Frogs and toads belong to a group of amphibians called Anura, and there are thousands of species of them. They are sorted into families, including Ranidae (true frogs), Hylidae (treefrogs) and Bufonidae (toads). True frogs, such as bullfrogs and pickerel frogs, are the largest of these, and unlike treefrogs, their front toes are not as webbed. Technically, a toad is a kind of frog, and scientists do not draw a firm line between the two: there is no scientific distinction between the names, although most frogs and toads are usually called one or the other. True toads of the family Bufonidae are unique among frogs in having a Bidder's organ, a rudimentary ovary that develops at the front end of the larval testes of males. Many scientists consider its persistence in adult males to be paedomorphic, a young feature kept into adulthood.

Frogs have nine or fewer vertebrae in front of the sacrum, usually eight, and the three or four behind it are fused into a rod called the urostyle. The forearm bones are fused to each other, as are the shank bones, and two ankle bones are greatly elongated, adding a level to the leg used in jumping. Frogs have long, strong back legs that they use for leaping, and the longer a frog's back legs are, the farther it can usually hop, while toads have short legs made for short hops and crawling. Adult frogs do not have tails, with one exception, the genus Ascaphus. Fossil frogs are known from the Jurassic of Europe, North America and South America, extending through the Pleistocene, but the fossil record is somewhat poor, because the group radiated widely and several families have no fossil representatives at all.

Frogs breathe with lungs and also through their moist skin, and a frog's skin has to stay moist, which is why most frogs live in or near water. Frogs take in water by absorbing it through their skin instead of drinking it, while a toad's thicker skin loses moisture more slowly, so toads can live farther from water than most frogs. Amphibians that have no lungs absorb all the oxygen they need through their moist skin, and as of November 2026, the only lungless frog species known is the Bornean flat-headed frog, Barbourula kalimantanensis. In dry regions, frogs may bury themselves underground or cut back on breathing through the skin, which also reduces water loss.

Frogs eat almost any live prey they can find, such as insects, snails, spiders, worms or small fish. In a study of how frogs catch insects, a frog's tongue captured an insect in under 0.07 seconds, five times faster than a human eye blink. A frog cannot see while its tongue is striking, so it can only make a catch by taking careful aim in advance. A frog's saliva is viscoelastic, which means it has properties of both a fluid and a solid, and in a study of frog tongues, low-viscosity (runny) saliva helped the tongue hit and release prey, while high-viscosity (thick) saliva helped it hold on to prey. Scientists measured that a frog tongue's work of adhesion is 60 times greater than for rigid plates coated with the same saliva. Frogs cannot chew, so prey is swallowed whole, and as the frog swallows its prey, the eyes sink through openings in the skull and help force the food down the throat, which is why frogs seem to blink as they eat.

Male frogs and toads call by breathing in air with their lungs and then bouncing the air between their vocal sacs and their lungs, and the vocal sacs work like echo chambers to amplify the sound of the call. A frog's ears, called tympanum, look like flat circles behind the eyes. The size of the tympanum and the distance between them determine which sounds a frog hears best, so its ears are tuned to its own species' calls, which helps a female find males of her species in a noisy pond. In male bullfrogs, the tympanum, the round circle on the side of the head near the eye, is much larger than the eye. During breeding, female frogs and toads listen carefully and hop closer to males with a deeper or more powerful call, which signals a larger size and a greater ability to survive and reproduce.

Most frogs have external fertilization and use a mating posture, amplexus, to ensure eggs and sperm meet. The primitive mode is many aquatic eggs with feeding larvae, but others include terrestrial eggs with aquatic larvae, direct development with no tadpole stage, live birth, and non-feeding aquatic larvae. Some toads lay eggs on leaves above water, and a few have tadpoles in torrential streams with suckers on their bellies to attach to the substrate. Most toads lay eggs that hatch into free-swimming larvae, but direct terrestrial development and live birth are known in toads too. The gastric brooding frog of Australia swallows her fertilized eggs, and the tadpoles stay in her stomach for up to 8 weeks until hopping out of her mouth as little frogs. A strawberry poison dart frog lays eggs on land, not in a pond, and the mother carries the tadpoles on her back to water-filled bromeliads in the trees.

