Frog

A frog is any member of a large group of short-bodied, tailless amphibian animals in the order Anura, a name that comes from Ancient Greek and means 'without tail'. Most frogs spend part of their lives in water. Frog species with rough, bumpy skin, caused by wart-like parotoid glands, tend to be called toads. However, the difference between frogs and toads is informal and only about appearance. It does not reflect taxonomy or evolutionary history.

Frogs live in many parts of the world, from the tropics to subarctic regions. Most kinds live in tropical rainforest and nearby wetlands, where the greatest species diversity is found. Frogs make up about 88% of living amphibian species. The oldest fossil "proto-frog," Triadobatrachus, comes from the Early Triassic of Madagascar, about 250 million years ago. Scientists think frogs split from other amphibians (divergence) even further back, to the Permian, about 265 million years ago.

Adult frogs have a stout body, protruding eyes, and a tongue attached at the front of the mouth. Their legs are folded underneath, and they have no tail. The "tail" of tailed frogs is actually an extension of the male cloaca. Frogs have glandular skin, and their secretions range from unpleasant-tasting to toxic. Their skin can be camouflaged, with brown, grey, and green colours that help them hide. Some frogs show bright red or yellow and black patterns that warn predators they are poisonous (aposematism). Adult frogs live in fresh water and on dry land. Some species are adapted for living underground or in trees. Because their skin lets water pass through, they can dry out, so they either live in moist niches or have special adaptations for drier habitats. Frogs make many sounds, especially during their breeding season, and show many complex behaviors to attract mates, defend themselves, and survive.

Frogs usually spawn their eggs in bodies of water. The eggs hatch into young called tadpoles, which have tails and gills inside their bodies. A few species lay eggs on land or skip the tadpole stage altogether. Tadpoles have special mouthparts for scraping food. The life cycle is completed when they metamorphose into adults that can move on land (terrestrial locomotion). Adults breathe using both lungs, aided by buccal pumping, and gas exchange across the skin. Their tail shrinks into their body as they grow. Most adult frogs are carnivorous and eat small invertebrates, especially insects, though some eat plants too. Frogs usually catch food by flicking out their sticky tongue and swallow it whole, often using their eyeballs and extraocular muscles to push food down the throat. Their digestive system is very efficient at turning food into body mass. Both tadpoles and adult frogs are an important food source for other predators and a vital part of the food web in many of the world's ecosystems.

Frogs, especially their muscular hindlimbs, are eaten by humans in many cuisines. They also have many cultural roles in stories, symbols, and religion. Frogs are environmental bellwethers, meaning that changes in frog populations can warn scientists about problems in the environment. Global frog populations and diversity have declined significantly since the 1950s. More than one third of species are considered threatened with extinction, and over 120 are believed to have disappeared since the 1980s. A fungal disease, chytridiomycosis, has spread around the world and harmed many frogs. Conservation biologists are working to solve these problems.

Etymology and taxonomy

The names frog and toad are not based on science. Scientists call every member of the order Anura a frog. Only members of the family Bufonidae are called "true toads." People often use frog for water-living animals with smooth, moist skin. They use toad for land-living animals with dry, bumpy skin. There are many exceptions. The European fire-bellied toad has slightly bumpy skin and likes water. The Panamanian golden frog is a toad but has smooth skin. The name of the order, Anura, comes from Ancient Greek and means "without tail." This comes from the alpha privative prefix and an Ancient Greek word for "animal tail." The origin of the word frog is uncertain. It first appears in Old English as frogga, and it is related to the older Old English word frosc. Similar words exist in other Germanic languages, such as Norwegian frosk, Icelandic froskur, and Dutch (kik)vors. These words likely come from a Common Germanic ancestor word. The Oxford English Dictionary says its origin is uncertain, though it may come from a Proto-Indo-European word meaning "jump." The word toad is unique to English, and its origin is also uncertain. It is the basis for tadpole, first written as Middle English taddepol. The change from frosc to frogga does not follow a regular sound-change.

