ZOO 138, Friday, February 7, 1997, 12:00 p.m.
Today I'm going to talk about the living reptiles, and these are found in several differentplaces in your classifications. The first group is in the subclass Anapsida, and this is the order --this lecture is covered in your Illustrated Notes. These are the turtles and tortoises and sea turtlesand so fourth. There are about 250 species, and they're found in tropical and temperate regionsfrom throughout the world.
The habitats in which we find turtles pretty much cover the full range from terrestrialhabitats, that is living on land all the time, that would include animals like the Mojave deserttortoise which we find in our deserts. Freshwater habitats include a lot of pond turtles, box turtles,and snapping turtles found in freshwater habitats.
And marine, would be the third of these environments, and that's where the find the bigsea turtles.
The oldest fossils of the turtles are found in the Triassic, but they are thought to bedescended from the captorhinomorphs having branched off from the captorhinomorphs fairly earlyon. And I think that nobody has any difficulty at all recognizing a member of this order. They allhave a shell that encases their body. The upper part of that shell is called a "carapace." And thelower part is the "plastron."
And both of these structures are composed of a couple of layers of material. If you lookat the Illustrated Notes, you can see that there is that outer layer which is labeled "horny plate."That terms "horny," in a scientific context, refers to the something which is composed of theprotein keratin.
Keratin is the protein the makes up the claws of animals' finger nails, covers the horns ofanimals like cows and sheep, hence the term "horny."
But it's one of the major proteins used by vertebrate animals in constructing structuresthat are on the outside of the body that need to be hard and resistant to wear. So the outer layerof both the carapace and plastron is composed of plates of keratin that are called scutes. And thenthere is an inner layer of these 2 structures, which is composed of dermal bone.
Dermal bone is different from haversion bone, which you have located at under amicroscope and studied the details of in lab, in that it does not have that wonderful elaboratestructure of the haversion systems, and it also has -- it's formed within the dermis and it justcomes out in layers.
So that's what the inner part of the bony plates are composed of.
And the carapace itself, if you look at this diagram, you can see that it is outside of theribs. So the carapace, the upper part does not include the vertebrae and the ribs. When you lookat the plastron, the one on the bottom of the animal, you will see that actually does have someelements of the pectoral girdle in it. And you'll have chance to take a look at those in lab whenyou study the skeletons of vertebrate animals.
Some other characteristics of turtles, of course, they are toothless. They have a beakcovered end with keratin, which is very similar to the beak of a bird, although it is independentlyevolved. So in a sense you could say that it's an example of convergent evolution.
They are certainly descended from reptiles that had teeth, and they abandoned teeth inorder to have this beak-like structure.
There is a third situation in which an animal apparently independently as well evolvedbeaks, and that is there are a number of Ornithischian dinosaurs, like triceratops that have beaksas well. This is something that vertebrate animals have appeared to evolved at least 3 differenttimes.
Probably the most bizarre thing about the anatomy of turtles is really can only beappreciated after you have studied the diversity of vertebrate skeletons for a while, but that is thefact that the pectoral girdle of these animals, the bones to which the front limbs are attached, thepectoral girdle is located inside the ribs and.
The pectoral girdles of all other vertebrate animals are located outside the ribs. Yourscapula, for example, is located outside your ribs. And it is extremely unusual for vertebrateanimals to reverse the position of 2 major skeletal structures like that.
In other words, if you assume as the old time zoologists did that natural selection was theonly mechanism of evolution, that things only changed in very, very gradual and in perceptivelysmall degrees, then you'd have a really hard time explaining how you can reverse the position ofthe vertebrate and the pectoral girdle.
Now, the pectoral girdle of turtles is so different from the pectoral girdles of othervertebrates that it's not clear that is it strictly homologous of that type of vertebrate animals. Buteven that is a pretty bizarre situation.
It's just not well understood how they did it.
There is quite of bit of variation in the size and extent of the shell of a turtle. It can bevery high-domed and kind of rough on the outside, that's what we see in the terrestrial types, likethe tortoises, the Galapagos tortoises or the desert tortoises.
The shell can be very smooth and streamlined and yet big enough for the animal to pull itshead and legs inside, that's what we see in the pond turtles. Or else the shell can be very, veryreduced and, in fact, it's not possible for the animal to pull its appendages inside, and that's whatwe see in the sea turtles and also in the snapping turtles.
But the shell, of course, is primarily a defensive structure. And in the really big animalsand really aggressive animals like the snapping turtles, they have essentially -- they don't need theshell for defense. And their own behavior and powerful beaks and muscles serve for defense.
And reducing the shell also makes the animal more agile. So a snapping turtle is a goingto be able to jump around and go after prey. So among highly predatory turtles, we're more likelyto see a reduction in the size of the shell.
There are turtles that are called box turtles, in which the plastron has essentially aconnective tissue hinge so that after they pull their head inside the bottom, plastron comes up andclosely abuts the carapace, so they are really sealed inside that box as a defensive mechanism.
The largest of all the living turtles is a big sea turtle called a "leather back," which gets tobe a couple of meters in diameter and weighs over 500 kilograms. Those are really big turtles.
Natural history of turtles, different species are herbivores. Some are carnivores like thesnapping turtles, for example. Many turtles are omnivores, those are all terms that you need to besure you are familiar with. Carnivore is a immediate eater. Herbivore is eating exclusively plantmaterial. And omnivore is like human beings are, at least in the primitive condition of eating, it's amixture of plant and animal materials.
Reproduction in turtles involves elaborate mating calls. We tend to think of turtles asbeing pretty quiet, but they actually make loud noises. And they also produce pheromones forsignalling between members of the species.
Fertilization is internal by means of a single copulatory organ. That's a characteristic thatdistinguishes them from lizards and snakes, because they have paired copulatory organs. Turtlesproduce amniotic eggs. I'll tell you more about that towards the end of the on lecture on Monday.But these eggs have a very thin shell, nothing like the hard calcium carbonate shell that you arefamiliar with in the bird's egg which is also an amniotic egg.
