Skin smooth, moist
Skin dry, warty-looking
Usually lives near water
May live away from water
Visible tympanum (eardrums)
Visible tympanum (eardrums)
No parotoid glands
Visible parotoid glands
Tree frogs have toe pads
No toe pads
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Toads don't cause warts
Frogs and toads are very alike, being different mostly in their habitat preferences and outward appearance: Most frogs have smooth, moist skin and prefer moist environments. Toads have dry, warty-looking skin and are adapted to drier habitats. So, although there are some exceptions, when you see a smooth-skinned, moist-looking frog, he's a frog. If he has warty-looking, dry skin he's called a toad. In the following, use of the term "frog" means toads, too, except where otherwise noted.
Frogs and toads are amphibians. Amphibians are animals who live part of their lives in water and part on land. Ninety percent of amphibians are frogs. Frogs belong to the order Anura (uh-NOO-ruh), which is from the Greek for "tail-less," along with newts, salamanders and caecilians
(suh-SILL-ee-un). There are over 5,000 described species of frogs in the world, classified into many different families. Only one family of frogs, Bufonidae (byoo-FAWN-eh-dee), is technically that of "true toads." So, it would be correct to refer to a toad as a frog, but frogs are never toads.
Amphibians evolved from fish who moved out of the water onto land about 400 million years ago (they still spent most of their time in water.) Frog/salamander-like animals date back as far as 290 millions years. The earliest "modern" frog, that is, one containing all the frog features of today, showed up about 125 million years ago. (See a fossil here
.) In 2008, scientists in Madagascar discovered a toad-like fossil that's 70 million years old. They named him Beelzebufo, or Devil Toad, because of his size: 10 pounds and 16 inches long!
Most frog species live in tropical rainforests, but they're native to everywhere there's a suitable habitat, except Antarctica and some islands. Some toads live in arid areas, burying themselves during dry spells and emerging when it rains. Frogs (as well as other amphibians) are threatened around the world from loss of habitat. Other forces are at play, too, such as chemical pollution of air and water, disease and over-harvesting for food. Many may become extinct in the near future; in some areas of the world certain species are already gone.
Frogs and toads are invaluable predators of insects, spiders, worms and slugs. Reportedly a single toad will eat about 10,000 insects over the summer. Some large toads and frogs even eat tiny rodents or reptiles.
In the U.S., the smallest frog is the Little Grass Frog (Limnaoedus ocularis) who, at 3/4-inch, fits on a penny. The largest U.S. frog is one you might guess: the Bullfrog, who's almost 8 inches in diameter and can weigh more than 1 1/2 pounds. The smallest toad in the U.S. is the Oak Toad (Bufo quercicus) who measures 3/4 to 1 1/4 inches.
The largest frog in the world is an endangered, appropriately named African species, the Goliath Frog (Conraua goliath), whose body can be more than a foot long. With his legs extended, he stretches to more than 2 1/2 feet. A Cuban frog called the Monte Iberia Eleuth (Eleutherodactylus iberi), and the Brazilian Gold Frog (Brachycephalus didactylus) are the world's smallest, with the former a fraction smaller than the other at 9.6 millimeters vs. 9.8 millimeters (3/8-inch).
Frogs are cold-blooded, meaning their body temperature matches the temperature of their surroundings. They're less active in cold weather, when their metabolism slows down. Some frogs produce high glucose concentrations in their vital organs which protects them in freezing weather. In spring or on cool mornings, you may find frogs sunning themselves to warm up their body temperature.
Head and senses
Frogs have a flat, wide head with eyes that sit on top, big and bulging. They have excellent night vision and, day or night, they see nearly everything: a wide field of vision allows them to see to the front, sides and partially behind. Frogs don't have a neck. This means they can't turn their head or move it up and down. So, the wide field of vision is an important asset for an animal who must detect a predator without moving his head. The upper and lower eyelids are underdeveloped and don't move much. In order to completely close their eyes, frogs draw their eyeballs deep into their sockets; this pulls the eyelids together. At other times they can move a transparent membrane up over their eyes. It protects and cleans the eyes.