A few frogs give birth to live young. Members of the African genus Nectophrynoides keep eggs in the oviduct and some nourish the young as they grow, so the young are born as miniatures of the adult. A tadpole is a young frog that lives in water and breathes with gills, and the change from a tadpole into a frog is called metamorphosis. During metamorphosis the hind legs emerge first, then the front legs. The gills are replaced by lungs, and the digestive system typically shifts from plant-based to carnivorous. The tail is gradually reabsorbed as a source of energy, and the young frog ventures onto land with a tail stub. Metamorphosis usually takes several weeks, although species that breed in temporary ponds may develop more quickly, because if the pond dries up too soon the underdeveloped tadpoles will perish. By contrast, a bullfrog tadpole can take two winters, or almost three years, to turn into a frog.

Some frogs can change their color, like chameleons, according to light, temperature, humidity, or even mood. A frog that changes color does it with pigment grains inside its cells: when the grains are packed together in the center of each cell the frog is light colored, and when they spread out the frog's color darkens. Many dull-looking frogs have hidden flash colors, bright patches on the belly, groin or backs of the thighs, and a fire-bellied toad's plain brown back matches the ground, but when threatened it shows its red underside, which can startle predators or buy the toad time to escape. The Chilean four-eyed frog has a bright pair of spots on its rump that look like enormous eyes, hidden by its thighs when it sits, and when a predator approaches, the frog lowers its head and lifts its rump, creating the illusion of a much larger and more intimidating head.

Bright colors on tropical frogs warn predators that they are poisonous, but frogs are not above bluffing: some nonpoisonous species have adopted the colors of poison dart frogs to trick predators into giving them a wide berth they do not deserve. Certain frogs of Central and South America have especially strong poisons in their skin, and there are about 170 kinds of poison dart frogs. In the wild, they are too toxic to touch. Golden poison frogs do not act aggressively toward people; the danger lies in their very toxic skin. The golden poison frog's skin toxins are steroidal alkaloids, including batrachotoxin, and they act on voltage-gated sodium channels, keeping the channels open and irreversibly depolarizing nerve and muscle cells. Captive golden poison frogs tend to lose their toxicity without the wild prey that are the source of the toxin, and the Panamanian golden frog gets its toxins from the insects it eats in the wild.

A toad's skin has glands that make a poison, which helps protect the toad from predators, and the large glands behind the eyes are called parotoid glands. The poison is called bufotoxin, and it can kill small animals and cause allergic reactions in people. A toad's poison can irritate people, so it is best not to handle a toad, and nobody should lick or eat one. A toad may also puff up its body to look bigger and inedible when a predator is near. You may have heard that touching a toad gives you warts, but in fact, touching a toad does not give you warts.

Wood frogs spend the winter frozen and thaw out when the weather warms. When blood freezes in most animals, nothing delivers oxygen and nutrients to the organs, so extreme metabolic damage occurs. At the start of a wood frog's winter freeze, its liver makes large amounts of glucose that flush into every cell in its body. A frozen wood frog thaws from the inside outward. As of November 2026, wood frogs are the only frogs that live north of the Arctic Circle, and scientists do not yet understand what starts a wood frog's heart again after it has been frozen for the winter.

Bullfrogs have a much higher critical thermal maximum than most other frogs, so they can thrive in higher water temperatures. They also have a longer breeding season and a higher rate of survival before metamorphosis, which helps them succeed against other frogs. As of November 2026, bullfrogs that people have introduced may be driving native frogs to extinction in some areas. People also moved toads: people brought cane toads to Australia in 1935 to kill sugarcane beetles, and several attempts to control insect pests failed when Bufo marinus, the cane toad, was introduced into regions where toads are not native, including Australia and Papua New Guinea. Cane toads have voracious appetites and do not restrict themselves to eating unwanted insects.

As of November 2026, a 2004 global assessment found that nearly one-third of the world's amphibians are threatened, representing 1,856 species. AmphibiaWeb adds that, in the last three decades before its 2021 update, nearly 168 species are believed to have gone extinct and over 43% more have declining populations. As of November 2026, most scientists agree that there is a worldwide decline in frog numbers, and habitat loss and invasive species are among the biggest threats to frogs and toads. As of November 2026, a fungus disease called chytrid is causing severe declines in many frog populations.

The disease called chytridiomycosis is caused by the chytrid fungus Batrachochytrium dendrobatidis. As of November 2026, AmphibiaWeb says the fungus is associated with the global loss of hundreds of amphibian species, a spectacular loss of biodiversity that some say is the worst in recorded history. As of November 2026, some frogs and salamanders have an innate resistance to the deadly chytrid fungus. Amphibians react strongly to changes in their surroundings, so scientists use them as signs of how healthy a place is.

Words to know

• urostyle: fused rod
• paedomorphic: keeping young features
• amplexus: mating posture
• batrachotoxin: golden frog toxin
• tympanum: frog ear