Some turtles, in fact, have no calcium carbonate at all. Some have a leathery kink ofcovering for the egg, and some have a very thin almost parchment-like calcium carbonatecovering.
The eggs are usually buried in the ground, frequently in fairly large groups. And you mayhave seen some of the natural history films that show the synchronized hatching of sea turtleshatchlings where they break out of their eggs all at the same time. It takes the combined effort ofall of them to dig their way out of the whole out in the sand where the female deposits the eggs inthe nest.
So those are the members -- those are the living Anapsida, the only living Anapsidareptiles.
The only living Archosaurs in the subclass archosauria, is the order crocodilia, thecrocodiles. And there are also alligators. And a couple of species of small crocodile-like animalscalled "caimans."
There are only 21 species.
They are found in tropical regions throughout the world.
There are only a couple of species alligators. Most of the members of this order areplaced in the family that includes the crocodiles.
Alligators, there is 2 species of alligators, they are found in freshwater habitats.
And the crocodiles are found in both freshwater and the marine habitats.
So the big Nile crocodile is a freshwater crocodile. The largest of all the crocodiles is asea-going Indo-Pacific (spelling) salt water crocodile. And those are the big ones. They get to beup to 21 feet in length. That's probably the guy who was chasing Capital Hook, by the way, sincehe was in the ocean when he was sailing his ship.
First fossil crocodiles are known from the Triassic. And they had somewhat larger hindlimbs than their front limbs. Even modern crocodiles have a somewhat larger hind limb, and that'sa remanent of that bipedal ancestry of the Thecodonts that I told you about. They stood on theirhind legs and had bigger hind limbs. And we see that remanent of characteristic in all of thedescendants of the Thecodonts.
The tail the laterally compressed. For swimming, they beat the tail from side to side. Theyhave an eardrum, but you can't see it because the opening in the skull, at the base of which theeardrum is located, that opening is covered with a flap, and they can actually seal off thatopening so that when they dive in the water, not much water gets down in there next to theeardrum.
The nostril, of course, is located out at the tip of the snout of the animal, but it can beclosed. They have a ring of muscle around the nostril, so that when they dive in the water, theydon't have their nasal cavities filling up with water which would happen otherwise.
And they have a structure that we're going to see in mammals but not in birds, which iscalled a "secondary palate."
The secondary pallet is a plate of bone that separates a nasal cavity off from the oralcavity. Now, you know, put your tongue up against the roof of your mouth, that is a secondarypalate. You have a cavity above that which is attached to your nostril through which you canbreathe. So you can breathe when you have your mouth shut.
And mammals have evolved a secondary palate. And crocodiles have the same thing. It isnot found in fish or in amphibians or in birds. In those animals, the air that comes in through thenostrils is actually in the same cavity as the mouth cavity up at the front of the mouth cavity.
STUDENT: It is a separate nasal cavity from the oral cavity.
INSTRUCTOR: When you look at the -- when you study your rat in detail, you are goingto see that they have -- you can actually cut through that to see the 2 cavities.
So above the secondary palate is a nasal cavity, below is the oral cavity or your mouthcavity.
STUDENT: You said that it's similar to mammals and not like birds. Are there any otherreptiles that also have a secondary palate?
INSTRUCTOR: No.
Now, the secondary palate, therefore, here is an example of convergent evolution, we seeit in crocodiles, we see it in mammals, we don't see it in birds, and we'll talk some more about whywe don't see it in birds. But, basically, the reason that a secondary palate evolves is so at that ananimal can breathe while it is chewing food.
And crocodiles don't really chew food, but their means of capturing food involvesgrabbing onto an animal and holding onto it. Like if a crocodile is trying to catch an antelope or adeer, it will lie and wait by the water, looking like a log, and the animal comes down to take adrink water and it will grab it by the leg and pull it out into the water, and then the animal twists,and it's either trying to yank the leg off so that it bleeds to death or just drown it in the water.
So it's spending some period of time with the leg of an animal in its mouth. And in orderto be able to breathe while it is doing that it has evolved this secondary palate which maintains anasal cavity free for the passage of air.
And that's basically the same reason that mammals have it. Not that they are trying toyank the legs off antelopes, but because mammals have evolved a behavior of chewing their foodwhich is something we don't see in any other group of vertebrate animals.
So that's why a secondary palate evolved. Crocodiles are also the only reptiles to have a4-chambered heart. Most reptiles have a 3-chambered heart.
And, although, the hearts of most reptiles are able to function almost as well as a4-chambered heart, these are the only reptiles that anatomically have a 4-chambered heart.
All the crocodiles are carnivores. Sometimes you'll here it said that you can tell whetheran animal is a crocodile or an alligator by the shape of the snout. But turns out that that's not true.The generalization that you will hear, which is incorrect, is that alligators have narrow snout andcrocodiles have a broad snout.
But, in fact, there are crocodiles that feed on fish that have a narrow snout. And so thereal relationship is not a distinction between alligators and crocodiles; the real relationship is thatanimals that feed and birds and mammals tend to have broad snout, and the animals that feed onfish tend to have a narrow snout. Some crocodiles have a narrow snout and some have a broadsnout.
Courtship in crocodiles involves loud mating calls. The males will set up territories thatthey defend from other males in which the females come into when they are ready for breeding.
Fertilization is internal via a single copulatory organ which is an outgrowth of the cloacaof the animal. They mate in the water.
They produce amniotic eggs that have a very heavy calcium carbonate shell. And theyhave thicker shell than birds eggs. They don't have the same kind of microscopic structure as birdeggs, which I will tell you about on Monday. But they do have a heavy carbonate calcium shell.
They make a nest. And in many cases the nest is composed of rotting vegetation. And thesignificance of that rotting is that there is heat production by the vegetation as it rots by themicroorganisms that are decomposing the vegetation. And that serves to warm the eggs a little bitand accelerates the development of the embryos.
The parents will defend the nest and the eggs in the nest. The parents -- when the youngare ready to hatch, they start sort of scratching at the inside of the shell. And there are actuallyobservations of female crocodiles and alligators picking up their eggs and sort of crunching them,helping the babies hatch out of the eggs.
