Biology Lesson 12: From Fishes to Birds
This stunning bird is a peacock. Do you know why he is spreading out his big colorful tail feathers like a fan? He is trying to attract a female for mating. Both the feathers and the behavior evolved because they increase the chances that males of the species will reproduce and pass their genes to the next generation. Many other vertebrates have similar behaviors for attracting mates. Even fish display some remarkably sophisticated mating behaviors.
Fish and birds are both vertebrates. Vertebrates are a diverse and fascinating group of animals. In many ways they are very different from the invertebrates described in previous chapters. Elaborate mating behaviors are just one way they differ. This chapter describes many other differences between vertebrates and invertebrates. It also describes in detail the classes of vertebrates from fish to birds.
Section 1: Overview of Vertebrates
Section Objectives
Vocabulary
Introduction
Vertebrates are a subphylum of the phylum Chordata. Like all chordates vertebrates have a notochord a dorsal hollow nerve cord pharyngeal slits and a post-anal tail. What other characteristics do vertebrates have? What traits set them apart from invertebrate chordates?
Characteristics of Vertebrates
The main distinguishing feature of vertebrates is their vertebral column or backbone (see Figure below). The backbone runs from the head to the tail along the dorsal (top) side of the body. The vertebral column is the core of the endoskeleton. It allows a vertebrate to hold its shape. It also houses and protects the spinal (nerve) cord that passes through it. The vertebral column is made up of repeating units called vertebrae (singular vertebra). In many species there are shock-absorbing discs between the vertebrae to cushion them during movement.
Human Vertebral Column and Vertebrae. The human vertebral column consists of 33 vertebrae. Two vertebrae are shown here enlarged.
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Vertebrate Endoskeleton
Another distinguishing feature of vertebrates is an endoskeleton made of bone or cartilage. Cartilage is a tough tissue that contains a protein called collagen. Bone is a hard tissue that consists of a collagen matrix or framework filled in with minerals such as calcium. Bone is less flexible than cartilage but stronger. An endoskeleton made of bone rather than cartilage allows animals to grow larger and heavier. Bone also provides more protection for soft tissues and internal organs. As shown in Figure below the vertebrate endoskeleton includes a cranium or skull to enclose and protect the brain. It also generally includes two pairs of limbs. Limb girdles (such as the human hips and shoulders) connect the limbs to the rest of the endoskeleton.
Vertebrate Endoskeletons. The vertebrate endoskeleton includes a vertebral column cranium limbs and limb girdles. Can you find these parts in each endoskeleton shown here?
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Other Vertebrate Traits
There are several additional traits found in virtually all vertebrates.
Nervous System (Human). The vertebrate nervous system includes a brain and spinal cord. It also includes a body-wide network of nerves called peripheral nerves. They connect the spinal cord with the rest of the body.
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Vertebrate Reproduction
Vertebrates reproduce sexually and almost all of them have separate male and female sexes. Generally aquatic species have external fertilization whereas terrestrial species have internal fertilization. Can you think of a reason why aquatic and terrestrial vertebrates differ in this way? Vertebrates have one of the following three reproductive strategies: ovipary ovovivipary or vivipary.
Vertebrate Classification
There are about 50000 vertebrate species and they are placed in nine different classes. Five of the classes are fish. The other classes are amphibians reptiles birds and mammals. Table below lists some of the distinguishing traits of each class.
Table 19.2 The majority of vertebrate classes are fish. Fish were also the first vertebrates to evolve.
Classes of Vertebrates
Class
Distinguishing Traits
Hagfish
They have a cranium but no backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.
Lampreys
They have a partial backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.
Cartilaginous Fish
They have a complete backbone; they have jaws; their endoskeleton is made of cartilage; they are ectothermic.
Ray-Finned Fish
They have a backbone and jaws; their endoskeleton is made of bones; they have thin bony fins; they are ectothermic.
Lobe-Finned Fish
They have a backbone and jaws; their endoskeleton is made of bones; they have thick fleshy fins; they are ectothermic.
Amphibians
They have a bony endoskeleton with a backbone and jaws; they have gills as larvae and lungs as adults; they have four limbs; they are ectothermic
Reptiles
They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with scales; they have amniotic eggs; they are ectothermic.
Birds
They have a bony endoskeleton with a backbone but no jaws; they breathe only with lungs; they have four limbs with the two front limbs modified as wings; their skin is covered with feathers; they have amniotic eggs; they are endothermic.
