Reptiles and Birds

Characteristics of Reptiles

• Members of the class Reptilia live throughout the world in a wide variety of habitats, except in the coldest regions, where it is impossible for ectotherms to survive.

• Reptiles share certain characteristics, features they retain from the time when reptiles replaced amphibians as the dominant terrestrial vertebrates.

Characteristics of Reptiles• Reptiles have a strong, bony skeleton,

and most have two pairs of limbs. – Snakes and some lizards are legless.

• The legs of reptiles are positioned more directly under their body than are the limbs of amphibians.

• Reptiles have toes with claws, which are used for climbing and digging.– Claws also enable reptiles

to get a good grip on the ground, allowing many reptiles to run quickly for short distances.

Characteristics of Reptiles• The nervous system of a reptile is very

similar to that of an amphibian.• Like their dinosaur

ancestors, modern reptiles have a brain that is small in relation to their body.– An alligator about 2.5 m (8 ft) long has a

brain that is about the size of a walnut.

• Despite this small brain size, reptiles are capable of complex behaviors, including elaborate courtship.

Ectothermic Metabolism• Reptiles’ ectothermic metabolism is too slow to

generate enough heat to warm their bodies, so they must absorb heat from their surroundings.

• As a result, a reptile’s body temperature is largely determined by the temperature of its environment.

• Many reptiles regulate their temperatures behaviorally, by basking in the sun to warm up or seeking shade to cool down.

Ectothermic Metabolism

• At very low temperatures, most reptiles become sluggish and unable to function.

• Intolerance of cold generally limits their geographical range and, in temperate climates, forces them to remain inactive through the winter.

Water Retention• Amphibians such as frogs cannot be

considered fully terrestrial because they lose too much water through their skin.

• Amphibians must stay moist to avoid dehydration, and their method of reproduction requires a moist environment.

• Reptiles have evolutionary adaptations that free them from the water requirements of amphibians.

Watertight Skin• Terrestrial animals face a serious

problem of water loss as water evaporates through their skin.

• Modern reptiles have evolved a skin made of light, flexible scales.

• These scales overlap and form a protective, almost watertight skin that minimizes water loss.

Watertight Eggs• For a reptile living on dry land,

reproduction presents another serious water-loss problem.

• Without a watery environment, both sperm and eggs will dry out.

• A reptile’s fertilized eggs need a moist environment in which to develop.

• A reptile’s amniotic egg solves the problem of a reptile needing a moist environment in which to develop.

Watertight Eggs• An amniotic egg contains both a water

supply and a food supply and is key to a reptile’s success as a terrestrial animal.

• Because the egg’s tough shell makes it essentially watertight, it does not dry out, even in very dry habitats.

• Most reptiles, all birds, and three species of mammals reproduce by means of amniotic eggs with shells.

• The formation of amniotic eggs with shells suggest that these three groups of animals evolved from a common ancestor.

The Amniotic Egg• Both reptiles and birds have amniotic eggs,

which are very much alike internally.• Although a reptile’s eggshell is leathery and

a bird’s is hard, both are almost watertight.• However, the shells are porous enough to

allow oxygen to enter the egg and carbon dioxide to leave.

• The shell and the albumen (egg white) protect and cushion the developing embryo.

• The albumen is also a source of protein and water for the embryo.

The Amniotic Egg• Within the egg, four specialized membranes

(the amnion, yolk sac, allantois, and chorion) play important roles in maintaining a stable environment in which the embryo can develop.

• The amnion encloses the embryo within a watery environment.

• This membrane creates a little ‘pond’ that substitutes for the water in which amphibians lay their eggs.

• This watery enclosure protects the embryo by cushioning it.

The Amniotic Egg• The yolk sac contains the yolk, the

developing embryo’s food supply.• The embryo absorbs nourishment from the

yolk through blood vessels connecting its gut and the yolk sac.

• The allantois is a sac that stores waste products from the embryo.

• It also serves as the embryo’s organ for gas exchange.

• Blood vessels in the walls carry oxygen and carbon dioxide from the embryo.