And then there is frequently very extensive parental care following the birth of the babyalligators and crocodiles. The parents will carry the babies in there mouth out to the riversomeplace where they can swim in the reeds and are safe from potential predators. The Americanalligator stays with its mother for a couple of years, and she finds food and shares it with them.
So these are really interesting animals that have an elaborate parental care that lasts forquite a long period of time.
I forgot to tell you something that is really bizarre about turtles, if you would go back tothe page on turtles.
It turns out that the sex of a turtle is determined by the incubation temperature of the egg.You're probably used the idea that the sex of a vertebrate offspring is determined by, at least inmammals, it's determined by the presence of the chromosome in the sperm. In mammals, thefemales have 2 X chromosomes, and males have an X and a Y chromosome. So the sperm eitherhas an X chromosome or a Y chromosome, and that determines the gender of the offspring.
But that's not true in all vertebrates. In fact, in birds, the X Y is found in females and notin males.
And in turtles, there are no sex chromosomes. Every embryo, early in its development hasall the genetic information that it needs to be either a female or male. And if the egg is beingincubated at a temperature above about 30 Celsius in the middle of incubation, then the embryowill develop into a male. And if the temperature of the egg is below 30 degrees, then the embryowill develop into a female.
So not all vertebrate animals have sex chromosomes, and this business of temperaturedetermines the sex of the offspring was only discovered recently, and it was really, really shockingwhen they discovered it.
STUDENT: Would that mean that all the eggs would come out to be one sex more orless with the same temperature?
INSTRUCTOR: Right at 30 -- obviously, I said above 30 you get females. If it happensto be right at 30, then you are going to get a mix of one or the other. And what happens becausethese are laid in nests where there is an accumulation of these eggs, you may get -- the wholething may be a little bit warmer in the middle than it is on the outside due to the metabolism of theembryos, so you might get males developing from the more interior eggs and the females from theexterior.
But the ecologists got very interested in this issue because under some environmentalconditions you might be more successful in producing grandchildren if your children were allfemales. And so the possibility of a female turtle determining the sex of her offspring by choosingthe place in which she laid her eggs.
And I haven't kept up on the literature to find out what they discovered about that. But itwas a very interesting discovery.
Any other questions about that?
STUDENT: You said that crocodiles and alligators were not distinguished by the shapeof the snout. So how are they distinguished?
INSTRUCTOR: Well, There are other technical features of their anatomy that I'm noteven familiar with that cause them to be placed in one family or the other. That's the kind ofquestion that Dr. Stuart could probably answer in explicit detail.
All the remaining living reptiles are placed in the subclass "Lepidosauria." And in theLepidosauria there is only a single living order. There are some extinct orders, but there is only asingle living order, and that's the order "Squamata." The root for this word Squamata where wesometimes refer to these animals as the squamate reptiles is the same as in squamous epithelium,which is simply a reference to a plate-like structure or a scale. So a squamous epithelial cell is areally flattened cell.
Squamate reptiles have scales in their skin, which is not something that is particularlyunusual, but that is basically the term.
There are 4 suborders within the order Squamata. The first one is a really unusual one,and it is only a single species. And this is the suborder "sphenodontia."
Among living reptiles there is only a single species, an animal called a "tuatara," which isin this suborder. In some classifications, this suborder will not be a suborder, but the tuatara isplaced in a different order.
But this is the classification that Dr. Stuart uses in the herpetology classes, so that'ssimplifies matters a little bit.
The tuatara, it basically looks like a good-sized lizard. They are about a meter and a halfin length. If you just look at one from any distance, you would just think you were seeing agood-sized lizard. They have a number of features of their biology that are the basis for placingthem in the separate suborder from all of other lizards.
They have a very primitive structure of the skill. So that just has to do with details ofwhat bones are present and how they relate to one another. We won't go into the details of that.
They have an organ called a "pineal eye," which is a light sensitive structure that's on theroof of the skull. It doesn't have a lens. It doesn't look like a cyclops kind of eye. It's unpaired, butit's a little sort of slightly discolored area on the surface of the head of the animal which issensitive to light, and which is connected directly to the brain of the animal and provides theanimal with information about day length and what season it is and whether it's time to breed andso forth. So that's one structure besides the weird structure of the bones of the skull.
These guys are found -- were originally found on the island of New Zealand, but thedestruction of the habitat and the presence of goats and cats and things like that have -- nowthey're found only on a couple of small islands off the coast of New Zealand.
One of the things that is peculiar about these guys is that they are active with very, verylow body temperatures. Active at night, which is also somewhat unusual but not peculiar. Theyare not nocturnal, and they're active with body temperatures as low as 8 degrees Celsius, which isreally pretty low for a reptile.
They live very long in captivity. They have been known to survive in captivity for 70years. They feed on insects and also on other vertebrates, so they are insectivores and carnivoresin terms of feeding habits.
And the other thing that is weird about them is they have the longest incubation period ofany egg-laying vertebrate animal. Their eggs take a whole year to develop. That's the incubationperiod, the length of time between the laying of an egg and hatching of the young is the incubationperiod, and it equals a full year in the tuatara.
The incubation period of reptile eggs are longer than the incubation periods of bird eggs.Bird eggs go anywhere from 11 days to 60 days. And reptile eggs usually take at least a couple ofmonths, anywhere from, you know, 45 to 120 days.
But most reptile eggs will hatch in less than 4 months; these guys take a whole year.
So you get this overall picture of this relic, this ancient animal which is very similar to agroup of animals which were much more numerous early on in the evolution of squamates.
Low body temperatures, active at night, found only on this tiny isolated island. NewZealand is one of the most -- in terms of zoogeography, it's one of the most isolated places on theplanet, separated by deep trenches from Australia and all the rest of the major land masses of theworld.
They have low body temperatures. They are active at night. They have a long incubationperiod. They live for a really long time. They are kind of a terrestrial equivalent of a coelacanth, ifyou will.
