Today's lecture is on the characteristics ofthe mammals as a group and also the distinctions between the 3major subgroups of mammals and it is covered in your illustrativenotes.
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The Class Mammalia has 2 subclasses, SubclassPrototheria and there are a variety of different fossil speciesthat are known that are placed in this subclass, only a couple ofliving species. And then the other subclass includes the majorityof the mammals and that's the Subclass Theria which has 2infraclasses, Metatheria and Eutheria.
The living Prototherians belong to an ordercalled Order Monotremata so we just generally refer to theseanimals 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 rest ofthe mammals or they may even be in some ways considered to berelectual therapsids in some ways, they are very much like thetherapsids. But the majority of the mammals are placed in theSubclass Theria and we have the marsupials like the kangaroos andkoala bears and things like that and then the rest of them thatinclude ourselves the placental mammals.
There are a total of 4,060 mammals alltogether, living species. Three living species of Monotremes, 242Metatherians, 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 about generalcharacteristics of the Class Mammalia. These are characteristicsthat apply in general to all 3 groups.
The first one that we will mention is thePresence of Mammary Glands for which the class is named. This isa unique feature of the class, of course the function of themammary glands is lactation - that is to provide milk to thedeveloping young, particularly important during the early phasesof their development when they don't have any teeth.
The second characteristic which is peculiar tothe class is the Presence of Hair or Fur at least evolvingoriginally as a form of insulation necessary as part ofadaptation for homeothermy - constant body temperature throughendothermy.
Third, mammals also Have a number of otherstructures/glands in their skin - that is in addition to mammaryglands. The skin glands have different purposes. There areEccrine glands that produce sweat which serves as a means ofcooling, some species of mammals have enough sweat glands so thatthey can evaporate water off their skin and cool. In some animalsthat don't cool by sweating, they may still have a small numberof these eccrine glands like in the pads of their feet to givethem a little extra traction. There are Apocrine glands and theseproduce scent. Pheromones are very important in mammals and theseapocrine glands serve to produce the scents used for territorialmarking, for communicating reproductive status, and greatlyincreased in size to be used as a defensive mechanism in skunksand some other sorts of animals like that. And then there areSebaceous glands which are the oil glands that primarily serve tomaintain their quality of the hair and the fur. So a variety ofdifferent kinds of skin glands with the exception of the preengland in birds, we don't find any other types of skin glands inthe reptiles, birds and mammals. Of course, amphibians do alsohave a variety of different kinds of skin glands as well.
The fourth characteristic of the group is thatthey are Homeothermic. They tend to have high and fairly stablebody temperatures most of the time. And part of being ahomeotherm, you will remember I told you that in order to be ahomeotherm, you need to have some kind of a control mechanism.You have to some means of cooling your body whether that'ssweating or gular flutter or something like that. You have tohave sensors that monitor the temperature of the skin and thetemperature of the core of the body. The controller and sensorcapabilities and even the panting as a cooling mechanism arefound in modern reptiles. Modern reptiles have very good abilityto regulate their body temperature. They can pant to cool theirbodies a little bit through evaporative water loss. But what theyprimarily don't have is endothermy.
The fifth characteristic is Endothermy. Thismeans having a high metabolic rate and just as with the birds,you will remember that the minimum metabolic rate that we canmeasure in a mammal is likely to be in the neighborhood of 6times as high as the resting metabolic rate in a reptile undercomparable conditions, the body size and body temperature. Sothey have evolved much higher rates of oxygen consumption, foodconsumption and so forth.
So there are a whole series of additionalanatomical and physiological capabilities of mammals that we canreally consider to be parts of this evolution of homeothermy.
1) One of these is the 4-chambered heart. Twoatria and 2 ventricles. Birds have this and crocodiles have this.
2) More complex lung - that means more complexthan modern reptiles. Much larger surface area and so forth.Birds lungs are more complex (they have air sacs that the airgets pulled into and the lungs themselves are not very highlyflexible) and crocodiles don't have this.