About 87% of amphibian species are classified in the order Anura. These include over 7,700 species in 59 families. The families with the most species are Hylidae, Strabomantidae, Microhylidae, and Bufonidae, which are the richest in species.

The Anura include all modern frogs and any fossil species that fit the definition. Adult anurans have a short body with no tail. Their bones and body parts include a radius and ulna, a tibia and fibula, long ankle bones, a hyoid plate, and a lower jaw without teeth (except for Gastrotheca guentheri). Their tadpoles have a single spiracle and mouthparts with keratinous beaks and denticles. Frogs and toads are grouped into three suborders: Archaeobatrachia, Mesobatrachia, and Neobatrachia. Neobatrachia is the largest group and includes most common frogs around the world. It is divided into the superfamilies Hyloidea and Ranoidea, based on features such as morphological traits, the pectoral girdle, and tadpole shape. Although scientists widely accept this grouping, they still debate how the families are related. Some anurans hybridise easily. The edible frog is a mix of the pool frog and the marsh frog. The fire-bellied toads Bombina bombina and B. variegata also form hybrids. These hybrids are less fertile than their parents, which creates a hybrid zone where hybrids are common.

Evolution

Scientists are still working out how the three main groups of amphibians are related. A molecular phylogeny study from 2005, based on rDNA, suggests that salamanders and caecilians are more closely related to each other than either is to frogs. The groups may have split apart in the Paleozoic or early Mesozoic. This was before the supercontinent Pangaea broke up, and soon after they separated from the lobe-finned fishes. A 2011 study concluded that lissamphibians first appeared about 330 million years ago. Another study placed their origin in the Permian, less than 300 million years ago.

In 2008, scientists found Gerobatrachus hottoni in Texas. This fossil is about 290 million years old. It has many features like those of frogs and salamanders, so it was called a missing link. Frogs and their close fossil relatives make up a group called Salientia. The oldest known frog-like amphibians, such as Triadobatrachus massinoti from Madagascar, lived about 250 million years ago.

The earliest known "true frogs" lived in the Early Jurassic period. One of them, Prosalirus bitis, was found in the Kayenta Formation of Arizona. It was well adapted for jumping. The oldest tadpoles known as of 2024 belonged to Notobatrachus degiustoi, which lived in the middle Jurassic. It had features that helped it filter-feeding. Modern frogs likely had fully evolved by the Jurassic period.

Genetic studies suggest that the families Hyloidea and Microhylidae, and the group Natatanura, diversified around 66 million years ago. Natatanura includes about 88% of living frogs. This happened soon after the Cretaceous–Paleogene extinction event, which is linked to the Chicxulub impactor.

Frog fossils have been found on every continent. In 2020, 40 million year old helmeted frog fossils were discovered on Seymour Island in the Antarctic Peninsula. These show that this region was once home to frogs related to those living in South American Nothofagus forest. A cladogram showing how the families of frogs are related is presented as a tree in the table above.

Morphology and physiology

Adult frogs have no tail, but tadpoles do. Most frogs have long back legs, webbed toes, no claws, large eyes, and smooth or bumpy skin. Frogs vary greatly in size. The tiny Paedophryne amauensis of Papua New Guinea is about 7.7 mm long. The goliath frog of central Africa can grow to about 35 cm. Many frogs are strong jumpers, and their bones and muscles are shaped for leaping. The shape of a frog's feet and legs depends on where it lives. Frogs that live in water often have webbed toes. Tree frogs have pads on their toes that help them grip smooth surfaces. Frog skin protects the body, helps it take in water, and helps control body temperature. Glands in the skin make a slippery substance that keeps the frog moist and makes it harder to catch. Some of these substances are toxic. Many frogs can camouflage themselves by changing colour or blending in with their surroundings. Frogs can also take in oxygen through their skin. They have hearts with three chambers.