They have do have internal fertilization and produce an amniotic egg, but they do nothave copulatory organ. They achieve fertilization by means of a cloaca kiss.
So that's the suborder sphenodontia, the first of four suborders in the order Squamata.
The next one is also one that you are probably not familiar with. This is the suborderamphispenia (spelling). These guys are called worm lizards. There are about a 130 species ofthese so-called worm lizards as the name implies. And you can see from the illustration these guyshave no limbs.
They're found in tropical and subtropical areas of South America and Africa. And in theirhabits they are fossorial. That is they spend is a good portion of their life burrowing through thesoil the way a worm does. And they're fairly highly specialized.
They are carnivores. They have fairly sharp teeth, and they come out and they either feedon invertebrate types of organisms that they find as they are borrowing through the soil, or theycan come out and attack things on the surface.
But they spend most of their life wedging their way through the soil. They have awedge-shaped head that they push themselves through the soil with. And so they have the fullcomplement of fossorial adaptations they we see among fossorial vertebrates.
They have lost their limbs. These guys have vestigial limb girdles. So it's are pretty clearthat these guys have descended from some type of more typical reptile that had limbs, but theyhave lost the limbs.
And it's interesting once an animal loses a structure like a limb so they there is nothingsticking out from the side slowing it as it's trying to wedge its way through the soil, it's notsurprising particularly that the limb girdles, which in fact are completely useless without any limbsattached to them, but they still have vestigial limb girdles because there is no disadvantage inhaving that limb girdle.
And if there is no disadvantage in having a structure, then natural selection isn't going toget rid of the structure very quickly. So these guys still have these vestigial limb girdles.
We are going to see that fairly frequently when we look at other types of vertebrates thathave lost the limbs. Whales, for example, have a vestigial pelvic girdles even though they don'thave pelvic appendages.
They have vestigial eyes and ears. The eyes are completely beneath a continuous layer ofskin. And they also have an extremely shortened tail.
Now, many of these features should sound a lot like the caecilians, a group of fossorialamphibians that exhibited pretty much this same similar suite of characteristics. And thesimiliarities are in no limbs, vestigial limb girdles, vestigial eyes.
Some of the differences are that the reptiles have short tails. Remember the caecilians hadno tail at all, they had a terminal anus. The anus was exactly the back end of the animal's body.There was no tail extending posteriorly beyond that.
Obviously, these guys lay amniotic eggs. And you'll remember that -- and they're alloviparous. Remember I talked about typhlonectes which is viviparous caecilian that the littleembryo with the gills that looked like a fancy feather headdress.
So the those are the amphispenians (spelling) or the worm lizards. There is a 130 speciesof tropical animals. You are not likely to be particularly familiar with them.
The next suborder is the largest suborder in terms of the number of species. This is thesuborder lacertilia. These are the lizards. And there are about 3000 species of lizards.
They are a fairly diverse group. They have a world-wide distribution, and that's sort of arelevant term. World-wide, when I use that term, does not include Antarctica where there arealmost no species of animals that live in Antarctica.
They are found in all the other major land masses of the world and in every habitat. Theyare really amazingly diverse. The majority of them are terrestrial, living on land.
But some lizards are fossorial, so there are burrowing lizards. Some of them are -- well,at least, partially aquatic. That means living in water. And it's probably not strictly correct to saythat there are no lizards that live their whole entire lives in water the way most fish do. But thereare many lizards that at least forage in the water looking for food.
There are even some lizards that are at least semi-marine, which means living in theocean. And, again, they don't live their whole entire life in the ocean, but when you watch the"Life on Earth" videotape, you'll see pictures of the Galapagos marine iguanas, which are greatbig lizard like guys that feed algae, seaweed, which they harvest between beneath the surface ofthe ocean in the Galapagos.
These guys swim out in this very cold ocean water, dive down to the bottom and forageon plants that there are harvesting beneath the subtitle zone in the ocean. So they are not fullymarine. They don't live their whole lives in the ocean, and there are some snakes that do. They arenot fully marine, but they certainly do utilize the marine habitat.
There are some lizards live their entire lives in trees. They might briefly be on the groundgoing from one tree to the next. The term is arboreal, that means living in the trees.
And there is even one lizard that's a glider.
And that's one thing that is interesting to see that is in every one of the major groups ofterrestrial vertebrates, there have been at least one genus that has developed someway of sort ofenhancing ability to glide from one tree to the next.
I didn't mention it, but there are frogs, and you can see the frogs on the "Life on Earth"videotape, they have really huge feet. And they leap out of the tree and stick their arms and legsout that like and they open up huge feet and they glide through the air to get two from one to thenext.
Well, there is a lizard that has really long ribs. And most of the time these ribs are layingback along the body the way you would expect. But when they jump out of a tree, they open outthese ribs and look like a big pancake with arms and legs sticking out.
This guy is in a genus called draco which is Latin for dragon. They are not particularlydragon-like, but they, nevertheless, can fly through the air because they have ribs that stick outmake a great big flat surface.
We can be a little bit generous and say that they are aerial. And at least there are a glider.They are not capable of power flight like a bird or a bat, but they are good gliders.
The oldest fossil lacertilians (spelling) are from the Triassic. So it's a pretty ancient group.
And, well, there are 3000 species, that's a pretty good number. However, most of themare pretty small. 80% of all living species of lizards weigh less than 20 grams. That's sort ofabout the body weight of a mouse.
So the kinds of lizards that you'll see running around campus, the fence lizards that yousee on buildings, that's about the -- that's a pretty average body weight for most members of thissuborder.
Of course, some of them do get to be larger than that. The largest of all the living lizardsis an animal called a Komodo dragon. A really interesting animal. It is doesn't breathe fire. Itdoesn't fly. But, nevertheless, they get to be about 3 meters in length. 9 or ten feet long. Theyweigh like 75 kilograms. That's as much as -- that's like 160 pounds. That's good sized humanbeing.
They are found on a small island off the coast of Indonesia. And they are a member of afairly widely distributed family of lizards that are called the Varanid lizards. The Komodo dragonis just the largest of all the Varanids.
