Chapter 34: Reptiles and Birds

Biology II

Key Characteristics of Reptiles: Anatomy

Strong, bony skeleton

Toes with claws Claws used for climbing and digging, but also allow for good traction

Most have 2 pairs of limbs, but snakes and some lizards are legless

Legs positioned more directly under body than those of amphibians

Dry, scaly, almost watertight skin

Almost watertight amniotic eggs

Key Characteristics of Reptiles: Physiology

Brain small in relation to body

Still capable of complex behaviors

Respiration through well-developed lungs

Ventricle of heart partly divided by septum

Internal fertilization

Key Characteristics of Reptiles: Ectothermic Metabolism

Reptile metabolism is too slow to generate enough heat to keep the body warm

Body temperature is largely determined by temperature of environment

Body temperature can also be regulated behaviorally, to an extent

Too cold – bask in sun

Too hot – seek shade

Though reptiles live in a variety of habitats, they cannot survive at very low temperatures

Become sluggish and unable to function

Reptilian Characteristics: Watertight Skin

Unlike amphibians, who must stay moist to avoid dehydration, reptiles have virtually watertight skin

Light, flexible scales overlap and form protective, almost watertight skin

Reptilian Characteristics: Watertight Eggs

2 potential problems for terrestrial reproduction

Egg and sperm will dry out without watery environment

Internal fertilization

Fertilized eggs need moist environment to develop

Amniotic egg – contains both water and food supply

Essentially watertight

Most reptiles, all birds, and 3 species of mammals all reproduce by means of amniotic eggs (suggests common ancestor)

The Amniotic Egg Shell is porous to allow gas exchange

O2 in and CO2 out

Shell and albumen (egg white) protect developing embryo

The amnion encloses embryo in watery environment

Yolk sac contains food supply Absorb yolk through blood vessels connecting to gut

Allantois stores waste and contains blood vessels that work in gas exchange

Chorion is membrane that allows O2 to enter and CO2 to leave

Reptilian Characteristics: Lungs

Most reptiles have greater O2 requirement than amphibians

Reptile skin cannot breathe like that of amphibians

Lungs of reptiles have many internal folds to compensate

Increases surface area of lungs

Strong muscles also attached to rib cage, allowing more efficient gas exchange through the lungs

Reptilian Characteristics: Heart

Incomplete septum partly divides ventricles of reptile heart

Enables some separation between O2-rich and O2-poor blood

O2 delivered more efficiently in reptiles than in amphibians, who have no septum

Crocodilians have complete division of ventricles, making O2 delivery even more efficient

Reptilian Characteristics: Reproduction

Many reptiles are oviparous, meaning their young hatch from eggs

Includes most snakes/lizards, all turtles/tortoises, all crocodilians and birds, and 3 species of mammals

Fertilization occurs internally, unlike in amphibians

Parental care of eggs is rare

Reptilian Characteristics: Reproduction

Some species of snakes and lizards are ovoviviparous

Female retains eggs within her body until either shortly before hatching or after hatching

Embryos receive water and O2 from mother, but still receive nourishment from yolk sac

Eggs are less vulnerable to predators

Today’s Reptiles: Lizards

Lizards and snakes belong to order Squamata

Characterized by lower jaw that is very loosely connected to skull, allowing mouth to open wide enough to accommodate larger prey

Include iguanas, chameleons, geckos, and anoles

Most are carnivorous

Most are small, measuring <1 ft. in length, but the Komodo dragon can reach 10 ft. and 275 lbs.

Some species have evolved defense mechanism in which tail will break off when seized by a predator, regenerating later

Today’s Reptiles: Snakes

Snakes likely evolved from lizards during Cretaceous

Lack movable eyelids and external ears, as do many lizards

Like lizards, snakes periodically molt

Most lack pectoral girdle, the supporting bones for bones of forelimbs

Found in even legless lizards

Jaw has 5 points of movement, making it very flexible

Some snakes use various methods to subdue prey before swallowing, including constriction and venom

Timber Rattlesnake: External Structure

Rattle consists of 5-7 interlocking rings made of the protein keratin

Each time it molts, a new ring is added to base

Use pit organ located between each eye and nostril to detect infrared radiation

Can locate prey in total darkness

Timber Rattlesnake: Internal Structure

Modified salivary glands in upper jaw produce venom containing hemotoxins, proteins that attack the circulatory system

Venom injected through hollow, upper fangs

Jacobsen’s organs, 2 depressions located in the roof of mouth, detect odor of chemicals taken using forked tongue

Used to follow scent trail of prey

Spine made up of 100s of vertebrae, each with own pair of attached ribs

Today’s Reptiles: Turtles and Tortoises

Differ from other reptiles because of hard, protective shell

Many can pull head and legs inside

Provides support for all muscle attachments in torso

Made of fused plates of bone covered with horny shields or tough leathery skin and consists of 2 basic parts

