Frogs are a group of semi-aquatic, tailless amphibians that constitute the order Anura. One of the five most diverse vertebrate orders, they represent around 88% of all extant amphibian species. The adults are characterized by protruding eyes, an anteriorly attached tongue, and moist, glandular skin used for respiration when in water. In contrast, their aquatic larvae, known as tadpoles, possess a tail and respire through external gills, which are later internalized as they grow.
These amphibians range from the tropics to the subarctic regions, but they are particularly diverse in tropical rainforests. While the adults are primarily carnivorous, feeding on invertebrates, their tadpoles are herbivorous and mainly consume algae. As of 2025, there are around 7,800 species of frogs worldwide.
Most frog species range between 2 and 4 in (5 and 10 cm) in body length, measured from the snout to the vent. However, one of the smallest frogs, the New Guinea Amau frog (Paedophryne amauensis), is only around 0.3 in (7.7 mm). In contrast, the largest living frog, the goliath frog (Conraua goliath), reaches up to 14 in (35 cm) in snout-vent length.
They have a bilaterally symmetrical body divided into head and trunk regions. The head is broad and flat, with a wide mouth for catching prey, while the trunk is short and muscular. Their hindlimbs are long, powerful, and webbed (though the degree of webbing varies with how much time the species spends in water), while the forelimbs are relatively shorter.
Most frogs are excellent jumpers, and their skeleton shows specialized adaptations that cater to their powerful leaps. For example, the tibia, fibula, and tarsals in the hindlimb are fused into one strong bone, and the radius and ulna in the forelimb are also fused similarly. Additionally, the metatarsals are elongated, adding to the leg length and allowing the animal a stronger push-off during a jump.
The ilium is extended and forms a flexible joint with the sacrum, adding to the power of the jump. Unlike tailed tadpoles, in adult frogs, the tail vertebrae are fused into a rod-like urostyle, which remains retracted inside the pelvis and helps stabilize the skeleton during a jump.
The muscular system of frogs is equally well-developed, with enlarged leg muscles that account for over 17% of their total body mass.
Most frogs have thin, smooth, and moist skin. However, some groups (commonly referred to as toads) possess thick skin covered in wart-like bumps, which are underlying mucous or poison glands. Some groups, such as poison dart frogs (family Dendrobatidae), are brilliantly colored in shades of bright red, orange, or yellow, often with contrasting black markings. Such coloration helps them appear poisonous and keep predators away (aposematism).
These amphibians typically shed their exoskeleton in a few weeks, splitting the skin down the middle of the back and across the belly. In fact, many species, like the Northern leopard frog (Rana pipiens), consume the shed skin either to recycle nutrients or remove any traces that might attract predators.
Like lizards, frogs possess a three-chambered heart consisting of two atria and a single ventricle. Oxygenated blood is pumped from the heart to the rest of the body through the aorta, much like in humans, while deoxygenated blood from the body is carried to the sac-like lungs through the pulmonary artery. The lungs receive air through the nostrils, particularly when the frog is on land.
When resting on land or underwater, they switch to cutaneous respiration. Since their skin is moist, oxygen diffuses directly through it. Additionally, the moist inner lining of the mouth aids in gaseous exchange.
Frogs bear maxillary teeth along their upper jaw. However, these teeth are weak and incapable of gripping prey efficiently. To compensate for this inability, their tongue is attached at the front of the mandible or lower jaw. Although the tongue typically remains coiled inside the mouth, it is shot out rapidly when a target prey is within range. However, some species, such as the African clawed frog (Xenopus laevis), lack a tongue altogether.
The ingested food moves through the esophagus into the stomach, where it mixes with digestive fluids released from gastric glands. Once it is churned well, the food moves to the small intestine for further digestion and absorption. The residual food moves to the large intestine, where excess water is removed to produce nitrogenous waste.
Like mammals, frogs have a pair of kidneys that help remove waste from their bodies. Although tadpoles and aquatic frogs typically excrete their waste in the form of ammonia, terrestrial frogs release urea. Some species, such as the water-holding frog (Ranoidea platycephala), conserve water by eliminating up to 80% of their nitrogenous waste as uric acid.[1]
The nervous system of frogs comprises a brain, spinal cord, and peripheral nerves (spinal nerves and 10 pairs of cranial nerves). Almost like the human brain, a frog’s brain is organized into two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a relatively small cerebellum, and a medulla oblongata.
Their protruding eyes are located on either side of the head, providing binocular vision over a field of 100° to the front and a total visual field of almost 360°. However, in members of the family Pipidae, the eyes are located on the top of the head.
The color of their irises and the shape of their pupils vary with species. For example, the common toad (Bufo bufo) has golden irises and horizontal slit-like pupils, whereas the red-eyed tree frog (Agalychnis callidryas) has vertical slit pupils. Similarly, the fire-bellied toad (Bombina) has triangular pupils, while the tomato frog (Dyscophus) has circular ones.