Mammals
They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with hair or fur; they have amniotic eggs; they have mammary (milk-producing) glands; they are endothermic.
Vertebrate Evolution
The earliest vertebrates were jawless fish similar to living hagfish. They lived between 500 and 600 million years ago. They had a cranium but no vertebral column. The phylogenetic tree in Figure below gives an overview of vertebrate evolution. As more data become available new ideas about vertebrate evolution emerge.
Phylogenetic Tree of Vertebrate Evolution. The earliest vertebrates evolved almost 550 million years ago. Which class of vertebrates evolved last?
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Evolution of Fish
Not too long after hagfish first appeared fish similar to lampreys evolved a partial vertebral column. The first fish with a complete vertebral column evolved about 450 million years ago. These fish also had jaws and may have been similar to living sharks. Up to this point all early vertebrates had an endoskeleton made of cartilage rather than bone. About 400 million years ago the first bony fish appeared. A bony skeletal could support a larger body. Early bony fish evolved into modern ray-finned and lobe-finned fish.
Evolution of Other Vertebrate Classes
Amphibians reptiles mammals and birds evolved after fish.
Evolution of Endothermy
Until mammals and birds evolved all vertebrates were ectothermic. Ectothermymeans regulating body temperature from the outside through behavioral changes. For example an ectotherm might stay under a rock in the shade in order to keep cool on a hot sunny day. Almost all living fish amphibians and reptiles are ectothermic. Their metabolic rate and level of activity depend mainly on the outside temperature. They can raise or lower their own temperature only slightly through behavior alone.
Both mammals and birds evolved endothermy. Endothermy means regulating body temperature from the inside through metabolic or other physical changes. On a cold day for example an endotherm may produce more heat by raising its metabolic rate. On a hot day it may give off more heat by increasing blood flow to the surface of the body. Keeping body temperature stable allows cells to function at peak efficiency at all times. The metabolic rate and activity level can also remain high regardless of the outside temperature. On the other hand maintaining a stable body temperature requires more energyand more food.
Section Summary
Extra Practice
1. Create a time line of vertebrate evolution that shows how and when important vertebrate traits evolved.
2. Explain the significance of changes in the number of heart chambers during the course of vertebrate evolution.
3. Compare and contrast ectothermy and endothermy including their pros and cons.
Points to Consider
The earliest and simplest vertebrates are fish. Fish also have the greatest number of vertebrate classes. Think about some of the fish you are familiar with such as fish you eat or fish you may have seen in aquariums.
Section 2: Fish
Section Objectives
Vocabulary
Introduction
Fish are aquatic vertebrates. They make up more than half of all vertebrate species. They are especially important in the study of vertebrate evolution because several important vertebrate traits evolved in fish.
Structure and Function in Fish
Fish show great diversity in body size. They range in length from about 8 millimeters (0.3 inches) to 16 meters (about 53 feet). Most are ectothermic and covered with scales. Scales protect fish from predators and parasites and reduce friction with the water. Multiple overlapping scales provide a flexible covering that allows fish to move easily while swimming.
Adaptations for Water
Many structures in fish are adaptations for their aquatic lifestyle. Several are described below and shown in Figure below.
General Fish Body Plan. A fish has a stream-lined body with gills and fins. (1) operculum (gill cover) (2) lateral line system (a sensory organ system) (3) dorsal fin (4) fat fin (5) caudal peduncle (narrow body region that connects the main body to the caudal fin) (6) caudal fin (7) anal fin (8) photophores (light emitting organs found on some fish) (9) pelvic fins (paired) (10) pectoral fins (paired).
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Fish Organ Systems
Fish have a circulatory system with a two-chambered heart. Their digestive system is complete and includes several organs and glands. Jawed fish use their jaws and teeth to grind up food before passing it to the rest of the digestive tract. This allows them to consume larger prey.
Fish also have a centralized nervous system with a brain. Fish brains are small compared with the brains of other vertebrates but they are large and complex compared with the brains of invertebrates. Fish also have highly developed sense organs that allow them to see hear feel smell and taste. Sharks and some other fish can even sense the very low levels of electricity emitted by other animals. This helps them locate prey.
Fish Reproduction and Development
Nearly all fish reproduce sexually and most species have separate sexes. Those without separate sexes avoid self-fertilization by producing sperm and eggs at different times. Each fish typically produces a large number of gametes. In most fish species fertilization takes place externally. These fish are oviparous. Eggs are laid and embryos develop outside the mothers body. In a minority of fish including sharks eggs develop inside the mothers body but without nourishment from the mother. These fish are ovoviviparous.
