Today's lecture is on the characteristicsof the mammals as a group and also the distinctions between the3 major subgroups of mammals and it is covered in your illustratednotes.
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The Class Mammalia has 2 subclasses,Subclass Prototheria and there are a variety of different fossilspecies that are known that are placed in this subclass, onlya couple of living species. And then the other subclass includesthe majority of the mammals and that's the Subclass Theria whichhas 2 infraclasses, Metatheria and Eutheria.
The living Prototherians belong toan order called Order Monotremata so we just generally refer tothese animals as the "monotremes". These are egg-layingmammals which are in many ways much more like reptiles than therest of the mammals. And there are even some theories that theymay be independently derived from the therapsids from the restof the mammals or they may even be in some ways considered tobe relectual therapsids in some ways, they are very much likethe therapsids. But the majority of the mammals are placed inthe Subclass Theria and we have the marsupials like the kangaroosand koala bears and things like that and then the rest of themthat include ourselves the placental mammals.
There are a total of 4,060 mammalsall together, living species. Three living species of Monotremes,242 Metatherians, and around 3,790 Eutherians. So there are reallyprofound differences in the sizes of these 3 major groups.
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But let's start by talking aboutgeneral characteristics of the Class Mammalia. These are characteristicsthat apply in general to all 3 groups.
The first one that we will mentionis the Presence of Mammary Glands for which the class is named. This is a unique feature of the class, of course the functionof the mammary glands is lactation - that is to provide milk tothe developing young, particularly important during the earlyphases of their development when they don't have any teeth.
The second characteristic which ispeculiar to the class is the Presence of Hair or Fur at leastevolving originally as a form of insulation necessary as partof adaptation for homeothermy - constant body temperature throughendothermy.
Third, mammals also Have a numberof other structures/glands in their skin - that is in additionto mammary glands. The skin glands have different purposes. There are Eccrine glands that produce sweat which serves as ameans of cooling, some species of mammals have enough sweat glandsso that they can evaporate water off their skin and cool. Insome animals that don't cool by sweating, they may still havea small number of these eccrine glands like in the pads of theirfeet to give them a little extra traction. There are Apocrineglands and these produce scent. Pheromones are very importantin mammals and these apocrine glands serve to produce the scentsused for territorial marking, for communicating reproductive status, and greatly increased in size to be used as a defensive mechanismin skunks and some other sorts of animals like that. And thenthere are Sebaceous glands which are the oil glands that primarilyserve to maintain their quality of the hair and the fur. So avariety of different kinds of skin glands with the exception ofthe preen gland in birds, we don't find any other types of skinglands in the reptiles, birds and mammals. Of course, amphibiansdo also have a variety of different kinds of skin glands as well.
The fourth characteristic of thegroup is that they are Homeothermic. They tend to have high andfairly stable body temperatures most of the time. And part ofbeing a homeotherm, you will remember I told you that in orderto be a homeotherm, you need to have some kind of a control mechanism. You have to some means of cooling your body whether that's sweatingor gular flutter or something like that. You have to have sensorsthat monitor the temperature of the skin and the temperature ofthe core of the body. The controller and sensor capabilitiesand even the panting as a cooling mechanism are found in modernreptiles. Modern reptiles have very good ability to regulatetheir body temperature. They can pant to cool their bodies alittle bit through evaporative water loss. But what they primarilydon't have is endothermy.
The fifth characteristic is Endothermy. This means having a high metabolic rate and just as with thebirds, you will remember that the minimum metabolic rate thatwe can measure in a mammal is likely to be in the neighborhoodof 6 times as high as the resting metabolic rate in a reptileunder comparable conditions, the body size and body temperature. So they have evolved much higher rates of oxygen consumption,food consumption and so forth.
So there are a whole series of additionalanatomical and physiological capabilities of mammals that wecan really consider to be parts of this evolution of homeothermy.
1) One of these is the 4-chamberedheart. Two atria and 2 ventricles. Birds have this and crocodileshave this.
2) More complex lung - that meansmore complex than modern reptiles. Much larger surface area andso forth. Birds lungs are more complex (they have air sacs thatthe air gets pulled into and the lungs themselves are not veryhighly flexible) and crocodiles don't have this.
3) A diaphragm, which is associatedwith the ventilation of the lungs. That sheet of muscle thatseparates the thoracic cavity from the abdominal cavity and itis dome shaped and it pulls down and helps ventilate the lungs. Neither birds nor crocodiles have this.
4) Another characteristic that wesee is that mammals have only a single aortic arch and that'sthe one called the aorta. It's an evolutionary modification fromthe condition that is seen in reptiles and amphibians in whichthere actually are both right and left aortas so the blood comesout of the heart and goes to both right and left sides and thencirculates around and goes down and connects up into a singleaorta back in the abdominal cavity. So the presence of only asingle aorta is something that is a specialization from that conditionof having 2 separate aortic arches. Left in mammals, right inbirds and 2 (both right and left) in crocodiles.