The Amniotic Egg• Surrounding the amnion, yolk sac, and

allantois is a membrane called the chorion.

• The chorion allows oxygen to enter the egg and carbon dioxide to leave.

Reptilian Lungs• A reptile’s scaly skin does not permit gas

exchange, so reptiles cannot use their skin as an additional respiratory surface.

• The lungs of most reptiles have many respiratory folds.

• These folds greatly increase the respiratory surface area of a reptile’s lungs.

• Reptiles also have strong muscles attached to their rib cage.

• The action of these muscles helps to move air into and out of the lungs, increasing the lungs’ efficiency.

Reptilian Heart• In most reptiles, the septum which

divides the heart extends into the ventricle, partly dividing it into right and left halves.

• The septum enables a much better, but still incomplete separation of oxygen-rich and oxygen-poor blood.

• As a result, oxygen is delivered to the body cells more efficiently than in amphibians.

Reptilian Heart

Reptilian Heart• Unlike most reptiles, crocodilians have a

heart with a completely divided ventricle that consists of two pumping chambers.

• This arrangement fully separates the lung circulation from the body circulation.

• The delivery of oxygen throughout the crocodilian body is further improved in these animals.

Reptilian Reproduction

• Unlike the eggs of most amphibians, reptilian eggs are fertilized within the female, during a process called internal fertilization.

• The male reptile introduces his semen and fluid sections.

• Internal fertilization protects the gametes from drying out, even though the adult animals are fully terrestrial.

Reptilian Reproduction• Many reptiles are oviparous, meaning

the young hatch from eggs.• In most cases, the eggs are not

protected by the parents.• Most snakes and lizards, all turtles and

tortoises, and all crocodilians are oviparous.

• All birds and three species of mammals are also oviparous.

Reptilian Reproduction

• Some species of snakes and lizards are ovoviviparous, which means the female retains the eggs within her body until shortly before hatching or the eggs may hatch within the female’s body.

• Although the embryos receive water and oxygen from the female, their nourishment comes from the yolk sac.

• In ovoviviparous reptiles, the eggs are less vulnerable to predators.

Modern Reptiles

Lizards and Snakes• Snakes and lizards belong to the order

Squamata.• A distinguishing characteristic of this order

is a lower jaw that is only loosely connected to the skull.

• This allows the mouth to open wide enough to accommodate large prey and explains how an anaconda (the world’s largest snake) can swallow a jaguar.

• This ability is a contributing factor to the success of snakes and most lizards as predators.

Lizards• Common lizards include iguanas,

chameleons, geckos, anoles, and horned lizards (often mistakenly called “horny toads”).

• A few species of lizards are herbivores, but most are carnivores.

• Most lizards are small, measuring less than 30 cm (1 fl) in length, but lizards that belong to the monitor family can be quite large.

• The komodo dragon of Indonesia is the largest monitor lizard.– It can be up to 3 m (10 ft) in length

and weigh up to 125 kg (275 lbs).• The tail of some species of lizards, such as the

gecko breaks off easily when seized by a predator, allowing the lizard to escape.– Lizards can regenerate a new tail,

but it does not have any vertebrae in it.

Snakes• Snakes probably evolved

from lizards during the Cretaceous period.• The close relationship between lizards and

snakes is reflected in their many similarities.– It is often difficult to distinguish the legless

species of lizards from snakes.

• Snakes lack movable eyelids and external ears, as do several species of lizards.

• Both snakes and lizards molt periodically, shedding their outer layers of skin.

Snake Body Structure• The skeleton of snakes is unique.

• Most snakes have no trace of a pectoral girdle (the supporting bones for the bones of the forelimbs), which is found even in legless lizards.

• The snake’s jaw is very flexible because it has

five points of movement.– Human jaws have only one movement point.

• One of these points is the chin, where the halves of the lower jaw are connected by an elastic ligament.

• This ligament permits the lower jaw to spread apart when a large meal is being swallowed.

Snake Feeding• While many snakes simply seize their prey and

swallow it whole, some snakes use other methods to subdue their prey.

• Many very large snakes (such as Anacondas, Boas, and Pythons) are constrictors as are some smaller species (such as King Snakes).