Spawning
In many species of fish a large group of adults come together to release their gametes into the water at the same time. This is called spawning. It increases the chances that fertilization will take place. It also means that many embryos will form at once which helps ensure that at least some of them will be able to escape predators. With spawning there is no way for fish parents to know which embryos are their own. Therefore fish generally dont provide any care to their eggs or offspring. There are some exceptions however including the fish described in Figure below.
Mouth Brooding. Some species of fish carry their fertilized eggs in their mouth until they hatch. This is called mouth brooding. If you look closely you can see the eggs inside the mouth of the African tilapia fish pictured here.
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Fish Larvae
Fish eggs hatch into larvae that are different from the adult form of the species (see Figure below). A larva swims attached to a large yolk sac which provides the larva with food. The larva eventually goes through metamorphosis and changes into the adult form. However it still needs to mature before it can reproduce.
Salmon Larva.
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Classification of Fish
There are about 28000 existing species of fish and they are placed in five different classes. The classes are commonly referred to as hagfish lampreys cartilaginous fish ray-finned fish and lobe-finned fish (see Table 19.2 in the previous Section).
Hagfish
Hagfish are very primitive fish. They retain their notochord throughout life rather than developing a backbone and they lack scales and fins. They are classified as vertebrates mainly because they have a cranium. Hagfish are noted for secreting large amounts of thick slimy mucus. The mucus makes them slippery so they can slip out of the jaws of predators.
Lampreys
Like hagfish lampreys also lack scales but they have fins and a partial backbone. The most striking feature of lampreys is a large round sucker lined with teeth that surrounds the mouth (see Figure below). Lampreys use their sucker to feed on the blood of other fish species.
Sucker Mouth of a Lamprey. The mouth of a lamprey is surrounded by a tooth-lined sucker.
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Cartilaginous Fish
Cartilaginous fish include sharks rays and ratfish (see Figure below). In addition to an endoskeleton composed of cartilage these fish have a complete backbone. They also have a relatively large brain. They can solve problems and interact with other members of their species. They are generally predators with keen senses. Cartilaginous fish lack a swim bladder. Instead they stay afloat by using a pair of muscular fins to push down against the water and create lift.
Cartilaginous Fish. All of these fish belong to the class of cartilaginous fish with jaws. (a) Oceanic whitetip shark (b) Ray (c) Ratfish
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One of the most important traits of cartilaginous fish is their jaws. Jaws allow them to bite food and break it into smaller pieces. This is a big adaptive advantage because it greatly expands the range of food sources they can consume. Jaws also make cartilaginous fish excellent predators. It youve ever seen the film Jaws then you know that jaws make sharks very fierce predators (see also Figure below).
Jaws of a Shark. Sharks have powerful jaws with multiple rows of sharp saw-like teeth. Most other fish are no match for these powerful predators.
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Ray-Finned Fish
Ray-fined fish include the majority of living fish species. including goldfish tuna salmon perch and cod. They have a bony endoskeleton and a swim bladder. Their thin fins consist of webs of skin over flexible bony rays or spines. The fins lack muscle so their movements are controlled by muscles in the body wall. You can compare their ray fins with the fleshy fins of lobe-finned fish in Figure below.
Fins of Bony Fish. The fins of ray-finned and lobe-finned fish are quite different. How is the form of the fins related to their different functions in the two classes of fish? Ray Fin (left) Lobe Fin (right)
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Lobe-Finned Fish
Lobe-fined fish are currently far fewer in number than ray-finned fish. Their fins like the one shown in Figure above contain a stump-like appendage of bone and muscle. There are two groups of lobe-finned fish still alive today: coelacanths and lungfish.
1. Coelacanths are ancient fish with just two living species. They are at risk of extinction because of their very small numbers.
2. Lungfish have a lung-like organ for breathing air. The organ is an adaptation of the swim bladder. It allows them to survive for long periods out of water.
Evolution of Fish
Invertebrate chordates use their gills to filter food out of water not to absorb oxygen. In the early evolution of fish there was a switch to using gills to absorb oxygen instead of to filter food. Gills consist of many thin folded tissues that provide a large surface area for oxygen uptake. With more oxygen absorbed by the gills fish could become much larger and more active.
Timing of Fish Evolution
Ancestors of hagfish are thought to have been the earliest vertebrates. Their fossils date back to about 550 million years ago. Fossils of cartilaginous fish with jaws resembling living sharks first appeared in the fossil record about 450 million years ago. They were followed about 50 million years later by the bony fish.
