Toads and Frogs

AMPHIBIANS WITHOUT TAILS

The toads and frogs that lived before the Jurassic period probably had long bodies and long tails. Biologists believe that thisbody form changed suddenly. The most obvious change was the disappearance of the tail in the adult. Other, less obvious changes made these animals better suited to life on the land. Their hind legsdeveloped an extra joint. And their ankle bones became longer. Thesechanges gave their legs great jumping power. Front legs stayed short.But this is suited for taking up the shock of landing from a jump. Modern frogs and toads have wide mouths. Their sticky front-hinged tongues can catch insects with lightning speed. Frogs and toads have existed for over 200 million years. They live in many places all overthe world. Biologists consider them among the most successful vertebrates.

Frogs and toads are similar in structure. But frogs differ from toads in many ways in their anatomy and behavior. Of all amphibians, the toad is most able to survive on land. It leaves the water early in life. It only returns to water to lay eggs. The toad starts life as a small black tadpole. The tadpole soon grows legs and resorbs its tail. It hops onto land as a small brown creature with warty skin. Adults of the common toad, Bufo, are usually red-brown on the top and gray-yellow underneath. There is no truth to the old tale that peoplecatch warts from toads. Warts are a viral condition that has nothing to do with toads.

Toad Leopard Frog

Toads often live in areas with loose, moist soil. They dig in and hide from enemies and summer heat. They can dig quickly with their hind legs. If a toad is unable to bury itself when disturbed, it may crouch low to the ground and remain still. The color and texture of its skin provide good camouflage. But poison glands in theskin are a toad’s best defense. These glands secrete an irritating, bad tasting substance. This causes most animals to leave the toad alone. But this defense does not keep snakes from eating the toads. Snakes are the toad’s major enemy.

The toad has been called the “gardener’s friend.” Toads feed upon insects, worms, and other forms of food harmful to plants. Toads

can easily bury themselves in moist, loose garden soil. It is probably this soil that attracts toads to gardens.

Unlike toads, frogs usually live very near water. You often see them around ponds. The most common frog in the United States is the leopard frog. These frogs live in almost every pond, marsh, and roadside ditch. They often travel a long way from water. Sometimes they can be seen hopping through grassy meadows. They are grayish-green with large dark spots surrounded by yellow or white rings. This colorationgives them their name. The soft underbelly of the leopard frog is creamy white. This makes it hard to see the frog when it is in the water. Its back color blends in with a grassy pond. And its belly blends with the sky when seen from below.

The bullfrog is names for its loud bellowing sound. It is the mostaquatic of frogs. It seldom leaves the water except to sit on the bank of a lake at night. The color of bullfrogs ranges from green to nearly yellow. But most of them are greenish-brown. The underbelly is gray-white with dark splotches. Bullfrogs are excellent swimmers because of their large, webbed hind feet. Their legs are strong and well developed. Their legs can be 25 centimeters long. Bullfrogs eatinsects, worms, crayfish, and small fishes. Big bullfrogs will sometimes even eat a small duckling.

The tree frogs of the genus Hyla are interesting amphibians. Mostof them have amazing protective coloration. Several of them have the ability to change their color. Some members of the genus live in trees. These frogs have a sticky disc on each toe. This enables themto cling to vertical surfaces. The spring peeper is a member of the genus Hyla. This frog lives in swamps and bogs rather than in trees.

Peeper eggs are laid in early spring. The tiny tadpoles feed on algae and protozoans. But the adults help us by eating mosquitos and gnats. A swamp or bog will be filled with the noise of peepers on a spring or summer night.

THE ECONOMIC IMPORTANCE OF FROGS

A major part of a frog’s diet is insects. This makes frogs valuable because they help control the insect population. Many stateshave laws to regulate frog hunting for this reason. These laws forbidthe capture of frogs during the breeding season.

Bullfrogs hind legs are thought to be a food delicacy. They are raised for this purpose on farms in marshy areas. Smaller frogs are often used as fish bait. Frogs are a favorite in biology labs as specimens for dissection. The frog’s internal organs have the same basic arrangement as the human’s. Thus, dissecting frogs is an excellent introduction to human anatomy.

Anatomy of the Frog

THE EXTERNAL STRUCTURE

The frog’s body is short, broad, and angular. It lacks the streamlined shape of fishes. For this reason, frogs do not swim as well as fish do. And the frog’s hopping is not as graceful as the movement of most land animals. But the frog is able to move about in both of these environments.

Frog skin is thin, moist, and loose. It is richly supplied with blood vessels. Glands in the skin secrete mucus. This reaches the skin’s surface through tiny tubes. This skin slime makes frogs hard to hold. Frog skin lacks protective growths such as the scales of fishes and reptiles.