Buccal pumping: Frogs breathe in an unusual way, through a method called buccal pumping. It's a "two-stroke" process: When they lower the floor of their mouth, it draws air in through their nostrils and pulls air from their lungs into their buccal (mouth) cavity. When they raise the floor of their mouth, it does the opposite, pushing air out, as well as into the lungs.
art: Mokele / Wiki
Frogs have a good sense of smell. Their nose sits on top of the head. With eyes and nose both above, frogs can sit in water with only the top of their head exposed above the surface. (All the better to catch unsuspecting insects with a quick flick of their tongue.) The eyes serve another, rather amazing, function: When a frog catches prey, his eyes close and drop down into the roof of his mouth and help push the food down his throat.
Most, but not all, frogs have teeth, albeit very short ones. They're located on the upper jaw only and used for grinding food. True toads, those in the family Bufonidae, don't have teeth; they swallow their food whole. Frogs have a mucus gland in their mouth that produces a sticky substance that coats their tongue. Their tongue is long and the base of it is fastened at the front of their mouth rather than at the back. This allows them to hold the tongue folded with the tip of the tongue facing the throat. When an insect happens by, the tongue can be flicked out quickly and accurately. Mucus helps to "glue" the insect to the tongue.
Frogs have an external ear on each side of their head. They're large, circular membranes called a tympanum, or eardrum. There's a muscular membrane at the end of the inner ear that's thought to help protect their' ears from extremely loud noises. These noises are made by the frogs themselves, some with calls so loud they can be heard a mile away. In 2008, scientists discovered a species of frogs living near a noisy area in China who can tune their hearing to different frequencies, like a radio knob, effectively tuning out objectionable noises or even the calls of other frog species.
Frogs communicate with others of their species. Their "calls" fall into categories of attraction calls and aggression calls, as well as release calls, which are used by both males and females to signal non-readiness to mate. Some frogs emit their attraction calls separately and others join a group in an immensely loud "chorus" of calls. For some frogs, making a call inflates and pulsates a conspicuous vocal sac located on their throat, which you can see in the photo to the right.
Green Tree Frog with inflated
(Phil Morley, Clearview Images)
Most of the nighttime sounds of spring and summer are intriguing and mysterious. Insects are most of the noise-makers; if you live near water, frogs are chiming in, too. We're all pretty familiar with the deep croak of Bull Frogs, but frog calls also include chirps, trills, twitters, peeps, clicks, twangs and other sounds that sort of defy description. In order to attract the right mate, each species has his own special call. As for the frog who goes, "ribbit, ribbit, ribbit," he's the Pacific Tree Frog. You can listen to him here: You can hear other frog calls here and here.
Frogs have two arms and two legs, actually pretty similar to those of humans: The arms have a humerus, radius and ulna (the radius and ulna are fused together, however.) The legs have a femur and fused tibia and fibula bones. The arms have four fingers and the legs have five toes. The shoulder blades and collarbone are shaped similar to a human's, too. Some frogs lack a tongue and use their fingers to catch prey and place it in their mouth. Frogs who spend a lot of time in water have longer hind legs than more terrestrial species. Also, their feet have more webbing, which aids in paddling through water. By comparison, the webbing on the feet of tree frogs, who spend all their time on land, covers only a half or less of the length of their toes.
The toes of tree frogs differ in another way, too: They have mucus-tipped, bristly pads on their toes that help them stick horizontally or even hang from the underside of leaves. There's an unusual group of frogs, called Chinese Gliding frogs, who stretch their toes wide and the webbing serves as wings, allowing them to glide from high places to lower places. They "fly" from tree to tree in this manner. "Spadefoot" toads, who live in dry habitats, have claw-like growths on their hind feet which they use for digging burrows. Their method of execution is interesting: as they dig, they move backwards in a circle until the burrow is deep enough for them to disappear into it.
Some species of frogs walk, but most hop as a standard method of locomotion on land and they're powerful jumpers. Frogs' arms are shorter and little use in jumping, but they help cushion the landing. The hind legs are long and strong.
Note the round toe pads on this
Red-eyed Tree Frog (Agalychnis
callidryas; native to C. Amer.)
(Carey James Balboa/ Wiki)
Skin and coloration: Frogs usually have thin, smooth skin. Toads usually have dry, bumpy skin, but not always. Some frogs have spines or tubercles that camouflage them so well they look like moss or lichen. Most frogs have dull coloration -- browns, greens, grays -- to help camouflage them in their environment.
