Amphibians
Frog
Anura · also called anuran
A frog is one of the most successful designs in the animal world: a tailless body, a short spine, and two big folded legs that turn it into a jumping machine. There are more than 7,700 species, they live nearly everywhere there is fresh water, and their thin, breathing skin has made them the animal we watch to know whether an ecosystem is healthy.
Overview
Frogs are the amphibian order Anura, the largest by far. A frog has no tail as an adult, long jumping hind legs, moist skin, bulging eyes, and, in the males, a vocal sac for calling. Most frogs eat live prey as adults — insects, worms, and anything else they can swallow — while their tadpoles graze algae. They are found on every continent except Antarctica.
Two lives
Most frogs hatch as a tadpole, a gilled, tailed, plant-eating animal that lives in water, then transform, growing legs, switching to lungs, and rebuilding the gut for meat, before the tail is reabsorbed and the young frog leaves the water. About one species in six skips the tadpole entirely.
Skin that tells a story
A frog breathes partly through its permeable skin, which also lets in water and pollutants, so frogs react early to problems in their environment. Across the group, frogs do extraordinary things — freezing solid and thawing alive, turning transparent, gliding between trees, brooding tadpoles in the stomach or on the back, and cocooning underground for years.
Status
Frogs as an order are Not Evaluated, but amphibians as a whole are declining faster than any other vertebrate group, with roughly two in five species threatened, driven by habitat loss, the chytrid fungus, pollution, climate change, and collection.
Frog: common questions
What is the difference between a frog and a toad?
There is no strict scientific line, because toads are simply one branch of the frog family tree, so every toad is a frog but not every frog is a toad. In everyday use, the word frog is kept for animals with smooth, moist skin, long powerful jumping legs, and a life spent in or near water, while toad is used for the dry, warty, short-legged, walking amphibians with large poison glands behind the eyes that live mostly on land. Some frogs called toads, such as the fire-bellied toads and the midwife toads, are not true toads at all. When a distinction really matters, biologists use the family or genus name rather than the words frog and toad.
How does a tadpole turn into a frog?
Through metamorphosis, driven by thyroid hormone. A tadpole hatches as a legless, tailed, gill-breathing animal with a long coiled gut suited to a diet of algae and a jawless mouth for scraping. Over weeks to months, depending on the species and the water temperature, the hind legs bud and grow, then the front legs emerge, the lungs develop and the tadpole starts gulping air at the surface, the gut shortens and rearranges for a meat diet, the mouth widens and gains a tongue, the eyes shift and enlarge, and finally the tail is reabsorbed, its tissue recycled into the growing body. The young frog then leaves the water, often in large numbers after rain.
Why are frogs disappearing?
Several pressures act at once, and frogs are unusually exposed to all of them. Their permeable skin and their reliance on both water and land for the life cycle make them vulnerable to habitat loss, drainage, pollution, and pesticides. Since the 1990s a fungal skin disease, chytridiomycosis, has spread around the world and caused the decline or extinction of hundreds of species, especially in the cool, wet uplands of Central and South America and Australia. Climate change is shifting breeding seasons and drying out the pools frogs need, and the international trade in frogs for pets, food, and dissection removes animals from wild populations. Together these have made amphibians the fastest-declining group of vertebrates.
Can frogs really survive being frozen?
Some can. The wood frog of North America, along with a few other northern species, spends winter in the leaf litter and lets much of the water in its body turn to ice, with its heart stopping and its breathing ceasing. It survives by flooding its cells with glucose and other cryoprotectants that stop the cells themselves from freezing and dehydrating fatally, while ice forms only in the spaces between them. When spring warmth thaws the frog, the heart restarts within hours and normal function returns. This freeze tolerance lets these frogs live and breed in places where the ground stays frozen for months and where burrowing below the frost line is not an option.