• Constrictors wrap their body around their prey, gradually squeezing tighter and tighter until the prey suffocates.

• The snakes then swallow their prey whole, even if the prey is very large.

• Like all snakes, constrictors have no teeth that are suited for cutting and chewing.

Snake Feeding• Some snakes kill their prey

with venom (poison).• Of the 13 or more commonly

recognized families of snakes, only four are venomous.–Cobras, Kraits, and Coral snakes–Sea snakes–Adders and Vipers–Rattlesnakes, Water Moccasins,

and Copperheads

Snake Feeding• In most venomous snakes, modified

salivary glands produce venom that is injected into the victim through grooved or hollow teeth.

• The African Boomslang and Twig Snakes produce venom but do not inject it.– Instead they bite their prey with fangs located

at the back of their mouth.– Grooved teeth direct the venom into their

victim’s wound.

Rattlesnake• Rattle – The rattle typically consists of

5 to 7 interlocking rings made of keratin, a protein. When shaken, it produces a rattling sound that serves as a warning. Each time the snake sheds its skin during molting, a new ring is added to the base of the rattle. The more rapidly the snake grows, the more rattles it accumulates during a given time.

• Pit Organ – Between each eye and nostril of the rattlesnake is an organ that can detect infrared radiation. The snake can locate a warm-bodied animal in a cool, nighttime environment by detecting the difference in infrared radiation emitted by the animal and the cooler background. This gives the rattlesnake the ability to hunt in total darkness.

• Jacobson’s Organs – Flicking the forked tongue into the air, the rattlesnake takes in chemical samples from the environment. These chemicals are transferred to two depressions in the roof of the mouth called Jacobson’s organs, which detect the odor of the chemicals. The snake uses these organs to follow the scent trail of prey.

Rattlesnake

• Reproductive Structures – This male rattlesnake produces sperm in his testes. Females are ovoviviparous. After the eggs hatch in the mother’s body, the live young are ejected and must fend for themselves.

Turtles and Tortoises

• There are about 250 or more species of turtles (which generally live in water) and tortoises (which live on land), all classified in the order Chelonia.

• Turtles and tortoises differ from other reptiles in that their bodies are encased within a hard, bony, protective shell.

• Many of them can pull their head and legs into the shell for protection from predators.

Turtles and Tortoises• While most tortoises have a dome-

shaped shell, water dwelling turtles have a streamlined, disk-shaped shell that permits rapid maneuvering in water.

• Turtles and tortoises lack teeth but have jaws covered by sharp plates, which form powerful beaks.

• Many are herbivores but some, such as the snapping turtle, are aggressive carnivores.

Turtles and Tortoises• Modern turtles and tortoises

differ little from the earliest known turtle fossils, which date to more than 200 million years ago.

• This evolutionary stability may reflect the adaptive aspect of their basic shell-covered body structure.

Turtle Evolution

Turtles and Tortoises• The shell is made of fused plates of bone

covered with horny shields or tough, leathery skin.

• In both cases, the shell consists of two basic parts.

• The carapace is the dorsal (top) part of the shell, and the plastron is the ventral (bottom) portion.

• The vertebrae and ribs of most species are fused to the inside of the carapace.

• The shell provides the support for all muscle attachments in the torso.

Crocodilia• The order Crocodilia is composed of 25

species of large, aquatic reptiles.• Of all the living reptiles, the crocodilians

are most closely related to the dinosaurs.• In addition to the crocodiles and

alligators, the order Crocodilia includes the alligator-like caimans and the long-snouted gavial.

Crocodiles and Alligators• Crocodilians are aggressive carnivores.

• Some are quite large.– American alligators can reach 5.5 m (18 ft) in

length, and Nile crocodiles can reach 6 m (20 ft) in length and weigh 750 kg (1,650 lb).

• Crocodilians generally capture prey by stealth, often floating just beneath the water’s surface near the shore.

• When an animal comes to the water to drink, the crocodilian explodes out of the water and seizes its prey.

• The crocodilian then hauls the prey back into the water to be drowned and eaten.