The Bony Fish
At first the lobe-finned bony fish were much more common than the ray-finned bony fish that dominate today. Lobe-finned fish were also ancestral to amphibians. Their stump-like appendages and lung-like organs evolved into amphibian legs and lungs. Ray-finned bony fish may have been the first fish to evolve in freshwater. They eventually became the most diverse and dominant class of fish.
Ecology of Fish
The habitats and diets of fish are varied. They live throughout the ocean and also in freshwater lakes ponds rivers and streams.
Fish Food
Most fish are predators but the nature of their prey and how they consume it differs from one class to another and even within classes.
Fish at Risk
Today more than 1000 species of fish are at risk of extinction. This is mainly because of human actions. Specific causes include over-fishing and habitat destruction caused by water pollution dam building and the introduction of non-native species.
Section Summary
Extra Practice
1. Assume that a new species of fish has been discovered deep in the ocean. It has a complete vertebral column made of cartilage. Which class should the new species be placed in? Name one other trait you would expect to find in the new species of fish. Explain your answers.
2. Explain why the practice of spawning is adaptive.
3. Fish with jaws may be very large. Infer how their jaws may be related to their large body size.
Points to Consider
Lobe-finned fish were the ancestors of amphibians which were the first vertebrates to live on land.
Section 3: Amphibians
Section Objectives
Vocabulary
Introduction
Amphibians are vertebrates that exist in two worlds. They divide their time between freshwater and terrestrial habitats. They share a number of features with air-breathing lungfish but they also differ from lungfish in many ways. One way they differ is their appendages. Amphibians are the first true tetrapods or vertebrates with four limbs. Modern amphibians include frogs salamanders and caecilians as shown Figure below.
Examples of Living Amphibians. In what ways do these three amphibians appear to be similar? In what ways do they appear to be different?
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Structure and Function in Amphibians
Amphibians have less variation in size than fish ranging in length from 1 centimeter (2.5 inches) to 1.5 meters (about 5 feet). They generally have moist skin without scales. Their skin contains keratin a tough fibrous protein found in the skin scales feathers hair and nails of tetrapod vertebrates from amphibians to humans. Some forms of keratin are tougher than others. The form in amphibian skin is not very tough and it allows gases and water to pass through their skin.
Amphibian Ectothermy
Amphibians are ectothermic so their internal body temperature is generally about the same as the temperature of their environment. When its cold outside their body temperature drops and they become very sluggish. When the outside temperature rises so does their body temperature and they are much more active. What do you think might be some of the pros and cons of ectothermy in amphibians?
Amphibian Organ Systems
All amphibians have digestive excretory and reproductive systems. All three systems share a body cavity called the cloaca. Wastes enter the cloaca from the digestive and excretory systems and gametes enter the cloaca from the reproductive system. An opening in the cloaca allows the wastes and gametes to leave the body.
Amphibians have a relatively complex circulatory system with a three-chambered heart. Their nervous system is also rather complex allowing them to interact with each other and their environment. Amphibians have sense organs to smell and taste chemicals. Other sense organs include eyes and ears. Of all amphibians frogs generally have the best vision and hearing. Frogs also have a larynx or voice box to make sounds.
Most amphibians breathe with gills as larvae and with lungs as adults. Additional oxygen is absorbed through the skin in most species. The skin is kept moist by mucus which is secreted by mucous glands. In some species mucous glands also produce toxins which help protect the amphibians from predators. The golden frog shown in Figure below is an example of a toxic amphibian.
Toxic Frog. This golden frog is only about 5 centimeters (2 inches) long but it
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Amphibian Reproduction and Development
Amphibians reproduce sexually with either external or internal fertilization. They attract mates in a variety of ways. For example the loud croaking of frogs is their mating call. Each frog species has its own distinctive call that other members of the species recognize as their own. Most salamanders use their sense of smell to find a mate. The males produce a chemical odor that attracts females of the species.
Amphibian Eggs
Unlike other tetrapod vertebrates (reptiles birds and mammals) amphibians do not produce amniotic eggs. Therefore they must lay their eggs in water so they wont dry out. Their eggs are usually covered in a jelly-like substance like the frog eggs shown in Figure below. The jelly helps keep the eggs moist and offers some protection from predators.