Frogs lack external ears or pinnae; instead, their tympanic membranes (eardrums) are directly exposed (or covered by a thin layer of skin in some species) and appear as a circular patch just behind the eye. When a sound wave strikes the membrane, the vibration gets transmitted to the middle and inner ear. While the middle ear contains semicircular canals, which help control balance, the inner ear has auditory hair cells.
Male frogs have a pair of testes attached to their kidneys. The semen released from the testes enters the kidneys and is drained through the urinogenital ducts. They lack a penis and hence eject the sperm directly from the cloaca.
Females possess a pair of ovaries adjacent to the kidneys. The eggs produced by the ovaries travel through a pair of oviducts and are eventually released through the cloacal opening.
The name of the order, Anura, stems from the Ancient Greek terms an- (meaning without) and ourá (meaning animal tail), referring to the tailless character of frogs. The group of frogs having warty skin texture is collectively referred to as ‘toads,’ though such a naming has no taxonomic or evolutionary basis.
According to the American Museum of Natural History, as of 2025, there are around 7,800 species of frogs. These species are classified into the following 54 families.[2]
Some of the families that represent toads are Bufonidae, Bombinatoridae, Calyptocephalellidae, Myobatrachidae, Pelobatidae, Rhinophrynidae, and Scaphiopodidae, among others.
These amphibians are found on all continents except Antarctica. They are particularly diverse in tropical regions such as the Amazon Basin in South America, Central Africa, and Southeast Asia, while the wood frog (Rana sylvatica) even extends into the Arctic Circle.
They prefer living close to freshwater environments, since their eggs and tadpoles develop in water. Their preferred habitats include ponds, lakes, rivers, marshes, and wetlands. While some species may prefer an underground lifestyle, others are arboreal, living on trees.
A few species, such as the desert rain frog (Breviceps macrops), are found in deserts. This species occupies a narrow strip of sandy coastline in Namibia and South Africa, relying on coastal fog for moisture.[3]
Most adult frogs are carnivores, feeding on a range of invertebrates, including insects, crabs, spiders, mites, worms, snails, and slugs. Some species, such as Ceratophrys varius, consume other frogs and small rodents.
In a population of the common toad frog (Leptodactylus mystaceus) from a deforested farm in Novo Progresso, Pará (Brazil), plant material was the most important dietary component, surpassing all insect prey. Similarly, the tree frog Xenohyla truncata is partly herbivorous, with its diet consisting of fruit, floral structures, and nectar. The green pond frog (Euphlyctis hexadactylus) switches from insectivory to folivory seasonally, with plants constituting around 79.5% of the diet by volume.[4]
Most tadpoles are herbivorous, feeding typically on algae, as well as diatoms filtered from the water.
Although most frogs are nocturnal, a few, such as poison dart frogs, are diurnal.
Different species adapt to various modes of locomotion, depending on the habitats they occupy.
Frogs are one of the best jumpers among all vertebrates. For instance, the striped rocket frog (Litoria nasuta) is capable of leaping over 6.5 ft (2 m), a distance over 50 times its body length. Similarly, the tiny northern cricket frog (Acris crepitans) moves across the water surface through a series of brief yet rapid jumps.
When a frog prepares to leap, it crouches, its leg muscles passively stretched for storing elastic energy in the tendons. The muscles contract first, transferring energy into the tendon, and right before takeoff, the muscles stretch again while the tendon releases its stored energy. This muscular mechanism is similar to that of a catapult.[5]
Some families, like Bufonidae, Rhinophrynidae, and Microhylidae, lack the long back legs that most frogs use for powerful jumps. Thus, they move by walking rather than leaping. Instead of springing long distances, they increase the pace of their limb movements to move faster.
Some frogs, such as the Kassina maculata (red-legged running frog), move in a unique running gait. The frog places one hind leg in front of the other in an alternating sequence, almost like a trotting horse.[6]
The Great Plains narrow-mouthed toad (Gastrophryne olivacea) moves in a combination of tiny runs and hops, proceeding just an inch or two at a time.
Frogs that venture into water, such as members of the family Pipidae, typically swim using their long, muscular back legs. Their large, webbed feet increase the surface area, making each kick more effective. Tadpoles, by contrast, swim using their tail fins, propelling themselves forward with side-to-side movements of the tail.
Several frogs have rounded bodies, short limbs, and small heads, features well-suited for burrowing and living underground. The purple frog (Nasikabatrachus sahyadrensis) of southern India, for instance, spends nearly its entire life underground, feeding mainly on termites. Similarly, the Plains spadefoot toad (Spea bombifrons) uses a flap of keratinised bone on one of its hind-foot metatarsals to dig the soil. While this species typically makes shallow burrows during summer, in winter it can dig tunnels much deeper, reaching depths of up to 14 ft 9 in (4.5 m).[7]
Many groups of frogs, such as tree frogs (family Hylidae), glass frogs (family Centrolenidae), bush frogs (family Hyperoliidae), some narrow-mouthed frogs (family Microhylidae), and the shrub frogs (family Rhacophoridae), live high in the canopy, moving around on the branches, twigs, and leaves, often not descending to the ground.