These guys are found in Africa and Australia. Some of them get to be pretty big. Andthey're not an uncommon pet. If you see somebody who has a pet lizards, it's likely to be bigIguana or one of these Varanid lizards. They have long necks, they are real active animals, theyhave really active predators.
They have the kind of ability to utilize oxygen, it almost begins to compare with amammal in terms of they're being really active, foraging active hunters and stuff like that. And theKomodo dragon is big Varanid.
STUDENT: Are the Monitors part of Varanids.
INSTRUCTOR: That's the common name for the Varanid is the Monitor. Monitor lizardis another name for a Varanid. You probably recognize it from the description of the behavior andsize.
So that's the Komodo dragon. And these guys have been studied, of course, because theyare really interesting because they are so big. They will attack goats. They have been known tokill small human beings that got in there way. They have even been known to attack waterbuffalo.
They are very intelligence active predators. They live on this little island where there aresome pretty steep hillsides and there is nice, lush, sort of forest down in the valleys between themountains. And then it's and grassy up on the top. There is a population of deer that live on thisisland. And each day the deer spend the nighttime down into the shelter, down in the forested partof the valleys, but they go to forage on the grass, that's up on tops of the mountains in thedaytime.
And these Varanid lizards -- I mean the Komodos will go and find a deer trail and gohide behind a rock that they know the deer are going to go on, then when the deer walks by theyjump out and attack them and kill them and eat them. So they are very intelligent and activepredators.
STUDENT: There is not many in captivity?
INSTRUCTOR: Occasionally you will see one.
STUDENT: I know the Indaianapolis Zoo has one.
INSTRUCTOR: I think they have one in San Diego, too.
STUDENT: Yeah, they do.
INSTRUCTOR: They are really pretty interesting critters.
Of course, lizards exhibit a number of anatomical characteristics that you are undoubtedlyfamiliar with. They have the ability to shed their tail. This is technically called autotomy. Youknow, "auto" means "self," and "tomy" you might have an appendectomy, it means cut to off toremove our lose.
Autotomy is losing your tail by yourself. Obviously, they don't just drop it for no reason.But particularly anybody who has ever gone out to try to catch lizards, sooner or later you havethe experience of grabbing a lizard and all you're left with is this tail, and the lizard runs away.
And it's a defense mechanism that these animals have evolved. They have special planes ofweakness in the vertebrae, that where the tail is going to be dropped. They have muscles in theartery of the tail that clamps down and prevents they're from being any significant blood loss oncethe tail is shed.
And there also are nerve reflexes in the tail that cause the tail to do this incrediblyspinning and flashing and trashing around. When the guy loses his tail, you're just shocked andyou are staring at this tail. And that's actually why this particular behavior as well as the anatomyand physiology that go along with it has evolved.
A predator goes to grab a lizard and he drops his tail, the tail goes like mad, and thepredator looks at the tail, and the lizard heads for cover with his gonads and survives toreproduce, which is what's really important in the life of a lizard.
They will regenerate the tail. It takes them a period of time to do that. The tail, once it'sregenerated, doesn't look exactly like the old tail. And you will frequently see lizards with apartially regenerated tail, it doesn't have all of the vertebrae and everything, but it still willregenerate the ability to shed that tail again after it has grown one.
They have paired pit-like sensory structures in the roof of their mouth, that are calledJacobsons organs. And these are chemosensory cells. They are like a high concentration of tastebuds in 2 little pits in the roof of their mouth.
So the tongue has 2 separate tips. And they'll stick that tongue out and waive it around inthe air, pick up any molecules or pheromones or scents that might be there and they might touch itto the ground. Touch the 2 tips to the grounds, pick up any molecules that are there.
And they pull their tongue back into their mouth and they stick it up in these little sensorypits in the roof of their mouth where there are these chemosensory cells that will test for thepresence of various kinds of interesting chemicals.
And because these have 2 they can even detect a difference is concentration from one sideto the next. And so that's why these animals, as well as snakes, who have the same adaptation,why they have a tongue with the 2 tips. It's not because they're liars.
They don't speak with a forked tongue; they actually taste with a forked tongue.
There are many different species of lizards that have evolved leglessness. You have apicture of one here on the page. And that appears to be an adaptation either for a fossorialexistence, that is burrowing through the soil, which many of them do, or for living in very, verydense grass land habitats.
So what we see frequently is that legless lizards live like in places like the great planeswhere the grass is very dense and very thick.
One thing that's interesting is when you see a legless lizard it has a very, very long tail.The length of the tail may comprise two-thirds of the total body length of the animal.
And so it appears that evolving leglessness, which is an adaptation for living glass land ora fossorial existence, it usually involves having a very long body. But in an evolutionary sense,what these animals have done is they have elongated their tail. When we look at other highlyfossorial legless animals like the snakes, what we find is a very short tail.
So it seems like, in an evolutionary since, that if an animal is going to become legless itwants to lengthen his body to locomote and still move around. It lengthens the tail, and then overthe course of evolutionary time, the species will progressively increase the length of the rest thebody and shorten up the tail. So this is like an early stage of the evolution of this leglessness.
Legless lizards have a real obvious opening where the eardrum is going to be located.This is called the "auditor meatus."
And this is a feature that we do not see in snakes. A legless lizard looks a lot like a snake.In fact, if you remember the old Indiana Jones movie, the first one, The Temple of Doom, whenIndy looks down in there, and there is the lost ark or the covenant is in the back. And he's goingto have to go down there and get the thing out. And he seems the room crawling with snakes andhe says, "Snakes, why do there have to be snakes?"
Well, if he had been a zoologogist instead of an archeologist, he would have known thatthe majority of those were European legless lizards, which didn't have any business being buried inthe ground in Egypt.
But, nevertheless, most of the animals down there had an external auditor meati. And ifhe just had a good eye, he would have known he didn't have anything to worry about.
I'll finish up talking about lizards and snakes on Monday.
31 ZOO 138, Monday, January ten, 1997, 12:00 p.m.
I need to finish today talking the classification of the natural history of the living reptiles. Ihave a little bit more to say about the lizards, and then I'll talk about the snakes. And then I'mgoing to talk about the about structure and function of the amniotic egg.