Frogs catch food with a sticky tongue that can shoot out very quickly. Food passes through a stomach and intestines. Their kidneys remove waste from the blood. Frogs have a well-developed brain. Their cerebellum helps control movement and balance. Most frogs have eyes on the sides of their heads, so they can see in front and to the sides. They can see in colour. Frogs hear through eardrums that are visible just behind the eyes. They do not have external ears. Each species has its own call. In most frogs, the males call mainly to attract mates. Some frogs spend cold months in a dormant state called hibernation. Some can even survive being frozen. The striped burrowing frog of Australia can aestivate. This means it stays dormant for nine or ten months of the year during the hot, dry season.

Locomotion

Frogs move in many ways. They can jump, run, walk, swim, burrow, climb and glide. For their size, frogs are considered the best jumpers of all backboned animals. The striped rocket frog, Litoria nasuta, can leap over two metres (6+1⁄2 feet). That is more than fifty times its body length of 55 mm (2+1⁄4 in). Jumping ability differs a lot between kinds of frogs. The Indian skipper frog (Euphlyctis cyanophlyctis) can leap out of the water from a floating position. The tiny northern cricket frog (Acris crepitans) moves across a pond with short, quick jumps. Strong jumpers such as the Cuban tree frog and the northern leopard frog use a stretchy tendon that works like a catapult. A similar method has been found in locusts and grasshoppers.

Frogs in the families Bufonidae, Rhinophrynidae, and Microhylidae have short back legs. They tend to walk rather than jump. The Great Plains narrow-mouthed toad (Gastrophryne olivacea) moves with a mix of running and short hops. In one experiment, Fowler's toad (Anaxyrus fowleri) was placed on a treadmill. Hopping was found to be a poor choice for long movement but useful in short bursts. The red-legged running frog (Kassina maculata) runs by moving its two back legs one after the other. At night, it climbs trees and shrubs to catch insects.

Frogs that live in or visit water have strong back legs and webbed toes. Members of the family Pipidae live entirely in water. They have a lateral line system that helps them sense movement in the water.

Some frogs are adapted for digging. The purple frog (Nasikabatrachus sahyadrensis) from southern India eats termites. It spends almost its whole life underground and comes out briefly during the monsoon to breed. Because of this fossorial way of life, it was first described in 2003, although local people already knew of it. The plains spadefoot toad digs backwards using a bony flap on one of its metatarsals. In winter, it rests in a deep underground chamber, where it takes in water by osmosis. The western spotted frog of Australia digs a burrow and lays its eggs in a foam nest. The green burrowing frog of Madagascar buries itself in leaf litter.

Tree frogs live high in the canopy, where they move around on branches and leaves. The "true" tree frogs belong to the family Hylidae. However, members of other families have also taken up tree living. These include the glass frogs (Centrolenidae), the bush frogs (Hyperoliidae), some narrow-mouthed frogs (Microhylidae), and the shrub frogs (Rhacophoridae). This is a case of convergent evolution. Most tree frogs have sticky toe pads. The pads are made of flat cells, with glands that release mucus. The mucus helps the pads grip wet or dry surfaces, including glass. Some frogs in the subfamily Phyllomedusinae have opposable toes. The reticulated leaf frog (Phyllomedusa ayeaye) has one opposed digit on each front foot and two on each back foot. This helps it grip bush stems.

Some rainforest frogs have also taken to the air. Wallace's flying frog (Rhacophorus nigropalmatus) from Malaysia and Borneo has webbed toes and flaps of skin on its limbs and tail. It can glide between trees over distances of up to 15 m (50 ft), but it cannot fly with power.

Life history

Frogs have two main ways of breeding. Most species gather at a pond, lake or stream at certain times of year. Many return to the same water where they grew up as tadpoles. Males usually arrive first and call to claim their territory. Females often choose a mate partly by how deep his voice is. Some frogs are called explosive breeders. They arrive quickly after rain, mate fast, and lay their eggs so the tadpoles can grow before temporary pools dry up. Frog eggs may be laid in clumps, films, strings, or singly. They can be in water, in plants, or on the ground. Tadpoles breathe with gills, swim with their tails, and mostly eat plants. When a tadpole becomes an adult, its body changes in about 24 hours. This change is triggered by the hormone thyroxine. Adult frogs mostly eat small animals such as insects. Many frogs are eaten by other animals in turn.