Some frogs become paler or darker in response to certain stimuli and many change their color altogether. The changes occur because of star-shaped pigment cells in the skin, called chromatophores. Different chromatophores contain different granules of color: some cells have red, some have yellow, some have black, and so on. How the changes occur is complicated, but they're triggered by a number of different variables: excitement, humidity, light, temperature or surroundings: the Pacific Tree Frog (Hyla cinerea), for example, changes skin color to match the color of the background, moving from shades of light-green to dark-brown as the need arises. Gray Tree Frogs can be even more dramatic. Their normal coloration varies, with some being yellow and others green, gray or brown. But some are almost black and can change their color to nearly white. There's a small African frog who is creamy-white and lives in the white blossoms of a particular lily. When the flowers die and turn brown, he turns brown, too.
The outer skin (epidermis) of frogs is moist. The moisture comes from mucus that's secreted by glands in the lower layer of skin (dermis). The skin of true toads secretes moisture, too, and they're able to retain it longer. Still, their skin is drier than that of frogs, allowing them to live farther from water; some toads even live in arid habitats, where they dig burrows to escape skin-drying conditions.
The moist skin of frogs does more than cover tissue and bone. Although frogs have lungs and take in air through their nostrils and mouth, they absorb additional oxygen through pores in their skin directly into their bloodstream. This is a process called cutaneous gas exchange. This is especially true when they're underwater, where 100 percent of their oxygen intake is through the skin. Frogs also "drink" through their skin rather than with their mouth. (It's this permeability that makes frogs so victimized by pollutants -- their skin provides them little protection.) Frogs shed (molt) their skin about once a week. They loosen old skin by twisting their body all around. Then they pull their skin off over their head in one piece, like a sweater. They don't leave their old skin laying about; they eat it.
The large, puffy bump behind this
toad's eye is a parotoid gland.
Toads and warts:
Toads don't have warts and don't cause warts. All frogs have glands in their dermis that secrete a mild poison. The glands are in different places on different species. The toxin is produced in warty-looking bumps on the body and, with toads, secreted through a puffy parotoid gland, located behind each eye. The secretion is a milky-white substance, called bufotoxin, which has a bitter taste, prompting most animals who catch a toad to drop him immediately. It's a toad's only defense against predators. The secretions seep out when the toad is handled too tightly and it may kill small predators, such as birds. The toxin reacts with the mucus lining of a predator's mouth and tongue and often causes foaming of the mouth, making the animal look rabid. It's a temporary, but effective, condition -- predators usually don't mess with a toad again.
Frog and toad secretions aren't usually harmful to people, but they can cause a rash or temporary shortness of breath. It's a good idea to always wash up after handling one.
Poisonous frogs: The U.S. doesn't have any dangerous frogs, but you might be interested in knowing there are such critters. In the tropics are some of the most eye-catching animals on earth: Brightly colored frogs dressed in vivid yellows, blues, reds, oranges and purple, some with lovely patterns of spots or stripes. These beauties have much to admire, but can't be touched. Their noticeable coloration serves as a warning to predators to stay away. (This is such an effective defense that some harmless frogs have also evolved colorful bodies to try to fool predators.)
The most poisonous animal on earth is probably the Golden Poison frog from Central and South America: There's enough poison in just one of these frogs to kill 10 people! Poisonous frogs tend to be diurnal (active in daytime) because they don't fear being seen. Many poisonous frogs are kept as pets because they're so beautiful. Interestingly, over time these captive frogs become less poisonous, some to the point where they can be safely handled. This is because the compounds that make them toxic are formed from substances in the bodies of the prey they eat in the wild. In captivity, these frogs are usually fed a diet that doesn't produce toxicity.
Happily for us in the U.S., wild frogs are generally safe to handle. Some people are sensitive to their skin secretions and may develop a rash or breathlessness. To avoid this, keep your hands away from your face and wash up after touching a frog. Kids love to catch and observe frogs, but they should always be released back into their habitat.
Most frogs are active at night, but some are active in daytime -- the Northern Cricket Frog, for example. Frogs have few defenses. Camouflage coloration helps them to hide in plain sight. Some change color to match the background they're on. Many play dead when approached by a predator, relying on most predators' preference for live food. Some frogs run, others may dive into the water. Some puff up to appear bigger. Poisonous frogs (see above) are protected by their bright skin coloration that warns predators not to touch.
Frogs become inactive when weather conditions are too dry or too cold. They enter a dormant state known as aestivation. This differs from hibernation in that it occurs during summer instead of winter and is a shallower "sleep." Many frogs who live in temperate areas go into hibernation during winter. Some may hibernate for up to eight months in such places as caverns, burrows, a crevice in a log, under a rock or under leaf litter and debris.