ADAPTATIONS OF THE FROG’S LEGS

The frog’s front legs are short and weak. Each foot has four inturned toes with soft, rounded tips. These feet are not webbed and are not used for swimming. Instead, the front legs are used to prop up the body when on land. They also break the fall when the frog lands from a jump. The inner toes, or thumbs, are enlarged on male frogs. These thumb pads become even larger during the breeding season for males.

The frog’s hind legs are well developed and strong. They are adapted for both swimming and jumping. The thigh and calf muscles arevery powerful. The ankle area and the toes are long. This forms a foot that is longer than the lower leg. A flexible web membrane connects the five long toes. This webbing makes each foot a very efficient swimming organ. When a frog rests on land, its hind legs fold against its body. In this position, a frog is ready to jump veryquickly.

THE HEAD

Frog eyes are very noticeable because they bulge up above the head. The colored iris surrounds an elongated black pupil opening. Muscles attached to the eyeball rotate the eye in its socket. The eyes can be pulled into the sockets and pressed against the roof of the mouth. This helps hold food in the mouth. When a frog’s eyes arepulled down, the upper and lower eyelids close.

The frog can float just below the water’s surface with only its bulging eyes showing. This allows the frog to see the surface when hiding in the water. The frog has a third eyelid called the nicitating

membrane. This joins the lower lid. This thin covering keeps the eyeball moist when a frog is on land. It also protects the eye when the frog is under water.

The nostrils are forward near the top of the head. This allows the frog to breath air when all but the top of its head is under water.

Frog have no external ears. But they do have eardrums or tympanic membranes. These are located on the body surface just behind the eyes.The cavity of the middle ear lies just below the tympanic membrane. Males tympanic membranes are usually darker in color especially duringmating season. A canal connects each middle ear with the mouth cavity. These canals are called the Eustachian tubes. The inner ears areembedded in the skull.

THE MOUTH – AN INSECT TRAP

The frog’s mouth extends from ear to ear. If you watch a frog catch a fly, you will see why the mouth is so large. It is a trap forinsects. The frog’s thick tongue is attached to the floor of its mouth at the front. This sticky tongue has two projections at its free end.

To catch an insect, a frog opens its mouth wide and flips its tongue over and outward. If the aim is good, the insect is caught on the tongue surface and is thrown into the mouth. The mouth snaps shutquickly. Then the frog swallows the insect. This all happens so fastit is hard to see. Two teeth project from bones in the roof of the mouth. They are called vomerine teeth. They aid in holding the prey. Small, cone-shaped maxillary teeth project from the upper jaw. These alsohelp hold on to prey. Frogs have no teeth on their lower jaws.

Frog’s tongue is well adapted for The frog’s mouth. Its relatively large size is an Catching insects in that it is both adaptation for obtaining food.Flexible and sticky. Note how it is Attached at the front of the mouth.

There are various openings inside the frog’s mouth. Internal nostril openings are located in the roof on either side of the vomerine teeth. Far back on the sides of the roof are openings of the Eustachian tubes. Openings to the vocal sacs are located at the back of the floor of the mouth of the male frog. When a male frog croaks, air is forced down these openings into the vocal sacs. This air forces the sacs to expand between the frog’s ears and shoulders. This action makes the croak louder and more powerful. When a frog croaks under water, air is forced from the lungs, over the vocal cords, into the mouth and then back to the lungs. The throat has two openings. The large gullet openings leads to the stomach. Below the gullet opening is the slitlike glottis. The glottis leads to the lungs.

THE DIGESTIVE SYSTEM

Adult leopard frogs usually feed on insects and worms. But they can swallow even larger meals because of their large, elastic gullet. The short gullet leads to a long stomach, an enlargement of the food tube. The upper end of the stomach is large, and it tapers at the lower end. At a point called the pylorus, the stomach links up with thecoiled, slender small intestine. At the lower end of the stomach, there is a muscle called the pyloric valve. This valve controls the movement of food from the stomach into the small intestine.

The small intestine is looped several times. It is supported by a fanlike membrane called the mesentery. The anterior area of the small intestine, which curves from the pylorus, is the duodenum. The middle portion is the coiled ileum. The lower end of the small intestine leads into a short, broad colon. The colon is also called the large intestine. The colon opens into a cavity called the cloaca. Tubes from the kidneys,the urinary bladder, and the sex organs also open into the cloaca. Waste materials and eggs or sperms pass out the cloaca through the cloacal opening.