Carapace – dorsal portion of shell

In most species, vertebrae and ribs fused to inside

Plastron – ventral portion of shell

Turtles and Tortoises (con’d) Most tortoises have dome-shaped shell, while many turtles have stream-lined, disc-shaped shell that permits rapid maneuvering in water

Tortoises are turtles particularly well-adapted to life on land

Lack teeth but jaws covered by sharp plates, forming powerful beaks

Many are herbivores, but some are carnivores

Today’s Reptiles: Crocodiles and Alligators

Members of order Crocodilia also include aggressive carnivores such as caimans, and gavials

Can be quite large, reaching 6 m and 1650 lbs

Bodies are adapted to stealth form of hunting

Eyes high on sides of head and nostrils on top of snout

Strong neck and enormous mouth

Valve in back of mouth prevents water from entering lungs

Characterized by parental care after hatching

Today’s Reptiles: Tuataras

2 living species of tuataras exist today

Members of genus Sphenodon

Native to New Zealand

Lizard-like reptiles up to 2 ft. in length

Most active at low temperatures

Bask in sun or burrow during day and feed on insects, worms, and small animals at night

Sometimes called living fossils because they are almost unchanged after 150 million years

Key Characteristics of Birds Forelimbs modified into wings

Body covered with feathers

Lightweight, hollow bones

Endothermic metabolism

Super-efficient respiratory system

Heart with completely divided ventricle

Birds: General Information Birds belong to the class Aves

Retain some reptilian characteristics:

Amniotic eggs

Feet/legs covered with scales

Show non-reptilian characteristics

Usually lack teeth

Tail greatly reduced in length

Presence of feathers and forelimbs modified into wings

Remember – all birds cannot fly!!!

Feathers Feathers are modified reptilian scales that develop from tiny pits called follicles

Birds molt and replace feathers

2 main types of feathers

Contour Feather

Down Feathers

Feathers can be important as camouflage or play a part in mate selection

Contour Feathers Contour feathers – cover bird’s body and give adult birds their shape

Has many branches called barbs

Several projections called barbules have microscopic hooks linking barbs together, providing continuous surface and sturdy but flexible shape for feather

Flight feathers – specialized contour feathers found on birds wings and tail that help provide lift for flight

Preening Preening – process in which bird pulls its feather through its beak

With use, connections between barbs become undone, but preening re-links these connections

Oil is also spread over feathers to clean and waterproof them

Preen Gland – specialized gland that secretes oil

Down Feathers Down feathers – cover the body of young birds and are found beneath contour feathers of adults

Provides insulation, conserving body heat

Bird Skeletons Birds are relatively lightweight for their size

Bones are thin and hollow

Many bones are fused, making skeleton more rigid than that of reptile

Fused sections form sturdy frame that anchors muscles during flight

Power of Flight Power for flight (or swimming underwater) comes from breast muscles

Can make up ~30% of bird body weight

Muscles stretch from wing to breastbone

Breastbone greatly enlarged and has prominent keel for muscle attachment

Muscles also attach to wishbone (fused collarbone)

No other vertebrates have wishbone or keeled breastbone

Endothermic Metabolism Birds are endotherms, meaning they generate enough heat through metabolism to maintain a high body temperature

Body temps range from 40–42

C (Humans: 37

C)

High temps results from high rate of metabolism, since flying requires more energy

Heart and lung structure also help meet this demand

Avian Heart Like crocodilians, the ventricles of birds are completely divided by septum

O2 delivered through body more efficiently, since there is no mixing of O2-rich and O2-poor blood

Unlike the fish heart, the sinus venosus is not a separate chamber in avian heart

Small amount of tissue remains in wall of right atrium; this is known as heart’s pacemaker and is point of origin of heartbeat

Avian Heart Structure

Highly Efficient Lungs

Because birds use considerable amount of energy, they need more efficient lungs, which they get through one-way air flow

Limits to increased surface area

Possible because air sacs that act as holding tanks are connected to lungs

Lungs exposed only to fully oxygenated air

Flow of blood in lungs runs in different direction than flow of air, increasing oxygen absorption

Avian Lung Structure

Internal Structure Birds rank 2nd among vertebrates in ratio of brain to body size (mammals 1st)

Makes possible the precise control of movement and balance necessary for flight

Excretory system is efficient and lightweight

No storage of liquid waste in bladder; rather nitrogenous wastes are converted to uric acid (white paste) and eliminated through cloaca

Eggs are also passed through cloaca

Digestive system consists of crop, an expandable lower portion of esophagus, and 2-chambered stomach, the second of which is the gizzard (grinds/crushes food)

Internal Structures

Birds Adaptations The 28 orders of birds that exist today are adapted for different ways of life

Beaks, legs, and feet are adapted for specific habitats

Stream-lined bodies adapted for gliding over water (gulls)

Low-light vision allows for nocturnal hunting (owl)

Bird Adaptations

Bird Adaptations