You can turn in your Illustrated Notes to the page and the lacertilia. These are the lizards.The snakes which is the last suborder, and it's almost as large in terms of the diversity. Snakeshave evolved from the lizards and so they have many characteristics in common with lizards.
As I mentioned, there are legless lizards that look like a quite a bit like snakes. There aresome anatomical characters which distinguish a legless lizard from a snake.
No snakes have legs, so that part it's not difficult to compare them or to distinguish asnake from a lizard with legs. But these are some characteristics that allow an anatomistherpetologist to tell it apart.
One of these deals is the phenomenon called skull kinesis. Kinesis refers to movement.You know, you have examined the skull of a snapping turtle and a cat in lab. And you know thatall the bones that make up that skull are very, very firmly connected to one another. There's notany one part of that skull that will move relative to another part of the skull.
However, that's not uniformly true of all vertebrate skulls. You may at some time oranother have observed a parrot. Have you ever noticed that a parrot's beak can actually moverelative to its head. That is skull kinesis, the motion of one part of the skull relative to the otherparts of the skull.
Lizards have a very small amount of skull kinesis. The snout region of the skull, which issort of the end of the nose and so forth, can bend upwards slightly relative to the part of the skullthat encases the brain and the eyes.
However, in snakes there is very extensive skull kinesis. So that's one characteristic thatwe can use to distinguish between the lizards on one hand and the snakes on the other.
Skull kinesis is modest. Which means there is small amount of it in lizards. And there is anextensive skull kinesis in the snakes.
Another characteristic which we can use to distinguish these 2 types of animals is thatlizards have movable eyelids. Snakes do not.
So that means it would not be a very good idea to get into a staring contest with asnake. Since they cannot blink, they don't have a movable eyelid, you're bound to end up losingthat contest.
The 2 halves of the lower jaw, each of those is a ramus of the lower jaw, the 2 jaw, rami,that's the plural of ramus. That term jaw rami, means the 2 halves of the lower jaw are fused toone another in lizards just as they are in yourself and in most vertebrate animals.
But you have undoubtedly at sometime or another seen a motion picture, video or Novaor something like that of a snake eating something that is larger in diameter and the snake itself is.And you have probably seen these things that show -- there is a common one, it's shows a snakeswallowing an egg, where the egg is bigger around than the whole head of the snake is.
And so this extensive skull kinesis is one of the consequences or really one of theadaptations of snakes that allows them to swallow something that is larger in diameter than theyare. And the fact that the jaw rami are unfused, just connected by fibrous connective tissue bysome kind of a ligament, so that the 2 halves of the lower jaw can move relative to one another isanother component of that set of adaptations.
Lizards have an external auditor meatus.
That is the opening in the side of the skull in the tympanic -- if you are looking at a cat'sskull for example. At the base of that tubular structure is the eardrum. And lizards have anexternal auditor meatus with an eardrum at the base and snakes do not.
It's okay to talk to your snake. It's all right to talk to snakes. They can hear sounds, butthey don't have an eardrum. They sense vibrations through the ground through their skin. Andthey have an internal air mechanism that will perceive sound waves that pass through their skulland stimulate the middle ear mechanism.
Those are some characteristics. And sort of the lizard's side of this set of characteristics,you might have it in your Illustrated Notes under lizard part. The snake part, I'll just refer to thatas being relative the characteristics of snakes.
But I'm putting them here to juxtapose them so you can see that they do allow you todistinguish between the to.
Natural history of lizards, most of them are insectivores, although there are some of themthat are carnivores. There are some really neat lizards that we have out in our desserts, forexample, that feed on other lizards.
There are larger lizards that feed on birds and other mammals. The monitor lizards likethe Komodo dragon, members of that Varanid are carnivores. Are lizards are herbivores. Someof the larger lizards are herbivores. Not only the Galapagos marine Iguana, but we have bigherbivore in our deserts as well, called the Chuckowalla, which is a good sized lizard that feedsand plant material.
In fact, there is probably a tendency for some of larger lizards to be herbivores. Youprobably can't make a living as a lizard if you are a little lizard and a herbivore.
Courtship in lizards involves various kinds of behavior. The push-up behavior that youmay see lizards doing sometimes, that's territorial displays that is directed towards another male ofthe same species. Sometimes that push-up behavior is a courtship behavior directed towards afemale of the species.
They also frequently develop brightly colored patches of skin in association withcourtship as part of that behavior that is associated with courtship.
Fertilization is internal in lizards, and they have paired copulatory organs or intromittantorgans that are called hemipeni. They have hemipenis. Each can function independently of oneanother. So instead of calling them hemi penis, which means half a penis is not an accurate term.
They produce amniotic eggs, and the majority of the time they have a leathery kind ofcovering. Although, there are some lizards called Gekos that have calcium carbonate shells. Onceagain, they are not as thick and not as elaborate in their as bird eggs.
Lizards exhibit the full range of embryonic development. The primitive condition, the kindwe see in the most primitive lizards, is an oviparous condition. But there are many differentspecies of viviparous lizards. But studies by herpetologist they estimated that viviparity may haveevolved something like 20 different times independently through convergent evolution anddifferent families in the genre of lizards. So that's something that lizards do fairly readily.
And obviously the intermediate evolutionary condition mean ovoviviparous. There arealmost no examples of any parental care among any of the lizards, even in the viviparous species.The baby just drops out the back and the mom keeps on walking along.
Those are the lizards in the suborder lacertilia.
The snakes are in the suborder Ophidia.
And there are 27,000 species of snakes in this suborder. So you can see that's almost aslarge a group as the lizards. They are found in tropics and temperate regions of the world. Andthey also exhibit really a wider range of habitats than the lizards do in a sense. The majority ofthe snakes are terrestrial, living on land. But there are some snakes that are very highly arborealliving virtually their entire lives up in trees.
