Frog

• 1. Introduction

  ›

  A frog is a short-bodied, tailless amphibian belonging to the order Anura, and frogs of one kind or another now make up close to 88% of all amphibian species alive today. Yet since the 1950s, global frog populations have fallen sharply, and more than a third of species are now considered at risk of extinction. Over 120 species are believed to have vanished since the 1980s alone, many of them killed off by a spreading fungal disease called chytridiomycosis. Frogs live nearly everywhere, from the tropics to the edge of the subarctic, with the greatest diversity concentrated in tropical rainforest and wetland. Despite how familiar the animal seems, a frog's body conceals a highly specialized set of adaptations, built up over a fossil record that stretches back more than 250 million years. Long before frogs became a symbol of ecological warning, they became one of evolution's most efficient jumping machines.

• 2. A Name Without Consensus

  ›

  The European fire-bellied toad has a slightly warty skin and prefers a watery habitat, while the Panamanian golden frog belongs to the toad family Bufonidae despite having smooth skin. Those two animals illustrate a rule that keeps breaking: the common names frog and toad carry no real taxonomic weight. Every member of the order Anura is technically a frog, and only the family Bufonidae counts as "true toads." In everyday use, frog usually refers to species that are aquatic or semi-aquatic with smooth, moist skin, while toad refers to terrestrial species with dry, warty skin, but exceptions like these two species are common.

  The word frog first appears in Old English as frogga, though the more usual Old English word was frosc, a term that survived in English dialects as frosh and frosk into the nineteenth century. Related words appear across the Germanic languages, including the German Frosch, the Norwegian frosk and the Icelandic froskur, allowing linguists to reconstruct a shared ancestor word, froskaz. How the older frosc gave rise to frogga is not fully understood, since the shift does not follow a regular sound change. One theory holds that Old English speakers had a habit of coining animal nicknames ending in -g, a pattern also behind dog, hog, pig and stag. The word toad, first recorded as the Old English tadige, is unique to English and just as mysterious in origin. It is also the root of tadpole, first recorded in Middle English as taddepol, apparently meaning "toad-head."

  Anura, the order's scientific name, comes from the Ancient Greek for "without tail," settling what the common names cannot: a reference to the one physical trait, missing in adults, that ties every member of the group together.

• 3. A Diverse, Ancient Order

  ›

  About 87% of all amphibian species are classified within the order Anura, which now includes more than 7,700 species spread across 59 families. Four families account for most of that diversity: Hylidae with 1,062 species, Strabomantidae with 807, Microhylidae with 758 and Bufonidae with 657. Anurans share a distinct set of traits, including nine or fewer vertebrae ahead of the sacrum, a fused tailbone called a urostyle, no tail at all in adulthood, and, in nearly every species, a lower jaw with no teeth. The single exception is Gastrotheca guentheri, the only known frog with teeth on its lower jaw.

  Frogs and toads split into three suborders. Archaeobatrachia holds four families of primitive frogs, Mesobatrachia holds five families considered evolutionary intermediates, and Neobatrachia, by far the largest of the three, contains most of the world's common species, further divided into the superfamilies Hyloidea and Ranoidea. Relationships between individual families inside these groups remain debated.

  The edible frog is a natural hybrid between the pool frog and the marsh frog, one of several anuran species that hybridize freely; the two fire-bellied toad species, Bombina bombina and Bombina variegata, cross in a similar way. These hybrids are less fertile than either parent species, but that has not stopped them from persisting in wide hybrid zones wherever the parent species overlap.

• 4. Deep Time

  ›

  Triadobatrachus massinoti, unearthed in Madagascar and dated to about 250 million years ago, is the earliest known animal more closely related to frogs than to salamanders. A similarly aged relative, Czatkobatrachus polonicus, turns up in Poland. Triadobatrachus already had a frog-like skull, broad with large eye sockets, but the rest of its skeleton tells a different story: a longer body with more vertebrae, a tail made of separate bones rather than a single fused urostyle, and a tibia and fibula that had not yet fused together, making it an unlikely candidate for serious jumping.

  An even older and stranger fossil turned up in Texas. Described in 2008 and dated to around 290 million years old, Gerobatrachus hottoni mixes traits of frogs and salamanders in a single skeleton. Some researchers hailed it as a missing link near the common ancestor of both groups, though others argue it belongs to an unrelated, extinct lineage entirely.

  The first fossils that count as true frogs appear in the early Jurassic. Prosalirus bitis, discovered in 1995 in Arizona's Kayenta Formation and dated to between 199.6 and 175 million years ago, had already lost nearly all of its tail and shows clear adaptations for jumping. Vieraella herbsti, another early Jurassic species, survives only as a single flattened impression about 33 millimetres long. Notobatrachus degiustoi, from the middle Jurassic and roughly 155 to 170 million years old, shows the body growing shorter still and the tail disappearing further. Tadpoles belonging to this species, dated to between 168 and 161 million years ago, are the oldest tadpole fossils known as of 2024, and they already show adaptations for filtering food from the water.