3) A diaphragm, which is associated with theventilation of the lungs. That sheet of muscle that separates thethoracic cavity from the abdominal cavity and it is dome shapedand it pulls down and helps ventilate the lungs. Neither birdsnor crocodiles have this.
4) Another characteristic that we see is thatmammals have only a single aortic arch and that's the one calledthe aorta. It's an evolutionary modification from the conditionthat is seen in reptiles and amphibians in which there actuallyare both right and left aortas so the blood comes out of theheart and goes to both right and left sides and then circulatesaround and goes down and connects up into a single aorta back inthe abdominal cavity. So the presence of only a single aorta issomething that is a specialization from that condition of having2 separate aortic arches. Left in mammals, right in birds and 2(both right and left) in crocodiles.
One thing that's interesting is that in mammalsthat'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, it isa right aortic arch. So we can compare some of these things withthe birds, they are also endotherms and so if only the birds andmammals are endotherms and if only the birds and mammals haveonly a single aortic arch, then one good interpretation would bethat for some reason this is an adaptation for endothermy. Onemight argue that it is something that they have inherited fromtheir common ancestors but the fact that in one group it's aright, and the other group it's a left would argue against thatcommon ancestor.
5) A secondary palate is also an adaptation forendothermy in mammals. Birds don't have this (because they don'tchew their food even) but crocodiles have this.
Crocodiles are not endotherms pose someinteresting comparisons above. So in some ways the crocodiles aresimilar even though they are not endotherms.
The sixth characteristic in mammals is Mostlack color vision. Almost all vertebrate animals have good colorvision. Fish, reptiles, amphibians, and birds all have good colorvision. But apparently mammals due to that nocturnal heritage,color vision is of very little value and in fact in some ways,color vision is a disadvantage in the dark (if you are nocturnal)because the photoreceptors that allow us to distinguish colorsare quite a bit less sensitive to light than are thephoto-receptors that simply detect the presence of light. Themajority of living mammals can be shown to not have color vision.The 3 biggest orders of mammals are the bats, rodents, and theinsectivores and all 3 of those major mammalian orders arelargely nocturnal. There are a few diurnal representatives ineach of these groups but most of them are nocturnal. So themajority of living mammals are also color blind.
There are examples of mammals such as ourselvesthat have become diurnal animals (day-active). There are forexample the ground squirrels are a group of rodents that areday-active rodents and they have partial color vision. They arered-green color blind but they can distinguish other colors. Andamong 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 mammals isDiphyodont Dentition. Diphyodonty having 2 sets of teeth so thatthere 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 of theanimal. That is made possible because the teeth have more roots,larger surface area and are more resistant to damage and loss.
I'm not listing heterodonty here althoughheterodonty is found in many groups of mammals because of thefact 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 the Jaw ormandible composed of just the dentary bone, this is a featureunique to the mammals. All other vertebrates except for thejawless vertebrates (agnathans) do have a jaw. Even birds have ajaw but 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 Jaw Joint.Dentary-squamosal jaw joint replacing the old quadrate-articularjaw joint of their reptilian ancestors which is also found in thebirds as well as amphibians although there are some differencesthere.
The tenth characteristic is the Three Ossiclesin the Middle Ear. And again associated with this, the presenceof 3 middle ear ossicles in mammals whereas all of the othervertebrates have only 1.
The eleventh characteristic is the Presence inthe Kidney of a Loop of Henle. I have talked about the way inwhich LOH creates the concentration gradient that is used by thecollecting duct to produce a concentrated urine. Birds actuallyhave a LOH. There's not a LOH found in the kidney of any othervertebrate animals. Now even though the birds have a LOH, theconcentration of dissolved solutes in the urine of a bird isalmost never higher than the concentration of the solutes in theanimal's blood.