Tiny gastric glands, in the stomach walls, secrete gastric fluid. Gastric fluid chemically digests some of the food. A large, three lobed liver partially covers the stomach. The liver stores digested food products, it also secretes bile and acts as a digestive gland. The bile collects in the gall bladder, which lies between the middle and right lobes of the liver. From the gall bladder, bile runs through the bile duct into the upper part of the small intestine. A second digestive gland, the pancreas, lies inside the curve of the stomach. Pancreatic fluid and bile pass through the common bile duct into the small intestine. All of the these fluids are necessary for digestion.

Mucous glands in the walls of the stomach and intestine secrete mucus. Mucus lubricates the passage for food.

Basically, a frog’s digestive system is like those of other vertebrates. It is a long food tube, generally called the alimentary canal. This canal has special regions for digestion and absorption of digested food. Increased length of the alimentary canal increases thegeneral efficiency of these processes.

THE RESPIRATORY SYSTEM

You now know that adult frogs are air breathers. Have you ever wondered how they can stay under water for a long time? And during the winter, a frog will lie buried in the mud on the bottom of a pond to hibernate. The skin of the frog is thin. And the skin has a lot of blood vessels. Thus, the frog’s skin directly absorbs dissolved oxygen from the water. It also gives off carbon dioxide. As long as a frog stays quiet, this type of respiration is enough to keep it alive. During hibernation, the frog’s body processes slow down. Therefore, its oxygen need is very low. But when a frog is active andswimming, it needs more oxygen. The frog then comes to the surface and breathes air.

We inhale and exhale air by increasing and decreasing the size ofour chest cavities. This is done by moving the ribs and diaphragm. The diaphragm is a muscular partition at the bottom of the chest cavity. The frog has no diaphragm, and thus no chest cavity. The frog does not even have ribs. Instead, frogs change the volume and pressure of air in their mouths. When a frog lowers the floor throughthe open nostrils. Then, when the floor of the mouth springs up, air passes out through the nostrils.

The lining of the mouth is also adapted for respiration. It is thin, moist, and has many blood vessels. Frogs can perform both mouth-breathing and lung-breathing. They may pump air in and out of their mouths for some time without using their lungs. When the lungs are used, the nostrils are closed by skin flaps as the floor of the mouth rises. The glottis opens and air enters the windpipe, or trachea.This leads into the lungs. Then, with the nostrils still closed, the mouth is trust down. This causes air to pass from the lungs into a partial vacuum. Then a sudden upthrust of the mouth forces air back into the lungs. After exchanging air between mouth and lungs a few times, the frog returns to mouth-breathing.

Frogs use their lungs only to assist mouth-breathing. As you might expect, frog lungs are small compared to lungs of higher vertebrates. Frog lungs have thin-walled sacs that lack the spongy tissue our lungs have.

THE CIRCULATORY SYSTEM

The circulatory system of the frog is a step more complex than that of the fish. This represents a step toward the higher vertebrates. One of these advances is the heart. The frog heart has three chambers. There are two atriums and a muscular ventricle. Deoxygenated blood flows into the right atrium from various parts of the body. When the lungs are used, oxygenated blood from the lungs flows into the left atrium. Both atriums contract at the same time. This forcesblood into the ventricle. The ventricle then contracts and pumps blood out of the heart. The blood leaves through a large vessel that lies against the front side of the heart. This is called the conus arteriosus. This large vessel immediately divides into two branches, theright and left truncus arteriosus. Each of these again branches into threearches. The anterior pair are the carotid arches. These carry blood to the head. The middle pair are the aortic arches. They transport blood around the right and left sides of the heart. They join below the liver to form the dorsal aorta. This great artery carries blood to muscles, the digestive organs and other parts of the body. The posterior pair of arches are called the pulmocataneous arches. They carry blood to the lungs, skin, and mouth.

The blood that returns to the frog’s heart, after a trip through the body, has lost most of its oxygen. This blood is loaded with carbon dioxide and other cell wastes. Three large veins carry blood back to the heart. These are called the venae cavae. They join a triangle-shaped, thin-walled sac, called the sinus venosus, at the back ofthe heart. The empties into the right atrium.

Part of the blood, returning from lower parts of the body flows through the vessels of the digestive system. There it absorbs digested food. This blood then flows through the hepatic portal vein which carries it to the liver. The liver picks up some of the food

substances carried by this blood and screens out some of the wastes. From the liver, this blood moves on to the right atrium.

During each complete circulation, some blood passes through the kidneys. The kidneys remove some water and nitrogen-containing wastesfrom cell activity. Pulmonary veins carry blood from the lungs to the left atrium. This blood is oxygenated when the frog is using its blood in air-breathing.