Crocodiles and Alligators

• The bodies of crocodilians are well adapted for this form of hunting.

• Their eyes are high on the sides of the head, and their nostrils are on top of the snout.

• As a result, they can see and breathe while lying nearly submerged in the water.

• Crocodilians have a strong neck and an enormous mouth studded with sharp teeth.

• A valve in the back of the mouth prevents water from entering the lungs when crocodilians feed underwater.

Crocodiles and Alligators

• Unlike other living reptiles, crocodilians care for their young after hatching.

• A female American alligator builds a nest of rotting vegetation for her eggs.

• After the eggs hatch, the mother may tear open the nest to free the hatchlings.

• The young alligators remain under her protection for up to a year.

Tuataras• The order Rhynchocephalia contains only

two species of tuataras.• The two living species of tuataras are members

of the genus Sphenodon and are native to New Zealand.

• Tuataras are lizard-like reptiles up to 70 cm (2 ft) long.

• Unlike most reptiles, tuataras are most active at low temperatures.

Tuataras• They burrow or bask in the sun during the

day and feed on insects, worms, and other small animals at night.

• Tuataras are sometimes called living fossils because they have survived almost unchanged for 150 million years.

• Since the arrival of humans to New Zealand about 1,000 years ago, the tuatara’s range has diminished, and their members are declining.

Characteristics of Birds• The birds we see today are the modern

members of the class Aves.• Unlike their reptilian relatives, birds usually

lack teeth and have a tail that is greatly reduced in length.

• Birds lay amniotic eggs that are very similar to those of reptiles, and the feet and legs of birds are covered with scales.

• Other characteristics unique to birds are the presence of feathers and the modification of the forelimbs into wings.

Feathers• Feathers are modified reptilian

scales that develop from tiny pits, called follicles, in the skin.

• Just as snakes and lizards replace their skin by molting, birds molt and replace their feathers.

• However, few birds shed all of their feathers at one time.

Feathers• Birds have two main types of feathers:

contour feathers and down feathers.

• Contour feathers cover the bird’s body and give adult birds their shape.

• Specialized contour feathers, called flight feathers, are found on a bird’s wings and tail.

• These feathers help provide lift for flight.

• A contour feather has many branches called barbs.

• Each barb has many projections, called barbules that are equipped with microscopic hooks.

• These hooks link the barbs to one another, giving the feather a continuous surface and a sturdy flexible shape.

Feathers

Feathers• With use, the connections of the hooks

and barbs become undone.• When you see a bird pulling its

feathers through its beak, it is relinking these connections.

• This process is called preening.• Most birds have a gland called

a preen gland which secretes oil.• When a bird preens, it spreads the oil

over its feathers, cleaning and waterproofing them.

Feathers• Down feathers cover the body of

young birds and are found beneath the contour feathers of adults.

• Their soft, fluffy structure provides good insulation for the bird, helping the bird conserve body heat.

Feathers• There are other reasons that

feathers are important to birds.• Their coloration may be protective

(as camouflage) or may be important in the selection of a mate.– The feathers of some birds allow them to

blend in with their surroundings.– In some species, the males

develop special plumage during the breeding season.

Avian Skeleton• The bones of birds are thin and hollow.• Many of the bones are fused, making a

bird’s skeleton more rigid than a reptile’s.

• The fused sections form a sturdy frame that anchors muscles during flight.

Avian Skeleton and Muscles• The power for flight (or for swimming

underwater, like penguins) comes from large breast muscles that can make up to 30% of a birds body weight.

• These muscles stretch from the wings to the breastbone.

• The breastbone is greatly enlarged and bears a prominent keel for muscle attachment.

• Muscles also attach to the fused collarbones (wishbone). – No other living

vertebrates have a keeled breastbone or fused collarbones.

Endothermic Metabolism

• Birds are endotherms; they generate enough heat through metabolism to maintain a high body temperature.

• Birds maintain body temperatures ranging from 40°C to 42°C (104°F to 108°F), which is higher than the body temperature of most mammals.

