavelengths that are influencedby color.
And there are some other things that will influence the amountof visible radiation that an animal absorbs and one of these isthe surface area exposed to the sunlight. If reptiles want tomaximize their absorption of solar radiation, they'll sort offlatten themselves out to get a maximum surface area exposed. If the lizard wants to minimize his absorption of direct solarradiation, they face directly at the sun and expose just the frontof their face and their head, the__ rest of the body then is inthe shadow part of the head. So they change orientation in orderto change the surface area exposed and that will influence theamt. of energy which is absorbed in the form of visible radiation. So that's only a way of gaining (+) heat. Animals cannot emitvisible radiation, they simply absorb it or fail to absorb it.
2. Infrared radiation (+/-) - Our eyes are not sensitive to infraredradiation and about 50% of the direct solar load is in the formof infrared radiation. So about half of the calories coming directlyfrom the sun to the surface of the earth are in this range ofwavelengths.
And of course that adds up to 100% and leaves out ultravioletbut there isn't a lot of heat energy in the form of ultravioletradiation although there is ultraviolet radiation. We're reallytalking about 50% of the calories that come directly in the formof radiation.
Now, infrared radiation is distinguished from visible radiationbecause it is completely absorbed by anything that has__ waterin it, and of course that includes all living organisms, so allof the infrared energy hitting an animal is going to be absorbedregardless of the color of the animal. That lizard that's changinghis color is only influencing the half of the radiation that'sin the visible spectrum, he's not able to do anything about theinfrared radiation that hits his body.
And the other things about infrared radiation is that all objectsin the environment give off infrared radiation including animals. So everything emits/gives off infrared radiation. And the amount,the number of calories per second that you are giving off or thatanything is giving off is proportional to the fourth power ofthe temp. on a Kelvin scale. So the hotter something is, themore infrared energy it gives off. And therefore you are sittingthere giving off infrared radiation but everything around youis emitting infrared radiation so that at the same time thatyour body is giving off infrared radiation, it's also absorbinginfrared _radiation. And therefore if the average temp. of thething surrounding you is cooler than your skin temp. then you'regoing to be giving off more heat than the env. is giving off toyou and vice versa.
Infrared radiation can be distinguished from visible radiationin that it can be either a means of gain or a means of loss (+). It's also proportional to the surface area so the more surfacearea exposed to the infrared radiation the more heat you're goingto gain. Animals take advantage of infrared exchanges by, ifthey want to cool off they might go down into a burrow beneaththe soil where the walls of the burrow are cooler than the skintemp. of the animal and so they will actually give off more infraredradiation in the walls of that burrow than the burrow gives tothem and so they will cool off. So going into the sun and goinginto the shade is another way of varying it's source of visibleand infrared radiation.
3) Conduction (+/-) Conduction is heat exchange due to physicalcontact with something. You are exchanging heat with anythingthat you touch, if it feels cooler or warmer than your skin. So conduction can be an avenue of either gain or loss dependingon whether the thing is hot or cold. So the amount of heat exchangedby conduction is going to be proportional to the difference intemp (delta means difference) so if something is very hot you'regoing to absorb a lot of heat from it and vice versa with cold. If it's just alittle bit warmer or cooler than your skin temp.you're only going to lose a little bit of heat by that avenue.It's also going to be proportional to surface area (SA) so themore of your body in contact with something the more heat you'regoing to exchange.
4) Convection (+/-) In some ways convection is like conductionbut the real difference is that convection is heat exchange withmoving air for a terrestrial animal (actually, in its most generaldefinition it is heat exchange with a moving fluid or liquid,because, from the standpoint of physics, air behaves like a fluid).Convection requires contact between the animal and the movingair and is like conduction in that the amount of heat exchangedis proportional to the difference in temp. and proportional tosurface area and it can be an avenue of gain or loss, that isyou can be warmed up by hot wind or you can be cooled off by acold wind. But the biggest difference between convection andconduction is that we're talking about moving air and the rateof movement will also influence the amt. of calories exchanged.
So the faster the wind moves the more heat you're going to exchangeby convection. Everything else being constant, surface area exposedto being constant, difference in temp. being constant , the fasterthe wind is blowing the more heat you're going to exchange bythat avenue. Wind chill factor - they measure the air temp. butthey also factor into that the influence of wind speed and whatthey are telling you is that your body is going to lose heat tothis wind. Animals can take advantage of convection as well,because it turns out that if you look at wind speed as a function,if this is the ground and here is the height above the ground,if we go out and me_asure wind speed but we're measuring windspeed of 4 or 5 feet off the ground, if we represent wind speedby the length of this vector what it turns out is that right atthe ground, within a centimeter or so of the ground the wind speedwill be very low. Even if you can measure 10 meters per secondof wind up here at waist height, down there within a centimeterof the ground where most vertebrate animals live, there's almostno wind at all. And there is a gradual increase in wind speedas you move away from the ground, it's very low, close to theground and reaches a maximum a few inches above the ground.
So how can an animal take advantage of that? Well, it can climbup in a bush. That's called wind shear - this change in windspeed of height above the ground. And an animal can take advantageof the influence of wind speed on convective exchange by gettingout of that wind shear at a very low speed.
5) Evaporation of Water (-) When water changes state from theliquid state to the gas state there is an amount of heat calledthe latent heat of vaporization which must be put into that waterto cause it to change state from the liquid to gas. That latentheat of vaporization comes out of the skin of the animal. Sosweating is an example of a thermal regulatory behavior in whichthe animal is exchanging heat by evaporation.
This one is unique in that it's only a means of heat loss undernormal biological conditions, animals only lose calories by evaporationof water from the surface of their skin and it's really just proportionalto the amt. of water that is lost.