Frog Eggs
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Amphibians generally lay large number of eggs. Often many adults lay eggs in the same place at the same time. This helps to ensure that eggs will be fertilized and at least some of the embryos will survive. Once eggs have been laid most amphibians are done with their parenting.
Amphibian Larvae
The majority of amphibian species go through a larval stage that is very different from the adult form as you can see from the frog in Figure below. The early larval or tadpole stage resembles a fish. It lacks legs and has a long tail which it uses to swim. The tadpole also has gills to absorb oxygen from water. As the larva undergoes metamorphosis it grows legs loses its tail and develops lungs. These changes prepare it for life on land as an adult frog.
Frog Development: From Tadpole to Adult. A frog larva (tadpole) goes through many changes by adulthood. How do these changes prepare it for life as an adult frog?
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Classification of Amphibians
There are about 6200 known species of living amphibians. They are placed in three different orders:
1. Frogs and toads
2. Salamanders and newts
3. Caecilians
Frogs and Toads
One feature that distinguishes frogs and toads from other amphibians is lack of a tail in adulthood. Frogs and toads also have much longer back legs than other amphibians. Their back legs are modified for jumping. Frogs can jump up to 20 times their own body length. Thats the same as you jumping at least 100 feet or more than the length of a basketball court. Think how fast you could move if you could travel that far on one jump!
Frogs and toads are closely related but they differ in several ways. Generally frogs spend more time in water and toads spend more time on land. As you can see from Figure below frogs also have smoother moister skin than toads as well as longer hind legs.
Frog and Toad. Frogs (top) and toads (bottom) are placed in the same amphibian order. What traits do they share?
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Salamanders and Newts
Unlike frogs and toads salamanders and newts keep their tails as adults (see Figure below). They also have a long body with short legs and all their legs are about the same length. This is because they are adapted for walking and swimming rather than jumping. An unusual characteristic of salamanders is their ability to regenerate or regrow legs that have been lost to predators.
Salamander and Newt. Salamanders and newts can walk or swim. Salamander on a leaf (left) newt swimming in the water (right).
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Caecilians
Caecilians are most closely related to salamanders. As you can see from Figurebelow they have a long worm-like body without legs. Caecilians evolved from a tetrapod ancestor but they lost their legs during the course of their evolution.
Swimming Caecilian. Caecilians are the only amphibians without legs.
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Evolution of Amphibians
Fossil evidence shows that amphibians evolved about 365 million years ago from a lobe-finned lungfish ancestor. As the earliest land vertebrates they were highly successful. Some of them were much larger than todays amphibians. For more than 100 million years amphibians remained the dominant land vertebrates. Then some of them evolved into reptiles. Once reptiles appeared with their amniotic eggs they replaced amphibians as the dominant land vertebrates.
Ecology of Amphibians
Amphibians can be found in freshwater and moist terrestrial habitats throughout the world. The only continent without amphibians is Antarctica. Amphibians are especially numerous in temperate lakes and ponds and in tropical rainforests.
Amphibians as Prey and Predators
Amphibians are an important food source for animals such as birds snakes raccoons and fish. Amphibians are also important predators. As larvae they feed mainly on small aquatic animals such as water insects. They may also feed on algae. As adults amphibians are completely carnivorous. They may catch and eat worms snails and insects as the frog in Figure below is doing. Unlike other amphibians caecilians are burrowers. They use their head to dig in the soil and they feed on earthworms and other annelids. Caecilians can be found in moist soil near rivers and streams in tropical regions.
Frog Predator. A frog eating its insect prey.
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The Threat of Amphibian Extinction
Currently almost one third of all amphibian species face the threat of extinction. The reasons include habitat loss pollution climate change and the introduction of non-native species. Most of these problems are the result of human actions.
Amphibians have permeable skin that easily absorbs substances from the environment. This may explain why they seem to be especially sensitive to pollution. Monitoring the health and survival of amphibians may help people detect pollution early before other organisms are affected.
Section Summary
Extra Practice
1. Assume that a certain species of toad appears to be dying out in a given ecosystem perhaps because of pollution. Many people think that the toad problem is unimportant because its just a toad. Write a letter to a hypothetical newspaper editor in which you explain why the health and survival of amphibians such as this toad species are important to all living things in an ecosystem.
2. Compare and contrast the three orders of living amphibians.
3. Explain why amphibians were able to become the dominant land vertebrates for millions of years.
Points to Consider
Amphibians gave rise to reptiles which replaced them as the dominant land vertebrates.
