Most frog species live in tropical rainforests, but they’re native everywhere, except Antarctica and some islands. Even some arid areas have toad species living there; they bury themselves during dry spells and emerge 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. Sadly, some are already extinct, some species now exist only in protective captivity and dozens more are under threat of extinction.
In the U.S, the largest native frog is one you might guess: the American Bullfrog (Rana catesbeiana), which is 3-3/4 to 6 inches long (9 to 15 cm) and can weigh up to 1-1/8 pounds (500 g). The smallest is the Little Grass Frog (Limnaoedus ocularis) which, at a mere 3/4-inch (19 mm), could sit on a penny. The largest native toad in the U.S. is the Sonoran Desert Toad (Incilius alvarius), which is 7-1/2 inches (190 mm) long. The smallest toad is the Oak Toad (Bufo quercicus) which measures 3/4 to 1-1/4 inches (19 to 31.8 mm) long.
Male and female frogs and toads mostly look alike, except that females are often much larger than their counterparts. But a frog can be distinguished from a toad. Once you know how it’s easy to tell between the two: Frogs have moist, smooth skin; a slender body; visible eardrums on the sides of their head; no parotid glands; and tree frogs have toe pads. Toads have dry, warty-looking skin, a heavier-looking body; visible eardrums; visible parotid glands; and no toe pads.
|Skin smooth, moist||Skin dry, warty-looking|
|Slender body||Heavier body|
|Usually lives near water||May live far away from water|
|Visible eardrums||Visible eardrums|
|Tree frogs have toe pads||No toe pads|
Frogs have a flat, wide head, with big, bulging eyes at the top, a nose, two eardrums, glands and a mouth. They have a brain, spinal cord, and highly developed nervous system.
Eyes: Due to the location of their eyes, species that spend much of their time in water can sit motionless with only their eyes above the surface as they watch for unwary insects. (Toads spend most of their time on land.) Frogs have excellent vision in both daylight and dark, with a wide field of view that allows them to see to the front, sides and partially behind. They don’t have a neck, which leaves them unable to turn their head or move it up and down, so their broad field of vision is important for spotting predators. The upper and lower eyelids are underdeveloped and move very little. In order to completely close their eyes, frogs must draw their eyeballs deep into their sockets, which pulls the eyelids together. They also have a transparent membrane that can cover their eyes; which protects and cleans them. Their eyes have an amazing function beyond vision — they help in swallowing: When a frog catches prey, its eyes close, drop down into the roof of the mouth and help push the food down its throat!
Senses: Frogs have two nostrils (nares) located on their head above the mouth, and a good sense of smell. They also have a good sense of taste and will reject foods they don’t like. They hear through a large, circular membrane on each side of their head, called a tympanum, or eardrum. Another membrane located at the end of the inner ear is thought to help protect their ears from deafening noises — which they produce themselves. Some frogs have calls that are so robust they can be heard a mile away! In 2008, scientists discovered a species of frogs living near a noisy area in China that can tune their hearing to different frequencies, like a radio knob, effectively tuning out objectionable noises or even the calls of other frog species.
Have you noticed that frogs pulsate their throat? It’s fairly conspicuous and demonstrates their unusual method of breathing. Called buccal pumping, it’s a “two-stroke” process: They lower the floor of their mouth, which draws air in through their nostrils and pulls air from their lungs into their buccal (mouth) cavity. Then they raise the floor of their mouth, which causes the opposite action of pushing air out, as well as into the lungs. They always breathe with their mouth closed.
A frog’s body contains the heart, two lungs, two kidneys, stomach, liver, small intestine, large intestine, spleen, pancreas, gallbladder, urinary bladder, and ureter. There’s also a urinogenital duct which serves as a passageway for waste products, sperm, and eggs to exit the body through the cloaca (anus). Males have two testes and females have two ovaries, in addition to other organs related to reproduction. Frogs also have fat bodies which store fat for use during hibernation.
Frogs have two arms and two long legs that are pretty similar to those of humans: Their arms have a humerus, fused radius and ulna bones and four fingers. Their legs have a femur, fused tibia and fibula bones and five toes. The shoulder blades and collarbone are shaped similar to a human’s, too. As adults, they have no tail
Legs: Frogs have long hind legs and webbing that stretches between their toes. Frogs that spend a lot of time in water have longer legs, and more webbing between their toes than more terrestrial species do. Webbing helps them when paddling through water. There’s an unusual group of “gliding” frogs that stretch their toes wide, and the webbing serves as wings, allowing them to jump from high places to lower. They “fly” from tree to tree in this way. (Watch them fly)
An example of frogs with less webbing are tree frogs. They spend all their time on land and webbing covers only half or less the length of their toes. Their toes 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. Toads don’t have webbed feet.