One thing that's interesting is thatin mammals that's a left aortic arch and I haven't heard any goodexplanations for why having a single aortic arch is an adaptationfor endothermy but there's only one other group of vertebrateanimals that has only a single aortic arch and that's the birdswho are also endotherms and the thing that's sort of surprisingis that in the birds, while there is a single aortic arch, itis a right aortic arch. So we can compare some of these thingswith the birds, they are also endotherms and so if only the birdsand mammals are endotherms and if only the birds and mammals haveonly a single aortic arch, then one good interpretation wouldbe that for some reason this is an adaptation for endothermy. One might argue that it is something that they have inheritedfrom their common ancestors but the fact that in one group it'sa right, and the other group it's a left would argue against thatcommon ancestor.
5) A secondary palate is also anadaptation for endothermy in mammals. Birds don't have this (becausethey don't chew their food even) but crocodiles have this.
Crocodiles are not endotherms posesome interesting comparisons above. So in some ways the crocodilesare similar even though they are not endotherms.
The sixth characteristic in mammalsis Most lack color vision. Almost all vertebrate animals havegood color vision. Fish, reptiles, amphibians, and birds allhave good color vision. But apparently mammals due to that nocturnalheritage, color vision is of very little value and in fact insome ways, color vision is a disadvantage in the dark (if youare nocturnal) because the photoreceptors that allow us to distinguishcolors are quite a bit less sensitive to light than are the photo-receptorsthat simply detect the presence of light. The majority of livingmammals can be shown to not have color vision. The 3 biggestorders of mammals are the bats, rodents, and the insectivoresand all 3 of those major mammalian orders are largely nocturnal. There are a few diurnal representatives in each of these groups but most of them are nocturnal. So the majority of living mammalsare also color blind.
There are examples of mammals suchas ourselves that have become diurnal animals (day-active). Thereare for example the ground squirrels are a group of rodents thatare day-active rodents and they have partial color vision. Theyare red-green color blind but they can distinguish other colors. And among the primates, ourselves, and a number of other diurnalprimates also have re-evolved good color vision. But by in largemammals lack color vision.
The seventh characteristic of mammalsis Diphyodont Dentition. Diphyodonty having 2 sets of teeth sothat there will be basically just 1 phase in the life of the animalwhen there is preprogrammed and symmetrical replacement of teethand then the remaining teeth will last throughout the life ofthe animal. That is made possible because the teeth have moreroots, larger surface area and are more resistant to damage andloss.
I'm not listing heterodonty herealthough heterodonty is found in many groups of mammals becauseof the fact that there are also quite a few good examples of modernmammals that have secondarily become homodont in their dentition. Heterodonty is a primitive feature of the mammals but it's notfound in all of the modern living groups of mammals so we're notlisting it.
The eighth characteristic is theJaw or mandible composed of just the dentary bone, this is a featureunique to the mammals. All other vertebrates except for the jawlessvertebrates (agnathans) do have a jaw. Even birds have a jawbut in all other groups of vertebrates the jaw is composed ofa dentary plus an articular or in many cases several additionalbones.
The ninth characteristic is the JawJoint. Dentary-squamosal jaw joint replacing the old quadrate-articularjaw joint of their reptilian ancestors which is also found inthe birds as well as amphibians although there are some differencesthere.
The tenth characteristic is the ThreeOssicles in the Middle Ear. And again associated with this, thepresence of 3 middle ear ossicles in mammals whereas all of theother vertebrates have only 1.
The eleventh characteristic is thePresence in the Kidney of a Loop of Henle. I have talked aboutthe way in which LOH creates the concentration gradient that isused by the collecting duct to produce a concentrated urine. Birds actually have a LOH. There's not a LOH found in the kidneyof any other vertebrate animals. Now even though the birdshave a LOH, the concentration of dissolved solutes in the urineof a bird is almost never higher than the concentration of thesolutes in the animal's blood.
So, technically speaking birds donot produce a concentrated urine and that's sort of a narrow viewof things because remember that in birds the primary nitrogenouswaste product is uric acid, and it's relatively insoluble sothat as the water is removed in the collecting duct in a mechanismthat's similar to what happens in mammals is the water is removedfrom the filtrate in the urine of the bird, the uric acid precipitatesout rather than staying in solution the way urea does and so asa consequence we can say that there is only like one species ofbird that's been shown to have a higher total solute concentrationin it's blood (Salt Marsh Sparrow) - this bird drinks a lot ofsalt water and so the dissolved solutes in that bird's urine aresalts and not nitrogenous waste products. So in general birdsdo not produce concentrated urine. But the LOH conserves waterlike in mammals.
