Toads and Frogs

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 this body 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 legs developed an extra joint. And their ankle bones became longer. These changes 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 over the 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 people catch 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 the skin 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 coloration gives 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 most aquatic 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 eat insects, worms, crayfish, and small fishes. Big bullfrogs will sometimes even eat a small duckling.

The tree frogs of the genus Hyla are interesting amphibians. Most of 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 them to 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 states have laws to regulate frog hunting for this reason. These laws forbid the 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 are very 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 very quickly.

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 are pulled 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 during mating season. A canal connects each middle ear with the mouth cavity. These canals are called the Eustachian tubes. The inner ears are embedded 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 for insects. 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 shut quickly. Then the frog swallows the insect. This all happens so fast it 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 also help hold on to prey. Frogs have no teeth on their lower jaws.

Frog’s tongue is well adapted forThe frog’s mouth. Its relatively large size is an

Catching insects in that it is bothadaptation 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 slitlikeglottis. 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 the coiled, 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 the general 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 and swimming, 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 of our 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 through the 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 forces blood 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, the right and left truncus arteriosus. Each of these again branches into three arches. 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.