Most frog species lack claws, but a group called spadefoot toads that 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 backward in a circle until the hole is deep enough for them to disappear into it.
Some species of frogs walk like a lizard (watch a walking toad). But most hop as a standard method of locomotion on land. Frogs are powerful jumpers with muscles and tendons that perform like springs. An American Bullfrog, cleverly named Rosie the Ribeter, set the record for the longest frog jump in the U.S. — 7.2 feet (2.2 m) — in a single bound at the Calaveras County (Calif.) Fair in 1986. Elsewhere, the African Tree Frog reportedly can jump 14 feet (4.3 m). How frogs jump so high. A frog’s arms are of little use in jumping, but they help to cushion the landing.
Some species become paler or darker in response to specific stimuli and others change their color altogether. The changes occur through 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. The process is complicated, but it’s triggered by a number of different variables: excitement, humidity, light, temperature or surroundings.
Here are some examples: The Pacific Tree Frog (Hyla cinerea) changes its 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 that’s creamy-white and lives in the white blossoms of a particular lily. When the flowers die and turn brown, it turns brown, too.
Toads (and some frogs, too) have warty-looking bumps all over their body, but they aren’t warts and they don’t cause humans to get warts by touching them, a common myth. tThe ‘warts” are a mix of two types of glands: mucous glands, which can sometimes be poisonous; and granular glands, which produce a much more toxic substance, called bufotoxin. Bufotoxin is secreted through two large, puffy glands, one behind each eye. A milky-looking poison, it gets released when a toad is squeezed or shaken (as dogs will often do). It can kill small predators and in some rare instances has killed small dogs.
In the tropics live 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 visually, but they can’t be touched. Their striking coloration serves as a warning to stay away. The frogs tend to be diurnal (active in daytime) because they don’t fear being seen.
The most poisonous animal on earth is probably the Golden Poison Frog inhabiting Central and South America: Just one milligram of its poison can kill up to 20 people. There are no frogs or toads in the U.S. that can be lethal to humans unless ingested. Even the poisonous Cane Toad (Rhinella marina) of the southern U.S. can be safely handled if done with care.
Communication: 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 species emit their attraction calls separately and others join a group in an immensely loud “chorus” of calls. With some species, making a call inflates and pulsates a conspicuous vocal sac located on their throat.
A lot of people are familiar with the deep croak of American Bullfrogs, but frog calls also include chirps, trills, twitters, peeps, clicks, twangs and other sounds that defy description. In order to attract the right mate, each species has its own special call. As for the frog that goes, “ribbit, ribbit, ribbit,” it’s the Pacific Tree Frog, Pseudacris regilla. Listen here.
Defenses: Frogs have few defenses, so most are active at night when they’re harder to see. Camouflage coloration allows them to sometimes hide in plain sight. Some change color to match their background. Another defense is playing dead when approached by a predator that prefers live food. Frogs also hop away from predators or dive into water. Some species puff up to appear bigger. Watch toad playing dead
All frogs move to water in their mating season. 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 the spring. Each species has a specific call. Females come to the callers, choosing a mate based on his call and, possibly, appearance. Usually, they mate 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 actual copulation, but rather a 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.
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.
Through the course of this stage, tadpoles make their change into frogs (called 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, some of the bones, kidneys, glands and gonads. Their legs start to grow, and finally, the tail is absorbed into their body. The gills are absorbed, too, as the lungs develop. 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. Watch development from egg to frog
All frogs live close to ponds, lakes and or streams. Tree frogs spend most of their time in nearby trees or tall vegetation, while other frogs may be seen on the ground or in water. Toads are willing to move farther from water
Frogs are considered to be 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, whether they’re in water or on land. When they start dying off or develop mutations, scientists take note. Broadly speaking, where frogs are safe, humans are safe.
Captive frogs are known to live 15 to 20 years, but just how long frogs live in the wild isn’t known, because they’re hard to track. Some are known to have lived at least five years, but predators probably eat most 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 more die during 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.