• These high temperatures are due to a high rate of metabolism, which satisfies the increased energy requirements of flight.

Avian Heart Structure• The ventricle of birds is completely

divided by a septum.• Oxygen-rich and oxygen-poor blood is kept

separate, meaning that oxygen is delivered to the body cells more efficiently.

• The sinus venosus, which is a prominent part of the fish heart, is not a separate chamber of the heart in birds (or mammals).–However, a small amount of tissue from it

remains in the wall of the right atrium.– This tissue is the point of origin of the

heartbeat and is known as the heart’s pacemaker.

Avian Heart Structure

Avian Lungs• Birds use a considerable amount of energy

when they fly.• Since birds often fly for long periods of

time, their cellular demand for energy exceeds that of even the most active mammals.

• To increase the efficiency of lungs, birds have air pass over the respiratory surface in one direction only.

• One-way air flow is possible in birds because they have air sacs connected to their lungs.

• There is no gas exchange in the air sacs, they act as holding tanks.

Avian Lungs• There are two important advantages to

one-way air flow.• First, the lungs are exposed only to air

that is almost fully oxygenated, increasing the amount of oxygen transported to the body cells.

• Second, the flow of blood in the lungs runs in a different direction than the flow of air does.

• The difference in direction increases oxygen absorption.

Avian Respiration

Avian Digestive System• Large meals are temporarily stored in the

crop, the expandable lower portion of the esophagus.

• The food then passes into a two-chamber stomach.

• In the first chamber, stomach acids begin breaking down the food.

• The partially digested food is then passed to the second chamber, the gizzard, where it is ground and crushed.

• Undigested material is eliminated through the cloaca.

Avian Excretory System• The excretory system is efficient and

lightweight.• It does not store waste liquids in a

bladder.• Instead, birds convert nitrogenous

waste to uric acid, which is concentrated into a harmless white paste.

• The uric acid travels to the cloaca and is eliminated.

External Avian Structures

Adaptations of Birds• There is great diversity among

the 28 orders of birds. • 60% of all birds species belong

to order Passeriforms. – These birds, also known as the songbirds,

number approximately 5,300 species and are by far the largest group of terrestrial vertebrates.

• Birds are adapted for different ways of life. • You can tell a great deal about the habitats

and diets of birds by examining its beak (bill), legs, and feet.

Adaptations of Birds• During the evolutionary history of

birds, their beaks, legs and feet have been adapted to the particular environment the birds live in.

• Some birds are more highly specialized than others, and many birds are highly flexible in their eating habits.

Adaptations of BirdsType of Bird Beak adaptations Foot Adaptations

Songbirds (cardinal, robin)

Seed-cracking: short, thick strong beak

Insect-catching: Long, slender beak for probing

Perching: Toes can cling to branches; one toe points backward

Hummingbirds Probing: Thin, slightly curved beak for inserting into flowers to sip nectar

Hovering: Legs so small the bird cannot walk on the ground; tiny feet

Woodpeckers Drilling: Strong, chisel-like beak

Grasping: Feet with two toes pointing forward and two pointing backward

Adaptations of BirdsType of Bird Beak adaptations Foot Adaptations

Parrots Cracking, Tearing: Short, stout, hooked beak used to crack seeds and nuts and to tear vegetation

Climbing/Grasping: Strong toes two pointing forward, two pointing backward, adapted for perching, climbing, and holding food

Birds of Prey Tearing: Curved, pointed beak for pulling apart prey

Grasping: Powerful, curved talons for seizing

and gripping prey

Adaptations of BirdsType of Bird Beak adaptations Foot Adaptations

Ducks Sieving: Long, flattened, rounded bill

Swimming: Three toes linked by webs for improved swimming

Long-legged Waders Fishing: Long, slender, spear-shaped beak for fishing

Wading: Long legs, toes spread out over a large area to support bird on soft surfaces

Other Bird Adaptations• There are many groups of

birds, each of which is adapted to its particular living conditions.–Gulls and Terns have

streamlined bodies that are adapted for flying over

the water in search of fish.–Owls’ excellent low-light

vision enables them to survive as nocturnal hunters.