So, technically speaking birds do not produce aconcentrated urine and that's sort of a narrow view of thingsbecause remember that in birds the primary nitrogenous wasteproduct is uric acid, and it's relatively insoluble so that asthe water is removed in the collecting duct in a mechanism that'ssimilar to what happens in mammals is the water is removed fromthe filtrate in the urine of the bird, the uric acid precipitatesout rather than staying in solution the way urea does and so as aconsequence we can say that there is only like one species ofbird that's been shown to have a higher total soluteconcentration in it's blood (Salt Marsh Sparrow) - this birddrinks a lot of salt water and so the dissolved solutes in thatbird's urine are salts and not nitrogenous waste products. So ingeneral birds do not produce concentrated urine. But the LOHconserves water like in mammals.
Now we can distinguish the 3 major groups ofmammals from each other. They all share in general these featuresso the monotremes have 3 middle ear ossicles and a jaw composedof just a dentary and so forth. And they have mammary glands andfur and everything.
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Characteristics of Living Prototherians
1. Reptilian Features of skeleton:
Here are some of the things that distinguishthe Prototherians (the living monotremes). In general they have anumber of features of their skeleton which is very reptilian innature.
For example they have ribs on their cervicalvertebrae, that's something that's seen in reptiles. It's notseen in mammals.
As a general statement, the limbs ofPrototherians have a reptilian kind of orientation. Instead ofhaving the limbs rotated underneath the body and more or lessvertical, they are more horizontal in that kind of permanentpush-up orientation that is seen in the reptiles.
And associated with that horizontal orientationof the limbs is a very reptilian plate-like pectoral girdleincluding among other things a bone that we don't find in thepectoral girdles of other mammals, a bone called an interclaviclewhich is located between the 2 clavicle bones.
And you may also remember that I told you thata modification in the mammalian scapula that is associated withthe development of the rotation of the limbs underneath the bodyis a scapular spine and so you won't be surprised to learn thatthey have no spine on the scapula. The supraspinous fossa inmodern mammals is a new area of the scapula, in other words, it'sonly the infraspinous fossa of the mammal which is homologouswith the scapula of a reptile. The spine of scapula and thesupraspinous fossa are new.
So there are a number of features of theskeleton of these Prototherians which is very reptile-like andnot like what we see in the other 2 major groups of mammals thatare much more closely related to one another.
2. Reproduction:
Reproduction is probably the area in which thePrototherians differ the most however.
a. Fertilization - They do have internalfertilization which is found in all amniotes. It is achieved by acolumn of erectile tissue that comes out of the cloaca of theanimal (see diagram). It's an extension of the floor of thecloaca but it has the same kind of components of an erectiletissue that are found in the penis of a rat for example. So theydo have internal fertilization by means of that.
b. Testes - Another things that has evolved inmammals and that you see in the rat is the testes that descend.But they don't do that in the Prototherians. There are somegroups of Eutherian mammals in which they also do not descend outof 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 lay alarge macrolecithal egg with a leathery covering that's very muchlike the egg of a reptile. They incubate that egg, the femalemonotreme develops a pouch in which she retains this leathery eggand incubates it and keeps it near her own body temperature.
d. Young - After they are born, the young arefed from mammary glands but even the mammary glands in thePrototherians are very different in that they don't have a nippleand in fact instead of having the mammary tissue sort oflocalized and exiting through a duct, what the marsupials have issomething in the neighborhood of 100 or more little tinyaccumulations of mammary tissue along the belly, each one ofthose little separate accumulations excretes the milk through apore onto the fur of the belly and the baby monotreme licks themilk off of the fur rather than hooking onto the nipple andsucking the milk out.
At one time, this particular arrangement in themonotremes was interpreted as being a primitive feature of theseobviously very unusually mammals and so it was thought that these100 or 120 little separate mammary glands looked a lot like sweatglands and lead to the generalization that mammary glands hadprobably evolved as a specialized type of sweat gland. But thecurrent interpretation is that is not the case, that this is apeculiar feature of the Prototherians and does not represent anearly stage in the development of the mammary glands.
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Characteristics of Living Metatherians
The second largest group are the Metatherians.Or as we frequently refer to them as the Marsupials.