There are snakes that are fossorial. We have snakes out in the sand dunes, out in theMojave, for example, that swim through the sand and almost never come out of the surface of thesand and are swimming through the sand all the time.
And, of course, some of the most interesting and highly venomous of snakes are the seasnakes. And they are truly marine. They do not need to come ashore at all. They are viviparousand spend their entire lives in the ocean.
And then just in case your stock of horror films for your nightmares isn't complete, thereis a glider snake as well. Which is a dorsoventrally flattened snake, and it strikes out of the trees.And as it goes gliding through the air, it's doing its wreathing thing. Fortunately, they only usethat as a means of getting from one tree to next. They don't use it as an attack modality or thatwould really be a good one for nightmares.
The oldest fossil snake is only from the Cretaceous, so that's probably the youngest groupof all these major groups of vertebrates that we have talked about. That's a boa constrictor typeof snake. The first snakes did not have poison, they killed their prey by constriction where theywrapped their body around the prey, individual, and prevent it from breathing.
Snakes range in size from about 20 centimeters in the shortest once, and that's sort of anadult body size and up to a maximum of about ten meters. Ten meters in length is the length ofthe longest of all the snakes. It's a snake called an snake 30 feet long. You know, that pretty muchwould cover the entire width of this room.
But it's not a particularly heavy-bodied snake. The really big heavy-bodied snakes like theanacondis (spelling) are not that long. But they are heavier in terms of total body mass.
Snakes are all legless. Although, most of them have -- at least, the primitive ones havevestigial limb girdles. And some of the more primitive ones have some vestigial limbs. Boas havelittle tiny spike-like structures located right near the cloaca. And those are vestigial limbs as wellas having the limb girdles inside.
They have a very large number of vertebrae, arranging anywhere from 160 to 400vertebrae. When you compare the formula for the number of vertebrae in your cat, did you learnthat? How many neck vertebrae are there -- how many thoracic? 13. Lumbar? 7. How many sacral? 3.
We didn't give numbers for caudal. 20. 40, 50. That's about 50 vertebrae in a cat,compare that would 400. So obviously in an evolutionary sense they have increased the number ofvertebrae substantially.
Again, they have a highly modified body. They have a lot of skull kinesis. They have noeardrum or tympanic membrane. Tympanum or tympanic membrane. No eyelid. They also cannotmove the eye. In addition to not being able blink, snakes cannot move their ice. You can't catch asnake looking out of the corner of its eye. Snakes can't do that.
The reason that you can look out of the corner of your eye and look up, and all that fancygestures you can do with your eyes is because you have muscles that are attached to the outsideof your eyeball. Those are your extrinsic eye muscles. Snakes don't have extrinsic eye muscles.They cannot move the eyeball within the orbit.
They also have a single lung. Sometimes it runs -- particularly in the sea snakes, the lungmay run the entire length or almost the entire length of the body. So they have these really, reallylong lungs. And in those sea snakes that do that, they do not ventilate most the posterior portionof the lung.
When they breathe in, the fresh air is only going up into front end of the lung. In fact,looking at the anatomy of the tissue of the lining lung, the more posterior portions of the lung arenot even well vascularized. They are not used for ventilation. So why would a snake have a long,long lung that runs the entire length the body cavity?
STUDENT: To float.
INSTRUCTOR: It's a hydrostatic organ. It's functioning the same way as a swim bladderin a fish.
What happens if you take a -- if you had a tube, you know, a tube of water as long as thistable. And it was half filled with water. And you lifted up one end, what would happen to thewater? It would all go down to that end. So that's what the circulatory system of a snake has tocontend with.
So there is this whole little study of what happens to snakes during tilting. They attach asnake to a board and then they tilt it. And find out how does the cardio vascular system of thesnake avoid hemorrhaging and other kinds of problems associated with having these very longbodies. And it's not a trivial problem, if you are an arboreal snake, obviously, you can be 20 feetlong and going up a tree and the blood pressure in your tail can be tremendously almosttwo-thirds of an atmosphere higher than it is at the front end of the snake.
There is a lot of interesting physiology to do with snakes as well.
There are so many weird things about the eyes of snakes that it's not surprising probablyto learn that these are interpreted as indicating that snakes have gone through a period in theirevolution when they had vestigial eyes. In fact, the general assumption is that snakes evolved fromsome kind of legless and fossorial lizard. And you will remember that it's a very common featureof the sleet of fossorial adaptations that these animals have vestigial eyes.
They also are some features of the embryonic development of eyes in snakes and somecharacteristics of the photo receptors in eyes of snakes that are peculiar, they are unique tosnakes.
And these all are interpreted or make sense if we assume that snakes have gone through aphase in their evolution when they had vestigial eyes. And that then they subsequently, most ofthe -- many of them have stopped being fossorial and are surface active or arboreal marine orwhatever. Had to, in essence, re-evolve the eye from a nonfunctional vestigial remanent of thewhat was formally a normal vertebrate eye.
And that's a very interesting observation. And it's an important thing to understand aboutevolution, and that is while it may be possible to, in essence, reverse evolution -- in other words,if you think about it, these guys had a good typical vertebrate eye like lizards. They becamefossorial. Their eyes became vestigial as an adaptation to being fossorial. And then theyre-invaded the land, the surface of the land where it was useful to have a functional eye. They hadto re-evolve the eye.
But they do not exactly reverse the process. And that's the generalization. That whenevolution does have to reverse a previous change, it frequently does not exactly reverse theprocess. So you don't end up exactly where started. If you started with a good eye, if you wentvestigial, you come back again, you can end up with a functional eye, but it may not work inexactly the same way.
One of the things that is different about the eyes of snakes and all other vertebrates is amethod of accommodation. Accommodation refers to the change in the mechanism of focussingthe image on the retina. You're probably all familiar with the idea that you your -- that you canfocus on something very close. If you hold your hand close to eye and focus on your hand, thenyou look across the room and you look back again, you can feel tension in your eyes when youare looking at something far away and then you are trying to focus on something close.
That's because there is muscles in your eye that are excerpting a pressure on the lens tochange its shape. And the lens needs to be rounder to see something that's close; and it needs tobe flatter to be able to focus on something that's far away.
