Amphibians
True Frog
Ranidae · also called ranid frog, typical frog
When you picture a frog — smooth wet skin, pointed face, a round eardrum, long folded legs ready to launch it into a pond — you are picturing a true frog, a member of the family Ranidae. This is the group that European and North American naturalists met first and used to define the word, so in a real sense these are the reference frogs and everything else is a variation.
Overview
True frogs are the family Ranidae, roughly 400 to 600 species, with the widest natural range of any frog family — every continent except Antarctica, and further north than any other amphibian. The family includes the common frog, the American bullfrog, the leopard and green frogs, and the edible frog. All are carnivores, catching live prey with a flicked tongue or a lunge.
Built to jump
Ranids have some of the longest hind legs relative to body size of any frogs, and a strong jumper can clear well over a metre — ten times its own length — in one leap, using it both to escape and to plunge straight into water. Most true frogs are semi-aquatic, rarely far from a still or slow-moving pond, ditch, or stream.
Frozen frogs and slow tadpoles
The northern wood frog survives winter frozen almost solid, up to two thirds of its water turned to ice, its heart stopped, its cells protected by glucose, then thaws in spring. True frog tadpoles are pond grazers; while many transform in a summer, bullfrog and green frog tadpoles in cool water can take two or three years, overwintering in the pond.
Farmed and invasive
The American bullfrog and edible frog are harvested at scale for frog legs. Escaped and released bullfrogs have become serious invasive predators across the western US, Europe, South America, and Asia, eating native wildlife and spreading the amphibian chytrid fungus and ranavirus.
Status
True frogs as a family are Not Evaluated, with a decreasing overall trend. Many species stay common, but the family also holds many threatened ones, hit by chytrid, drained wetlands, pollution, roads, the frog-leg trade, and introduced bullfrogs.
True Frog: common questions
What makes a frog a true frog?
Membership of the family Ranidae, a specific branch of the roughly 7,000 frog species. True frogs share a recognisable set of features, a smooth moist skin, a fairly pointed head, a visible circular eardrum behind the eye, a narrow waist, and long muscular hind legs with well-webbed feet, and most are semi-aquatic, living at the edge of fresh water. This is the body plan that European and North American naturalists saw first and used to define what a frog is, so true frogs are in a sense the reference frogs, and the many other frog families, such as toads, tree frogs, and narrow-mouthed frogs, are the variations.
What is the difference between a true frog and a toad?
Toads are a different family, the Bufonidae, adapted for a drier, more terrestrial life. Compared with a true frog, a typical toad has dry, warty skin, a broad squat body, short legs suited to walking and hopping rather than leaping, no obvious neck, large poison glands behind the eyes, and it lays eggs in long strings rather than clumps. True frogs are tied more closely to water, have smooth skin and long jumping legs, and lay eggs in rounded masses. Both belong to the wider group Anura, and neither name marks the whole of it, so many frogs are neither a true frog nor a true toad.
How can wood frogs survive being frozen?
The wood frog, a true frog of northern North America, spends winter on the forest floor under leaf litter and lets much of its body freeze. As ice starts to form in the body cavity, the frog's liver floods the blood and cells with glucose, which acts as antifreeze inside the cells and stops them shrinking and rupturing as water is drawn out into the surrounding ice. The heart stops, breathing stops, and up to about two thirds of the frog's water becomes ice for weeks. When spring warmth thaws it, the heart restarts within hours and the frog walks away. Only a handful of frogs and turtles can do this.
Why are American bullfrogs a problem outside their native range?
The American bullfrog is large, aggressive, long-lived, and produces up to 20,000 eggs per clutch, and outside eastern North America it often has no effective predators or competitors. Where it has been introduced, through the frog-leg trade, the pet trade, and stocking, it eats almost anything it can swallow, including native frogs, salamanders, snakes, fish, and hatchling birds, and it out-competes local amphibians for food and breeding sites. Bullfrogs also carry the chytrid fungus and ranavirus, often without being harmed themselves, and can seed outbreaks that devastate native species. They are now established across much of the western United States, parts of Europe, South America, and Asia.