Some frogs care for their young in many different ways. An estimated 20% of amphibian species may care for their offspring in some form. The strawberry poison-dart frog carries her tadpoles on her back to small water pools. She feeds them with eggs that have not been fertilised. The common midwife toad carries eggs attached to his hind legs until they hatch. The female Darwin's frog from Chile has her tadpoles looked after by the male. He keeps them and carries them inside his vocal sac. Some frogs live for many years. Captive cane toads have been known to live for 24 years.

Defence

At first, frogs can seem quite defenceless. They are small, move slowly, have thin skin, and have no spines, claws or teeth. Many use camouflage. Their spotted or streaked skin, in plain colours, helps them blend into their surroundings. Some make big leaps, often into water, to escape attackers. Many frogs also have chemical defences. Their skin contains mild bufotoxins that make them unpleasant to eat. Most toads and some frogs have large parotoid glands behind the eyes. These release mucus and toxins that make the frog slippery and unpleasant to eat, or poisonous. Poisonous frogs often use bright colours to warn predators. This strategy is called aposematism. The poison dart frogs are typically red, orange, or yellow with black markings. Allobates zaparo is not poisonous, but it looks like two toxic species that it lives alongside. This may fool predators. The European fire-bellied toad (Bombina bombina) shows its bright belly when attacked.

Some poison dart frogs are very toxic. Native peoples of South America have used their poison on weapons for hunting, although few species are strong enough. Two harmless frogs, Eleutherodactylus gaigei and Lithodytes lineatus, mimic the dart frogs' colours for protection. Some frogs get poison from the ants and other small creatures they eat. The Australian corroboree frogs can make alkaloids themselves. These chemicals may be irritants, hallucinogens, convulsants, nerve poisons or vasoconstrictors. Some frogs also use bluff. The European common toad (Bufo bufo) puffs up its body and raises its back end when attacked. The bullfrog (Rana catesbeiana) crouches with its eyes closed. Some frogs "scream" to startle predators. The grey tree frog (Hyla versicolor) makes a sudden loud sound that sometimes drives off the shrew Blarina brevicauda. Although many predators avoid toads, the common garter snake regularly eats them. Young American toads (Bufo americanus) often crouch and stay still to avoid being noticed.

Distribution

Frogs live on every continent except Antarctica. They are not found on some islands, especially those far from large areas of land. Many kinds of frogs are limited to small areas. This can be because of the climate or because of hard-to-cross places, such as seas, mountain ridges, deserts, cleared forests, roads, or other human-made barriers. There are usually more kinds of frogs in warm tropical areas than in cooler places like Europe. Some frogs live in deserts. The Australian genus Cyclorana digs underground and forms a cocoon that water cannot pass through. There it can aestivate during dry periods. When it rains, these frogs come out, find a temporary pool, and breed. Their eggs and tadpoles grow much faster than those of most other frogs. This means they can finish breeding before the pool dries up.

Some frog species are adapted to cold places. The wood frog (Rana sylvatica) lives as far north as the Arctic Circle. It buries itself in the ground during winter. Much of its body freezes during this time. However, it keeps a high level of glucose in its important organs, which helps protect them.

Conservation

See also: Decline in amphibian populations

Frog populations have dropped sharply since the 1950s. Many frog species are at risk of dying out, and more than 120 species are believed to have disappeared since the 1980s. These include the gastric-brooding frogs of Australia and the golden toad of Costa Rica. The golden toad's population crashed in 1987 in the Monteverde Cloud Forest Reserve. Losing homes, pollution, climate change, increased UVB radiation, and non-native predators and competitors all add to the decline. Diseases such as chytridiomycosis and ranavirus are also harming frog populations. Many environmental scientists believe amphibians are good indicators of the health of the wider ecosystem.