Now we can distinguish the 3 majorgroups of mammals from each other. They all share in generalthese features so the monotremes have 3 middle ear ossicles anda jaw composed of just a dentary and so forth. And they havemammary glands and fur and everything.
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Characteristics of Living Prototherians
1. Reptilian Features of skeleton:
Here are some of the things thatdistinguish the Prototherians (the living monotremes). In generalthey have a number of features of their skeleton which is veryreptilian in nature.
For example they have ribs on theircervical vertebrae, that's something that's seen in reptiles. It's not seen in mammals.
As a general statement, the limbsof Prototherians have a reptilian kind of orientation. Insteadof having the limbs rotated underneath the body and more or lessvertical, they are more horizontal in that kind of permanent push-uporientation that is seen in the reptiles.
And associated with that horizontalorientation of the limbs is a very reptilian plate-like pectoralgirdle including among other things a bone that we don't findin the pectoral girdles of other mammals, a bone called an interclaviclewhich is located between the 2 clavicle bones.
And you may also remember that Itold you that a modification in the mammalian scapula that isassociated with the development of the rotation of the limbs underneaththe body is a scapular spine and so you won't be surprised tolearn that they have no spine on the scapula. The supraspinousfossa in modern mammals is a new area of the scapula, in otherwords, it's only the infraspinous fossa of the mammal which ishomologous with the scapula of a reptile. The spine of scapulaand the supraspinous fossa are new.
So there are a number of featuresof the skeleton of these Prototherians which is very reptile-likeand not like what we see in the other 2 major groups of mammalsthat are much more closely related to one another.
2. Reproduction:
Reproduction is probably the areain which the Prototherians differ the most however.
a. Fertilization - They do haveinternal fertilization which is found in all amniotes. It isachieved by a column of erectile tissue that comes out of thecloaca of the animal (see diagram). It's an extension of thefloor of the cloaca but it has the same kind of components ofan erectile tissue that are found in the penis of a rat for example. So they do have internal fertilization by means of that.
b. Testes - Another things thathas evolved in mammals and that you see in the rat is the testesthat descend. But they don't do that in the Prototherians. Thereare some groups of Eutherian mammals in which they also do notdescend out of the body cavity into a scrotal sac.
c. Oviparous - But the biggest singledifference between the Prototherians and the rest of the mammalsis that the fact that the Prototherians are oviparous. They laya large macrolecithal egg with a leathery covering that's verymuch like the egg of a reptile. They incubate that egg, the femalemonotreme develops a pouch in which she retains this leatheryegg and incubates it and keeps it near her own body temperature.
d. Young - After they are born,the young are fed from mammary glands but even the mammary glandsin the Prototherians are very different in that they don't havea nipple and in fact instead of having the mammary tissue sortof localized and exiting through a duct, what the marsupials haveis something in the neighborhood of 100 or more little tiny accumulationsof mammary tissue along the belly, each one of those little separateaccumulations excretes the milk through a pore onto the fur ofthe belly and the baby monotreme licks the milk off of the furrather than hooking onto the nipple and sucking the milk out.
At one time, this particular arrangementin the monotremes was interpreted as being a primitive featureof these obviously very unusually mammals and so it was thoughtthat these 100 or 120 little separate mammary glands looked alot like sweat glands and lead to the generalization that mammaryglands had probably evolved as a specialized type of sweat gland. But the current interpretation is that is not the case, thatthis is a peculiar feature of the Prototherians and does not representan early stage in the development of the mammary glands.
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Characteristics of Living Metatherians
The second largest group are theMetatherians. Or as we frequently refer to them as the Marsupials.
Skeleton - And in these animals theskeleton is much more like the skeleton of the general Eutherians(cat). The Metatherians have the same number of vertebrae inthe various segments of the vertebral column, 7 cervical, 13 thoracic,7 lumbar and 3 sacral vertebrae just as do the majority of theplacental mammals that you are familiar with. They have a scapularspine and supraspinous fossa just as the Eutherian mammals do.
Reproduction:
a. Viviparous - They are viviparousbut it's in this area of reproduction that the marsupials reallydiffer the most from the placental mammals. They have a veryshort gestation period and a much more extended period when theyoung are nourished by lactation.
b. Fertilization & Development- The fertilization is internal and the testes descend into ascrotal sac just as they do in the majority of Eutherian mammals. But one thing that is different about these animals is that thepenis is located posterior to the testes rather than anterioras it is in Eutherian mammals. So that suggests that there'sa possibility that this business of having the testes descendactually evolved separately in the 2 groups.