Their toe pads consist of tightly packed, flat-topped hexagonal epidermal cells separated by fine grooves. Mucus secreted into these grooves keeps the pads moist, allowing the frogs to adhere to both wet and dry surfaces. In fact, they are quite acrobatic, sometimes hanging from a twig by a single toe while snatching insects.[8]
A few frogs living in the tropical rainforests are capable of gliding from tree to tree. For instance, the Wallace’s flying frog (Rhacophorus nigropalmatus) has large, webbed feet with the fingertips expanded into flat adhesive discs. When the frog is ready to glide, it splays its digits and stretches its limbs out, gaining the power to glide.[9]
Most male frogs communicate by producing a croaking sound by passing air through the larynx. Each species has a distinct croak, aimed at attracting mates. Some species, such as the American bullfrog (Lithobates catesbeianus), produce croaks that could be heard up to a mile (1.6 km) away. In contrast, a few species, like the coastal tailed frog (Ascaphus truei), do not vocalize at all.
These calls facilitate sexual selection, as females prefer males that produce sounds with greater intensity and lower frequency. However, they may also vocalize to ward off predators or alert their conspecifics of an imminent danger.
The lifespan varies widely across the different species of frogs. For instance, the Cuban tree frog (Osteopilus septentrionalis) lives between 5 and 10 years in the wild, whereas the southern cricket frog (Acis gryllus) typically survives less than a year. The African clawed frog survives around 15 years or more in the wild.
The Animal Ageing and Longevity Database reports that the common toad (Bufo bufo) has survived up to 40 years in captivity.
Most species undergo prolonged breeding, in which they gather seasonally at ponds, lakes, or streams, often returning to the same place where they developed as tadpoles. Their males typically arrive first at the breeding site, while females arrive later and leave soon after spawning. The males generally advertise themselves through loud croaks, while also warding off competing males.
Some species, such as the common frog (Rana temporaria), undergo explosive breeding, triggered by sudden events like heavy rainfall in dry areas.[10] Both males and females quickly gather in groups (known as an army) in temporary pools before they dry out. The frogs create a loud chorus, with males prioritizing rapid mating over prolonged courtship.
Mating typically takes place in water, where the male grasps the female from behind (amplexus). The female releases her eggs in the water, which are then fertilized externally by the sperm released by the male.
Although most species are oviparous, members of the genus Nimbaphrynoides are viviparous, giving birth directly to live young. Others, such as Limnonectes larvaepartus and Eleutherodactylus jasperi, are ovoviviparous.
Around half of all frog species lay their eggs in water as clumps, surface films, strings, or individually. A few, such as the yellow-striped pygmy eleuth (Eleutherodactylus limbatus), bury their eggs in soil, while others, like the smoky jungle frog (Leptodactylus pentadactylus), make a nest of foam in a hollow. The red-eyed treefrog (Agalychnis callidryas) lays its eggs on a plant above a pool, such that the hatched larvae slip into the water below.
The eggs hatch into tadpoles, which lack limbs or eyelids. Most tadpoles are fully aquatic, but some, like Nannophrys ceylonensis, live among wet rocks (semiterrestrial). The tadpoles have a cartilaginous skeleton, three pairs of external gills (covered by a gill pouch), and a muscular tail for swimming. They are also equipped with rows of small, tooth-like keratinized structures called keradonts, instead of teeth.
Towards the end of the larval stage, the level of the hormone thyroxine rises, leading to metamorphosis, which typically lasts about 24 hours. The gills are internalized, and lungs develop as an accessory breathing organ. Eventually, the jaw enlarges into a strong adult mandible, and the long, coiled herbivorous gut of the tadpole is replaced by a short carnivorous gut. At the end of metamorphosis, the hind and forelimbs develop, organs are formed, the lateral line system is lost, and the tail is absorbed, giving rise to the adult frog.
These amphibians are preyed upon by a range of predators. For example, birds such as herons, storks, egrets, kingfishers, hawks, and owls consume both adult frogs and tadpoles. Similarly, mammals, including raccoons, skunks, otters, minks, foxes, wild cats, and even some rodents, eat frogs.
Among reptiles, snakes are the most common predators of frogs. However, lizards, crocodiles, and turtles also target them, especially around water bodies. Some large fish, such as bass, pike, and catfish, consume tadpoles. A few frogs, such as the African clawed frog, cannibalize their own tadpoles.[11]
Among invertebrates, dragonfly larvae, beetles, spiders, and water bugs feed on frog eggs and tadpoles.
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