Section 4: Reptiles
Section Objectives
Vocabulary
Introduction
Reptiles are a class of tetrapod vertebrates that produce amniotic eggs. They include crocodiles alligators lizards snakes and turtles. The reptile class is one of the largest classes of vertebrates. It consists of all amniotes except birds and mammals.
Structure and Function in Reptiles
Reptiles have several adaptations for living on dry land that amphibians lack. For example as shown in Figure below the skin of most reptiles is covered with scales. The scales are made of very tough keratin and they protect reptiles from injury and also prevent them from losing water.
Crocodile Scales. These crocodiles are covered with tough waterproof scales.
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Reptile Respiration
The scales of reptiles prevent them from absorbing oxygen through their skin as amphibians can. Instead reptiles breathe air only through their lungs. However their lungs are more efficient than the lungs of amphibians with more surface area for gas exchange. This is another important reptile adaptation for life on land.
Reptiles have various ways of moving air into and out of their lungs. Lizards and snakes use muscles of the chest wall for this purpose. These are the same muscles used for running so lizards have to hold their breath when they run. Crocodiles and alligators have a large sheet of muscle below the lungs called a diaphragm that controls their breathing. This is a structure that is also found in mammals.
Ectothermy in Reptiles
Like amphibians reptiles are ectotherms with a slow metabolic rate. Their metabolism doesnt generate enough energy to keep their body temperature stable. Instead reptiles regulate their body temperature through their behavior. For example the crocodile in Figure below is soaking up heat from the environment by basking in the sun. Because of their ectothermy reptiles can get by with as little as one tenth the food needed by endotherms such as mammals. Some species of reptiles can go several weeks between meals.
Heat Transfer to an Ectothermic Reptile. This crocodile is being warmed by the environment in three ways. Heat is radiating directly from the sun to the animal.
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Other Reptile Structures
Like amphibians most reptiles have a heart with three chambers although crocodiles and alligators have a four-chambered heart like birds and mammals. The reptile brain is also similar in size to the amphibian brain taking into account overall body size. However the parts of the reptile brain that control the senses and learned behavior are larger than in amphibians.
Most reptiles have good eyesight and a keen sense of smell. Snakes smell scents in the air using their forked tongue (see Figure below). This helps them locate prey. Some snakes have heat-sensing organs on their head that help them find endothermic prey such as small mammals and birds.
Snake
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Reptile Reproduction
Most reptiles reproduce sexually and have internal fertilization. Males have one or two penises that pass sperm from their cloaca to the cloaca of a female. Fertilization occurs within the cloaca and fertilized eggs leave the females body through the opening in the cloaca. In a minority of species the eggs are retained inside the females body until they hatch. Then the offspring leave the mothers body through the cloaca opening.
Amniotic Eggs
Unlike amphibians reptiles produce amniotic eggs (see Figure below). The shell membranes and other structures of an amniotic egg protect and nourish the embryo. They keep the embryo moist and safe while it grows and develops. They also provide it with a rich fatty food source (the yolk).
Amniotic Egg. The amniotic egg is an important adaptation in fully terrestrial vertebrates. It first evolved in reptiles. The shells of reptile eggs are either hard or leathery.
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Reptile Young
Unlike amphibians reptiles do not have a larval stage. Instead newly hatched reptiles look like smaller versions of the adults. They are able to move about on their own but they are vulnerable to predators. Even so most reptile parents provide no care to their hatchlings. In fact most reptiles dont even take care of their eggs. For example female sea turtles lay their eggs on a sandy beach and then return to the ocean. The only exceptions are female crocodiles and alligators. They may defend their nest from predators and help the hatchings reach the water. If the young remain in the area the mother may continue to protect them for up to a year.
Classification of Reptiles
There are more than 8200 living species of reptiles with the majority being snakes or lizards. They are commonly placed in four different orders. The four orders are described in Table below.
Orders of Living Reptiles
Characteristics
Crocodilia: crocodiles alligators caimans gharials
They have four sprawling legs that can be used to gallop; they replace their teeth throughout life; they have strong jaws and a powerful bite; they have a more advanced brain and greater intelligence than other reptiles; they have a four-chambered heart.
Sphenodontia: tuataras
They are the least specialized of all living reptiles; their brain is very similar to the amphibian brain; they have a three-chambered heart but it is more primitive than the heart of other reptiles.
Squamata: lizards snakes
Lizards: most have four legs for running or climbing and they can also swim; many change color when threatened; they have a three-chambered heart.
Snakes: they do not have legs although they evolved from a tetrapod ance