Now the arrangement of the femalereproductive tract is also different as shown here at the bottomof the page. And what you can see is that rather than havinga single body of the uterus as in the rat, there are 3 differentstructures that lie in the same general area of the female reproductivesystem. There is a single sinus vaginalis in the center throughwhich the young are going to be born. And the embryo is actuallydeveloped within the uterine horn so that's similar to the placentalarrangement but what's different about the reproductive tractof these animals is that they have paired lateral vaginas. These2 structures that are located lateral to the sinus vaginalis serveto transmit the sperm from the urogenital sinus up to the uterinehorn so the sperm go up these 2 lateral tracts, the babies developin the uterine horn and then the babies are born down throughthe sinus vaginalis.
Now associated with this unusualspecial pair of structures for the sperm to get up to the uterinehorn is the fact that the male has a penis with 2 heads calleda bifid penis.
So the females have a double clitorisas well. And the sperm are ejaculated through the 2 separateheads of the penis into the 2 lateral vagina tracts so they canget up to the 2 horns of the uterus.
c. Cloaca - The Metatherians, livingmarsupials, have a cloaca which is a common opening of the urogenitalsinus and the rectum.
And as I said, the biggest differencebetween the Metatherians and the Eutherians is in the patternof development. Some zoologists about 15 to 20 years ago thoughtthat the marsupials were inferior, that they were an inferiorform of animal. In fact if you look at the names of the groups, the mammals are divided into the Prototherian - Therian is aLatin word that really means animals. But it's loosely translatedas meaning mammals. And Prototherians means like the proto-type,the first evolutionary effort at evolving the mammalian condition. And Eutherian means true mammals and Metatherian means some kindof middle or sort of approximation of the wonderful true placentalcondition of being a mammal. And so even the names of the groupsreflect this idea that the Metatherians are somewhat inferiorin their adaptations. Now there are some observations about theevolution of Metatherians that appear on the surface to be consistentwith this basic idea that the placentals are superior to the Metatheriansand one of the² lines of evidence is based upon the observationthat in South America, there used to be many more species of marsupials. South America after it separated from Africa was totally surroundedby water, a large island continent that had a very extensive faunaof marsupials including big carnivores and a tremendous varietyof those animals and when North America and South America beganto approach their present condition there was a land-bridge connectedbetween the 2, then there was the possibility for placental mammalsfrom North America to invade across the land-bridge into SouthAmerica and when that happened, the placental mammals out-competedthe marsupials and caused a vast extinction of marsupials in SouthAmerica. That observation is consistent with this idea that theplacentals are superior to the marsupials and in a similar waya number of different types of placental mammals introduced intoAustralia have really reeked havoc with the marsupial fauna ofAustralia. Sheep for example and cats have ¡just had a devastatingeffect on a number of different native species of marsupials inAustralia. And that's consistent with this observation as well.
Eutherians have a much longer periodof time where the young are taken care of by gestation and a relativelyshorter period of time where they are nourished through milk fromtheir mothers. The marsupial pouch young may have poorly developedhind limbs but for an animal that has only undergone a week or2 of embryonic development, it really has highly accelerated developmentof it's front limbs and claws on the digits because they haveto make the trek from the vagina, crawl up the belly of the motherand find the nipple and hook on all by themselves. The motherdoesn't transfer them by herself. She licks her fur to sortof make a little nice little freeway that goes to the right placebut other than that, the youngster has to get there under theirown power. They also have to have completely functional respiratorysystems, digestive systems, and fairly extensive neuromusculardevelopment and nervous system after only a couple of weeks ofgestation in order to be able to make this trek. So it's reallynot correct to say that they have poorly developed young. Theyhave just emphasized lactation as a way of nourishing the youngand they have actually accelerated the development of importantdevelopmental sequences in order to be able to allow the youngto be able to make that trip.
The gestation period in some of thesmaller marsupials can be assured as 11 days and the longest gestationperiod is only 4 weeks. Compare that with the gestation periodin human beings of 9 months and it's even longer in some largermammals. So it's really just a relatively greater period ofdevelopment outside of the womb than inside.
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Characteristics of Living Eutherians
Skeleton - Similar to Metatherians. There are some minor differences in the arrangement of the bonesof the skull. There are also some differences in the numbersof teeth that are found in Eutherians and Metatherians primarilyinvolving the number of incisors. Marsupials tend to have moreincisors than do placental mammals. But that's a minor difference.
Reproduction :
a. Viviparous - They are all viviparous. They all have internal fertilization by means of a penis. Inmost species the testes are descended but the penis is locatedanterior to the testes.
b. Placenta - There's a lot of variabilityin the structure of the placenta in placental mammals. Some havemuch more intimate contact between the maternal bloodstream andthe fetal bloodstream. And the whole process of development ofthe placenta undergoes a very accelerated sort of short-cut typeof development that speeds up the process of developing a placentain a placental mammal.