Frog mutations and genetic defects have increased since the 1990s, often including missing or extra legs. Possible causes include ultraviolet radiation, chemical pollution, and parasites such as the trematode Ribeiroia ondatrae. This parasite's life cycle includes the ramshorn snail. In a few cases, breeding programs in zoos and other protected places have worked well. The World Association of Zoos and Aquariums named 2008 the "Year of the Frog." The cane toad was brought to Australia in the 1930s as a biological pest control helper. In 1935, 3000 toads were released in the sugar cane fields of Queensland, Australia, to eat cane beetles. The toads later upset the local balance of nature, harming native frogs and other animals. Today they are seen as pests and invasive species.

Human uses

See also: Animal testing on frogs and Frogs in captivity

People eat frog legs in many parts of the world. Indonesia sends more frog meat to other countries than any other place, mostly to France, Belgium and Luxembourg. At first, the meat came from wild frogs. Too many were caught, so people began frog farming, which created a worldwide trade. The American bullfrog (Rana catesbeiana) is mostly raised on farms in China. The mountain chicken frog got its name because it tastes like chicken. It is now endangered, partly because people eat it. Mark Twain wrote about raccoon, opossum, partridges, prairie chicken, and frogs as foods people ate in America. In November 1970, NASA sent two bullfrogs into space for six days on the Orbiting Frog Otolith mission. They were studied to learn about weightlessness. Frogs are used for dissections in schools and universities, but this is becoming less common because of animal welfare concerns. "Digital frogs" are now available as a alternative. Frogs have been experimental animals throughout the history of science. Luigi Galvani found a link between electricity and the nervous system by studying frogs. He also made one of the first tools for measuring electric current using a frog leg. In 1852, H. F. Stannius used a frog's heart in a Stannius ligature. The African clawed frog was widely used in laboratories for pregnancy tests. A discovery by Lancelot Hogben showed that a pregnant woman's urine could make a female frog lay eggs. This happens because of a hormone called human chorionic gonadotropin. In 1952, Robert Briggs and Thomas J. King cloned a frog using somatic cell nuclear transfer. The same technique was later used to create Dolly the sheep. Frogs of the genus Xenopus are a model organism in developmental biology and embryology. Their embryos are large and easy to work with. The smaller Xenopus tropicalis is increasingly used instead of Xenopus laevis, because it becomes ready to reproduce much sooner. The genomes of several frog species have been recorded in the NCBI Genome database.

Frogs make many different toxins, so scientists sometimes call them a "natural pharmacy". The epibatidine is a painkiller that is much stronger than morphine. It is made by some poison dart frogs. Chemicals from frog skins may also help fight HIV. Some people long ago, the Mesoamericans, may have used a toxin from the cane toad as a hallucinogen. The Colorado River toad (Bufo alvarius) is more likely to have been used. Its bufotenin, a psychoactive compound, has been used as a recreational drug. Stories about people licking toad skin to get high may be an urban myth. Native Colombians have long used the skin secretions of the golden poison frog on the tips of blowgun darts. One frog has enough toxin to kill many mice. The Kokoe poison dart frog and the black-legged dart frog are used the same way, but they are less toxic. Frogs also appear in fairy tales and popular culture. In traditional Chinese myths, the world rests on a giant frog that tries to swallow the moon, which causes a lunar eclipse. The ancient Egyptians showed the god Heqet, protector of newborns, with a frog's head. The Mayans linked frogs to water, crops, and the god Chaac. In the Bible, Moses brings a plague of frogs on the Egyptians. Medieval Europeans linked frogs and toads with evil and witchcraft. The Brothers Grimm story The Frog Prince features a frog who becomes a handsome prince. In modern culture, frogs often play funny or unlucky roles. Examples include Mr. Toad in The Wind in the Willows, Michigan J. Frog of Warner Bros. Cartoons, Kermit the Frog of The Muppets, and the game Frogger.