When terrestrial frogs hibernate, their body temperature drops to the surrounding temperature. In freezing temperatures, their heart, respiration and muscle action stops. Ice crystals may form under their skin, in their bladder and elsewhere -- in other words, they freeze. And yet they don't die. A high glucose concentration in their body acts like antifreeze. When their body warms up, the ice melts, the organs spring into action!
Aquatic frogs hibernate underwater, partially burying themselves in the soil at the bottom. They may occasionally swim slowly around.
Frogs, including tree frogs, live near ponds, lakes and streams. If you live close to water or in an area with plenty of dew and rain, you'll encounter frogs. Tree frogs spend most of their time in trees or tall vegetation, while other frogs may be seen on the ground. If your environment is dry, you're unlikely to see amphibians. If any, they'll be toads, who are willing to move farther from water. You may be able to entice amphibians by following some of the suggestions on this page.
Frogs are considered bio-indicators because their disappearance in an area indicates that something unhealthy is going on in the environment. When their populations decline, something has gone amiss. Their permeable skin absorbs toxins easily while in water and on land. When they start dying off, or develop mutations, scientists take note. Broadly speaking, where frogs are safe, humans are safe.
Frogs are carnivores. They eat insects, spiders, snails, worms, small fish and small land animals, such as mice. A frog hunts by sitting motionless, rarely even blinking. When prey walks or flies by, his mouth opens and his tongue flicks out and back, faster than we can see.
Most male frogs become sexually mature at around 10 to 12 months of age. Females are usually sexually mature at two to three years. Male frogs start calling for females in spring. Each species has a specific call. Females come to the callers, choosing a mate based on call and, possibly, appearance. Usually they couple in the water, but depending on the species it may occur in trees above water or on the ground. The male climbs on the female's back and wraps his forelegs around her "waist," neck or head in an embrace called amplexus. It doesn't involve copulation, but rather pseudocopulation: There's no contact of sexual organs between the two. Instead, the male releases semen onto eggs as they leave the female's body. Frog eggs are subject to extreme predation. So, they're usually laid in huge batches of thousands of eggs (sometimes called "frogspawn.) This increases the chance that a few will survive. When the eggs touch water, even if it's just a tiny puddle in a cupped leaf, they soak it up and swell into gelatinous masses. Toads, on the other hand, usually lay their eggs in a long strand.
Mass of frog eggs. The black
"dots" are developing
(Maurice van der Velden)
Some frog parents watch over their eggs. Some males carry eggs from a moist place, such as the water in a cupped leaf, to a wetter place, like a pond. They may do this by placing the eggs on their back, in a pouch on their belly or even in their mouth. Some males wait for the eggs to hatch and then transport tadpoles.
The time it takes for eggs to hatch varies from two to 40 days -- the warmer the temperature, the shorter the time. Frogs hatch as tadpoles. This stage may last for days, weeks or years. The North American Bullfrog, for example, hatches in a few days and spends about three years as a tadpole (also called pollywogs or froglets). Through the course of this stage, tadpoles make their change into frogs (metamorphosis). They eat voraciously, first on algae and later on tiny water animals, small fish and insects. Their appearance is quite different from their parents. At first, tadpoles have an oval-shaped body with gills, a long tail and, like their parents, large eyes. Body parts begin to develop: their tongue, teeth, certain bones, kidneys, glands and gonads. Their legs begin to grow and finally the tail is absorbed. The gills are absorbed, too, as the lungs develop. Most tadpoles feed on algae, although a few are carnivorous species and eat small fish and insects. Tadpoles move through water the same way fish do.
After metamorphosis, frogs leave the water for a more terrestrial life or continue their water lifestyle, depending on the species. Captive frogs are known to live 15 to 20 years, but it isn't known just how long they live in the wild because they're hard to track. Some are known to have lived at least five years, but most are probably eaten by predators within days of leaving the water.
Humans are the biggest threat to frogs as a result of habitat destruction, pollution and pesticide use. Also, many people eat them and millions are used in scientific research or classroom experiments. Some are captured and kept as pets. Other predators are snakes, lizards, large spiders, fish, birds and foxes and other mammals. Automobiles also kill many frogs in spring and fall as they cross roads while migrating between shallow, summer breeding ponds and deeper ponds for hibernating.