So you have muscles that are called intrinsic eye muscles. And you'll look at someintrinsic muscles of the eye of a sheep in the last lab exercise. They change the shape of the lens.All vertebrate animals accommodate, that is focus on the things that are at different distances bychanging the shape of the lens. All terrestrial vertebrates do except for snakes.
In snakes, the lens actually moves frontwards and backwards within the eye itself. Andthat's also a peculiar feature. The lens of a snake does not change its shape. It changes its positionin order to accommodate.
The snake doesn't have an eyelid. But it's necessary to protect the cornea of the lens fromdesiccation. What they have is a scale similar to the scales that are in the skin of the rest of thebody. They have a scale that covers over the eye itself is which is clear.
And appropriately enough, it's called a spectacle. The same word that we use for a pair ofglasses would be a pair of spectacles. When a snake sheds its skin, it sheds the spectacle. And soif you find a snake skin that's been shed off the head of a snake and you look at it closely, you'llsee right where the eye opening should be, a little clear hemispherical scale that is the spectaclethat was shed with the skin.
If you want to know what the world looks like through the eyes of a snake, you can lookthrough that spectacle of a snake.
Natural history of snakes, they are all carnivores. Some are specialized for feeding ondifferent types of vertebrate foot. Some feed on lizards. Some specialize in birds and eggs. Someare specialize in fish. Others specialize in mammals. But all of the snakes are carnivores. There areno omnivores or herbivores. It's a fairly homogeneous group.
You have a lot of pictures here on the side of the page. I'm not going to go into a lot ofdetails here, but there is quite a diversity of arrangement of fangs. Some senior hinged. Some arenot. Some are at the front. Some are at the back. Not all snakes have big obvious hinged fangslike the rattlesnakes that you have undoubtedly seen.
Some of them have their fangs at the back. But the arrangement that we see in therattlesnakes is certainly the most threatening looking with such a big fang that the animalcouldn't close it's mouth if it weren't able to fold it back out of the way. Of course, that fang ishollow like a hypodermic needle, and it can inject poise.
There are 2 basically different types of poisons. Primitively, the most primitive poisonsare what we call hemotoxins. And a hemotoxin, the name suggests it's a blood poison. That itsfunction is to poison the blood. Actually, all that hemotoxins are proteolytic enzymes.
What does a proteolytic do?
STUDENT: Breaks up protein.
INSTRUCTOR: Protein splitting. These are basically digestive enzymes. The same basictype of enzymes that you have in your stomach. You have pepsin in your stomach as anappropriate proteolytic enzyme. And the reason that snakes originally evolved these poisons wasnot to kill the prey, but to accelerate the process of digestion.
In other words, if a snake swallows an intact animal that's bigger in diameter than thebody of the snake, the only area available for the attack by the digestive enzymes is the outside ofthe animals body. Because snakes like all reptiles and birds and fish and amphibians do not chewtheir food, only mammals chew their food.
It's only the outside surface area of that big hunk of meat in there is accessible to thedigestive enzymes. And so what happened was early on in the evolution of hemotoxins was thatinstead of having, you have in your saliva, you have an enzyme but it's a starch digesting enzyme.That's a fairly common feature of vertebrates.
Well, snakes started evolving to put proteolytic enzymes into their saliva. And whathappens is they bite the animal, inject the proteolytic enzyme into the blood, the heart continues tobeat for a little bit longer, circulates this proteolytic enzyme throughout the body of the animal,and then the animals dies and the heart stops beat, and the snake swallows it down.
But now the animal is being digested from the inside out, and it tremendously acceleratesthe process of digesting this big chunk of meat.
Now, by making them much more aggressive, they have also made them into a poison, sothat a rattlesnake can strike at a mouse and inject a little tiny bit of poison into the animal, andthen instead of having to grab ahold and take the chance of being scratched or bit by the prey theycan just inject the poison and let the poison do its deed. Which happens very, very quickly.
The other kind of toxins are called neuro toxins.
And neuro toxins function by blocking the neuro muscular junction. Hemotoxins functionby digesting a protein. If I were to tell you that one of those was very painful and one waspainless, which one do you suppose would be painful? Hemotoxin. It's no fun to be digested.Hemotoxins are painful and neurotoxins are not.
In fact, there was a story of a herpetologist who was working down in South Americaand he got bit by a highly toxic snake that produced a neurotoxin. And he knew, you know, heknew that he had about five minutes before he was going to be dead. And so he calmly sat downand started taking notes, so that we have a first-hand story of what it feels like to die from theneurotoxins. That's a dedicated scientist. He wanted to get one last publication out.
That's the published and perished version of being a scientist.
STUDENT: He died?
INSTRUCTOR: Yeah. It was his last publication. In fact, what happens when you get bitby a snake with a neurotoxic is that you get a very relaxed feeling because all of your skeletalmuscle, neuro muscular junctions are being blocked. And you get to kind of relax, and you don'tfeel any pain. But the problem is, of course, that the respiratory muscles are skeletal muscles, sopretty soon you stop breathing and pass out from hypoxia. It's a fairly painless way to go.
There is only a couple of snakes in the United States that produce neurotoxins. There is arattlesnake the Mojave desert called the Mojave Green that produces a neurotoxin. And a coupleof people a year actually die from bites of the Mojave Green. It's fairly rare.
But most of the rattlesnakes around here produce hemotixons, and very, very rarely arethey fatal. However, they do frequently result in permanent impairment. In other words, if you geta bite in your hand and you get a bunch of your hand muscles digested, you may have apermanent loss of some mobility and function in those muscles. But they are usually not fatal.
STUDENT: So are anti-venoms only for hemotoxins and not neurotoxins?
INSTRUCTOR: You can get anti venom for neurotoxins as well. The most lethal of allthe poisons are the sea snakes, and they are also neurotoxins.
Reproduction is similar to lizards. They are oviparous, viviparous, ovoviparous. The seasnakes and rattlesnakes are viviparous.
That's the classification and natural history of the living reptiles.