Frogs have a three-chambered heart. As you may remember, the fish heart has only two chambers. Blood passes through the fish heartonly once in its trip around the body. But frog hearts receive bloodfrom both the body and the lungs. And the frog heart pumps blood to the head and the body, as well as to the various respiratory organs.

THE EXCRETORY SYSTEM OF THE FROG

Frog skin is a vital excretory organ. Most of the carbon dioxideleaving the blood passes through the frog’s skin rather than through the mouth or lungs. The liver, too, removes certain wastes. The liver eliminates these wastes with bile or changes them chemically so the kidneys can remove them. The large intestine eliminates undigested food and other wastes. But the kidneys are the main excretory organs.

The kidneys are large, dark red organs. They lie on either side of the spine against the back body wall. Blood flows into kidneys,

through the renal arteries and out through the renal veins. The kidneys filterwastes and collect it as urine. The urine then flows to the cloaca through tiny tubes, called ureters. Urine may be excreted immediately. But frogs are also able to force urine from the cloaca through a small opening that leads to the urinary bladder. Here, it may be stored for a while before being excreted.

THE NERVOUS SYSTEM

The frog brain is more highly developed that the fish brain. Olfactory lobes lie at the anterior end of the brain. The long lobes of the cerebrum are proportionally larger than those of the fish. Next are the optic lobes. Behind the optic lobes is the cerebellum. This is small band of tissue lying at right angles to the long axis of the brain. The medulla oblongata lies at the back of the frog brain. It joins the short, thick spinal cord. The spinal cord extends down the frog’s back. The spinal cord is encased in bony vertebrae. As in thefish, spinal nerves branch from the cord to various parts of the body. Inaddition, ten pairs of cranial nerves extend from the brain.

Dorsal and Ventral View of the Nervous System

THE REPRODUCTIVE SYSTEM

In both sexes, the frog’s sex organs are internal. Thus, it is hard to tell the sexes apart, except during the breeding season. At this time, the thumbs of the males are enlarged and the “ears” are darker.

Male and female frogs mating. The mass of eggsIn the water indicates external fertilization.

The male reproductive organs are two oval testes. The testes area creamy-white or yellow color. They lie in the back, one on each side of the spine, in the anterior region of the kidneys. Sperm cellsdevelop in the testes. The sperms then pass through tubes, the vasa efferentia, into the kidneys. When the sperms are discharged, they pass through the ureters into the cloaca. Some frog species have an enlargement, the seminal vesicle, at the base of each ureter.

In the female frog, the eggs develop in a pair of large lobed ovaries. The ovaries lie along the back above the kidneys. During the breeding season, the eggs enlarge and burst through the thin ovarywalls. This frees the eggs into the body cavity. The abdominal muscles work the eggs toward the anterior. Here funnel-like openings to the oviducts gather the eggs. Oviducts are long and coiled, and are lined with ciliated cells. The cilia fan the eggs into the oviduct openings. Oviducts lead into the cloaca. Near the openings to the cloaca, the walls of the oviduct secrete a jellylike substance. This substance surrounds each egg. There is a saclike uterus at the base of each oviduct. The eggs are stored in the uterus until they are laid.

Frog Development

FERTILIZATION AND DEVELOPMENT OF THE EGGS

The female leopard frog lays up to 200 eggs between early April and mid-May. When the eggs are to be laid, the male clasps the female. The male uses its large thumb pads to press down on the female. This is called amplexus. It helps the female to expel the eggs.As the eggs pass from the female’s cloaca into the water, the male

spreads sperm over them. The sperm cells reach most of the eggs with this direct external fertilization.

The jelly-like coat that surrounds each egg swells up in the water. This binds the eggs together in a round mass. In this clump, the eggs look like small beads covered by transparent jelly. The jelly-like coat protects the eggs from injury and makes it hard for fish to eat them. It also helps to keep the eggs at a constant temperature. Later, the covering serves as food for the young tadpoles.

A frog egg is partly black and partly white. The white part is the yolk, a stored food material. This nourishes the tadpole during development. The dark part is the living protoplasm of the egg and a dark pigment. The yolk is heavier than the protoplasm. So frog eggs float with the dark side up. The dark pigment absorbs heat from the sun. The lighter lower half blends in with the light from the sky. This makes it hard to see the eggs from below. After eight to twenty days, depending on the weather and temperature, the leopard frog tadpoles hatch. They then wiggle away from the egg mass.