  The families Hyloidea and Microhylidae, together with the group Natatanura, which today includes about 88% of all living frogs, appear to have diversified at almost the same moment, around 66 million years ago, just after the asteroid impact that ended the age of dinosaurs, accelerating the modern frog radiation. In 2020, palaeontologists announced a final surprise: 40-million-year-old helmeted frog fossils recovered from Seymour Island, on the Antarctic Peninsula, related to frogs that still live in the Nothofagus forests of South America today.

• 5. Built to Jump

  ›

  Paedophryne amauensis, the smallest frog species known, measures just 7.7 millimetres from snout to vent in the forests of Papua New Guinea. At the other extreme, the goliath frog of central Africa grows to roughly 35 centimetres and about 3.3 kilograms, and some now-extinct prehistoric species were larger still.

  In the hind legs of most species, the tibia, fibula and ankle bones have fused into a single strong bone able to absorb the shock of landing, while the metatarsals have grown longer to give the leg extra length to push against the ground on takeoff. A frog's pelvis has its own trick: the ilium bone has grown long and forms a mobile joint with the sacrum, effectively adding an extra limb joint that powers the leap in strong jumpers such as the true frogs and tree frogs. The old tail has been reduced to a single fused bone, the urostyle, tucked inside the pelvis, where it helps transfer force from the legs into the rest of the body. Leg muscles alone can account for more than 17% of a frog's total body mass, and in especially powerful jumpers, such as the Cuban tree frog and the northern leopard frog, a stretched tendon releases energy like a catapult, producing bursts of power beyond what the muscle could generate on its own. The striped rocket frog puts all of this to use dramatically, leaping more than 2 metres in a single bound, over fifty times its own 55-millimetre body length.

  Toads and other species with short back legs tend to walk or run instead of jumping. The red-legged running frog moves by alternating its hind legs, keeping the same gait whether it is moving slowly or fast, and it climbs trees and shrubs at night to hunt insects. The Indian skipper frog can run across the surface of open water and launch straight into a leap from a floating position.

  Members of the fully aquatic family Pipidae have flattened, streamlined bodies, a fish-like lateral line system, and large webbed hind feet built for swimming. The purple frog of southern India spends almost its entire life underground, feeding on termites and surfacing only briefly during the monsoon to breed. Local communities had long known the animal, but it went unrecorded by science until 2003. North America's spadefoot toads dig backward into the earth using a hardened, blade-like structure on each hind foot, and in winter they can burrow as deep as 4.5 metres. Tree frogs climb using toe pads that grip wet or dry surfaces through mucus-assisted adhesion rather than suction. In the rainforests of Malaysia and Borneo, Wallace's flying frog takes the most extreme approach of all, spreading webbed fingers and flaps of skin along its limbs and tail to glide as far as 15 metres between trees, though it cannot manage true powered flight. That gliding membrane is really just an extension of the frog's skin, an organ that does far more than cover the body.

• 6. Skin That Breathes

  ›

  Oxygen can pass directly through a frog's skin, letting some species breathe even where open air is scarce, since a frog's skin does far more than protect the body beneath it. Most frogs lack ribs entirely, so their lungs fill through a pumping action of the throat rather than the expansion of a chest, and a frog that loses its lungs entirely can often still function using its skin alone. The Bornean flat-headed frog takes this to its logical extreme: it is the first frog species found to have no lungs at all. Frogs also have a three-chambered heart, a structure they share with lizards, in which oxygenated blood from the lungs and skin and deoxygenated blood from the rest of the body enter through separate chambers before mixing in one shared ventricle and being sorted back out by a spiral valve. The Titicaca water frog relies on its skin more than most. Its wrinkled surface increases the area available for gas exchange in the oxygen-poor water of Lake Titicaca, and the frog rarely uses its own rudimentary lungs, instead rocking its body rhythmically along the lake bed to keep fresh water flowing across its skin.

  Glands concentrated on the head and back release secretions ranging from merely distasteful to genuinely toxic, keeping the skin moist while making the animal harder for predators to hold onto. That same skin is shed every few weeks, splitting down the middle of the back and across the belly as the frog works its arms and legs free, before the animal pulls the discarded layer toward its head and eats it.

  The grey foam-nest tree frog can turn almost entirely white to avoid overheating in direct sun, since frog skin tends to darken in cool, damp conditions and lighten on hot, dry days, serving the same protective purpose as colour elsewhere in the animal kingdom. Some species coat their skin in a waxy secretion to cut down on water loss, while Woodhouse's toad instead sits in shallow water to rehydrate whenever it gets the chance after a dry spell. None of these defences matter much if a frog cannot sense a predator coming in the first place.

• 7. Senses and Sound

  ›

  A frog's eyes sit near the top of the head on either side, bulging outward to give a field of binocular vision spanning 100 degrees in front and almost 360 degrees in total. Each eye has closable upper and lower lids plus a nictitating membrane for extra protection, particularly while swimming, though members of the fully aquatic family Pipidae keep their eyes positioned on top of the head, better placed to watch for prey above. Iris colour and pupil shape vary widely across species. The common toad has golden irises with horizontal slit pupils, the red-eyed tree frog has vertical slits, the poison dart frog has dark irises, the fire-bellied toad has triangular pupils, and the tomato frog's pupils are simply round. A calling frog will fall silent at the sight of an intruder or even a moving shadow from a distance, yet the closer that same object gets, the less clearly the frog can make it out, since a frog sees distant objects far better than close ones. When a frog shoots out its tongue to catch an insect, it is actually reacting almost blind, since it closes its eyes the moment the tongue extends and must aim beforehand.