Skeleton - And in these animals the skeleton ismuch more like the skeleton of the general Eutherians (cat). TheMetatherians have the same number of vertebrae in the varioussegments of the vertebral column, 7 cervical, 13 thoracic, 7lumbar and 3 sacral vertebrae just as do the majority of theplacental mammals that you are familiar with. They have ascapular spine and supraspinous fossa just as the Eutherianmammals do.
Reproduction:
a. Viviparous - They are viviparous but it's inthis area of reproduction that the marsupials really differ themost from the placental mammals. They have a very short gestationperiod and a much more extended period when the young arenourished by lactation.
b. Fertilization & Development - Thefertilization is internal and the testes descend into a scrotalsac just as they do in the majority of Eutherian mammals. But onething that is different about these animals is that the penis islocated posterior to the testes rather than anterior as it is inEutherian mammals. So that suggests that there's a possibilitythat this business of having the testes descend actually evolvedseparately in the 2 groups.
Now the arrangement of the female reproductivetract is also different as shown here at the bottom of the page.And what you can see is that rather than having a single body ofthe uterus as in the rat, there are 3 different structures thatlie in the same general area of the female reproductive system.There is a single sinus vaginalis in the center through which theyoung are going to be born. And the embryo is actually developedwithin the uterine horn so that's similar to the placentalarrangement but what's different about the reproductive tract ofthese animals is that they have paired lateral vaginas. These 2structures 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 babiesdevelop in the uterine horn and then the babies are born downthrough the sinus vaginalis.
Now associated with this unusual special pairof structures for the sperm to get up to the uterine horn is thefact that the male has a penis with 2 heads called a bifid penis.
So the females have a double clitoris as well.And the sperm are ejaculated through the 2 separate heads of thepenis into the 2 lateral vagina tracts so they can get up to the2 horns of the uterus.
c. Cloaca - The Metatherians, livingmarsupials, have a cloaca which is a common opening of theurogenital sinus and the rectum.
And as I said, the biggest difference betweenthe Metatherians and the Eutherians is in the pattern ofdevelopment. 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 mammaliancondition. And Eutherian means true mammals and Metatherian meanssome kind of middle or sort of approximation of the wonderfultrue placental condition of being a mammal. And so even the namesof the groups reflect this idea that the Metatherians aresomewhat inferior in their adaptations. Now there are someobservations about the evolution of Metatherians that appear onthe surface to be consistent with this basic idea that theplacentals are superior to the Metatherians and one of the
Eutherians have a much longer period of timewhere 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 poorlydeveloped hind limbs but for an animal that has only undergone aweek or 2 of embryonic development, it really has highlyaccelerated development of it's front limbs and claws on thedigits because they have to make the trek from the vagina, crawlup the belly of the mother and find the nipple and hook on all bythemselves. The mother doesn't transfer them by herself. Shelicks her fur to sort of make a little nice little freeway thatgoes to the right place but other than that, the youngster has toget there under their own power. They also have to havecompletely functional respiratory systems, digestive systems, andfairly extensive neuromuscular development and nervous systemafter only a couple of weeks of gestation in order to be able tomake this trek. So it's really not correct to say that they havepoorly developed young. They have just emphasized lactation as away of nourishing the young and they have actually acceleratedthe development of important developmental sequences in order tobe able to allow the young to be able to make that trip.
The gestation period in some of the smallermarsupials can be assured as 11 days and the longest gestationperiod is only 4 weeks. Compare that with the gestation period inhuman 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 aresome minor differences in the arrangement of the bones of theskull. There are also some differences in the numbers of teeththat are found in Eutherians and Metatherians primarily involvingthe number of incisors. Marsupials tend to have more incisorsthan do placental mammals. But that's a minor difference.
Reproduction :
a. Viviparous - They are all viviparous. Theyall have internal fertilization by means of a penis. In mostspecies the testes are descended but the penis is locatedanterior to the testes.
b. Placenta - There's a lot of variability inthe 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 aplacenta in a placental mammal.