THE METAMORPHOSIS OF THE FROG- FROM TADPOLE TO ADULT

The hatched tadpole is a tiny, short-bodied creature. It has a little round mouth. It clings to the egg mass or to a plant. Yolk stored in the body nourishes the tadpole until it starts to feed. Soon the body grows longer and three pairs of gills develop at the outside of the head. The tail grows longer and develops a caudal fin.Then the tadpole’s mouth opens, and it begins to scrape the leaves of water plants with its rough lips.

Not long after the tadpole starts to swim around, its rough lips disappear. A long, coiled digestive tract develops and the tadpole starts to eat the scum of water plants. Then a flap of skin grows over the gills, leaving only a small opening on its left side. This allows water to pass out of the gill chambers. At this stage, the tadpole is like a fish. It has a lateral line, fin, two-chambered heart, and a one-circuit circulation. It also has a relatively long intestine coiled in a spiral.

The change to an adult frog is amazing. First, hind legs appear on the tadpole’s fishlike body. The front legs start to form at aboutthe same time. But they remain hidden under the tadpole’s fishlike operculum for a while. When the front legs do appear, the tadpole starts resorbing its tail. The tadpole does not shed or eat the tail.

Near the end of the metamorphosis, the mouth broadens and teeth develop. As these external changes happen, internal changes also occur. A saclike chamber, resembling the swim bladder of a fish, forms behind the tadpole’s throat. This divides into two sacs, which become the lungs. The heart develops three chambers. The gill arteries change into the carotids, aortic arches, and pulmoncutaneous arteries. Soon the gills stop functioning and the tadpole starts swimming to the water’s surface to gulp air. At this time, the tadpoles thin skin and broad, flat tail still play important roles in respiration.

Even before the tail is completely resorbed, the tadpole starts moving out on the land. From this point, the tadpole is considered a young frog. The young frog usually takes about a month to become a full-grown frog.

The metamorphosis of the leopard frog takes from two to three months. Adult leopard frogs usually appear around July first. Bullfrogs usually spend two winters as tadpoles. And it may be three years before the adult bullfrog is fully formed.

REGENERATION IN AMPHIBIA

Many amphibians, especially salamanders, can regrow injured or lost body parts. A foot, a part of a limb, or the tail may be lost inescaping from an enemy. Regeneration can occur rapidly. In their tadpole stages, frogs and toads also have this capability. But, as the tadpole matures, the capability disappears. No adult frogs and toads can regenerate body parts.

Frog BehaviorHIBERNATION AND ESTIVATION IN THE FROG

Like fishes and reptiles, frogs are cold-blooded vertebrates. This does not mean that the blood is always cold. It means that the body temperature varies with the temperature of the external environment.

With the coming of fall, a frog’s body temperature drops with thetemperature of the air. Soon the frog’s body is too cold to remain active. This is when it buries itself in mud at the bottom of a pond.Its heart slows down and blood hardly circulates in the vessels. Because it is so inactive at this time, the frog needs very little oxygen. Enough can be supplied through the moist skin. Body tissues stay alive by the slow oxidation of stored food. The food is stored in the liver and in the fat bodies above the kidneys. Nervous activity stops almost completely. The frog lies in its burrow in a stupor. This is the condition of the frog during hibernation. Hibernation is a winter rest.

With the coming of spring, the water gradually warms. The frog’sbody activities speed up. And soon the frog wakes from its winter sleep. After a while, it begins the activities of a normal life again.

The hot summer months bring other problems for the frog. Lackinga way to cool its body, the frog must escape from extreme heat. It may lie quietly in cool, deep water. Or it may bury itself in mud at the bottom of a pond. This inactivity during the heat of summer is called estivation. A small pond may dry up entirely during a mid-summer hot spell. Then the frog has to bury itself in the mud and estivate. Cool weather eventually returns, and the pond fills with water again. The frog comes out of estivation and continues normal activities untilhibernation.

SUMMARY

The vertebrate time scale shows an interesting history for amphibians. According to our reading of fossils, the earliest amphibians date back to the close of the Devonian Period, often calledthe Age of Fishes. It was probably then that the early ancestors of amphibians, the lungfish and lobe-finned fishes, crawled out of the primal seas onto the edges of the land. This move offered amphibians a new environment. They no longer had to compete with fish. Thus, amphibians suddenly began to flourish in the Carboniferous and Permianperiods. But during the Mesozoic Era, other vertebrates appeared who were more completely adapted to life on land. These newer creatures challenged the amphibians and greatly reduced their numbers. Today

there are not many amphibians compared to other land animals, especially the mammals.

Amphibians are limited to moist areas because they need water to lay eggs and develop into adults. They represent a kind of “in-between” stage in the evolutionary process. They live in two worlds, water and land. But fish surpass them in the water. And other vertebrates surpass them on the land.