  In bullfrogs, the circular patch just behind each eye, the tympanic membrane, reveals the animal's sex, since frogs hear both in air and underwater without any external ear structure and rely on that exposed or thinly skin-covered membrane instead. Males carry a tympanum larger than their own eyes, while in females the eye and the tympanum are roughly the same size. Because a frog's cochlea is short, the auditory system relies on electrical tuning to extend the range of frequencies it can pick out, sharpening its ability to recognise the calls of its own species.

  Some frog calls carry as far as a mile, produced by pushing air through the larynx and amplified by one or more vocal sacs, pouches of skin under the throat or at the corner of the mouth, and certain species have even been found using man-made drain pipes to amplify their voices artificially. The coastal tailed frog of North America is a rare exception that does not call at all. Calling exists mainly to attract mates, sometimes building into a full chorus where many males converge on a single breeding site, and females tend to prefer louder, lower-pitched calls as a signal of a male's fitness.

  The American bullfrog's croak is often rendered as "jug o' rum," while the Pacific tree frog produces the onomatopoeic "ribbit" familiar from countless films. In The Frogs, an Ancient Greek comedy by Aristophanes, the call of the marsh frog appears in print as "brekekekex koax koax," one of the oldest written renderings of any frog call.

  In colder regions, many frogs enter a dormant state called torpor for months at a time. Land-dwelling species such as the American toad dig a burrow and form a hibernaculum to wait out the cold, while aquatic species like the American bullfrog sink to the bottom of a pond, semi-immersed in mud but still able to draw on the oxygen dissolved in the water. The wood frog, the moor frog and the spring peeper go even further and can survive being frozen solid. Ice crystals form beneath the skin and inside the body cavity, but a high concentration of glucose protects the essential organs, and a frozen, apparently lifeless frog can resume breathing and restart its own heartbeat once conditions warm again. That same resilience carries into how frogs reproduce, an area where parental habits vary just as widely.

• 8. Life, Death and Parenthood

  ›

  Frogs reproduce in one of two ways. Most species follow prolonged breeding, gathering at a familiar pond, lake or stream at a set time each year, often after migrating there in numbers that can reach the thousands. Others practice explosive breeding, converging on a site the instant conditions trigger them, such as sudden rainfall filling a temporary pool in an arid region. Among prolonged breeders, males typically arrive first and defend calling territories, while females arrive later and leave soon after laying their eggs.

  During mating, the male grips the female tightly from above in a position called amplexus, sometimes holding on for several days, and fertilizes her eggs externally as she releases them. In the Great Plains toad, the male even uses his back feet to hold the eggs in place for about three minutes during fertilization. Members of the West African genus Nimbaphrynoides skip external fertilization altogether: they are viviparous, giving birth to fully developed juvenile frogs rather than laying eggs at all.

  Eggs are laid as clumps, sheets, strings, or singly, depending on the species, and only around half of all frog species lay them directly in water. The red-eyed treefrog deposits its eggs on a leaf hanging above a pool, so that hatching larvae simply fall into the water below. The wood frog packs its eggs with symbiotic algae that may supply the developing larvae with extra oxygen through photosynthesis, and the centre of its egg clusters can run up to 6 degrees Celsius warmer than the surrounding water, speeding up development.

  The tadpoles that emerge lack eyelids and limbs entirely, breathing first through external gills and then internal ones while a fin-edged tail powers their swimming. Most feed on algae, though some species turn carnivorous, and a few, including the Cuban tree frog, are outright cannibalistic, eating slower tadpoles that have not yet grown legs. Metamorphosis itself unfolds astonishingly fast, often complete within about 24 hours once triggered by the hormone thyroxine. Lungs develop as gills vanish, the jaw reshapes for a carnivorous adult diet, and the gut shrinks to match, all while the animal stops eating entirely and is at its most vulnerable to attack.

  Parental care after that point is rare among frogs, appearing in an estimated one in five amphibian species, but where it exists it can be remarkably elaborate. The female strawberry poison dart frog lays her eggs on the forest floor, where the male guards them and keeps them moist with water carried in his own cloaca. Once they hatch, the female carries her tadpoles one at a time on her back to separate water-filled plants, then returns to each one regularly to feed it unfertilised eggs until it is ready to transform. The male common midwife toad takes on the job himself, carrying fertilised eggs attached to his hind legs for three to six weeks and dipping into ponds to keep them damp, until he finally delivers them to the water to hatch. The tungara frog builds its own nursery instead, whipping up a floating nest of foam with natural antimicrobial properties, sometimes joining with other breeding pairs to build a single shared raft layered with alternating bands of foam and eggs.