How Do Tadpoles Turn Into Frogs? The Frog Life Cycle Explained

The stages of the frog life cycle

Every stage below is easy to watch in a pond in spring. The timings are typical for the common frog in western Europe and change with water temperature: warm water speeds everything up, cold water slows it down.

From frogspawn to hatchling

A frog’s egg is a single cell with a dark upper half and a pale yolk-filled lower half, wrapped in layers of clear jelly. The jelly swells with water after the egg is laid, protects the embryo and, in clumps of common frog spawn, traps the sun’s warmth.

Within days the round black dot divides into thousands of cells, lengthens into a comma shape and develops a head, tail and the start of a nervous system. At hatching, the young tadpole is only a few millimeters long. It cannot yet feed, so it hangs on the old jelly or on plants with a sticky gland under its head and lives on leftover yolk.

The tadpole stage: gills, mouth and gut

Newly hatched tadpoles breathe with feathery external gills that stick out from the sides of the head. After a few days, a fold of skin called the operculum grows over them, and the tadpole breathes with internal gills, letting water out through a small opening on one side, the spiracle. It can also take in oxygen through its thin skin, and older tadpoles often rise to the surface to gulp air into their developing lungs.

A tadpole is built for grazing. Its mouth has a beak and rows of tiny horny teeth for scraping algae from stones and plants, and its long, coiled gut can digest plant food. Tadpoles also scavenge on dead animals, and many become partly carnivorous as they grow. This is the main growth phase: the more a tadpole eats, the larger the froglet it eventually becomes.

Legs, lungs and the big change

The first visible sign of metamorphosis is a pair of tiny buds at the base of the tail, which grow into back legs with knees and webbed toes. The front legs develop at the same time but are hidden under the operculum. Near the end of the process they push out, one through the spiracle and one through the skin.

Inside, the changes are even bigger. The gills shrink and the lungs take over. The scraping mouthparts are shed and the mouth widens into a frog’s broad jaws with a sticky tongue. The long plant-digesting gut shortens into the short gut of a meat-eater. The eyes move up and outward, the skin thickens, and the ear and skeleton are remodeled for life on land.

During these final days the young frog stops eating. It relies on the tail, which is absorbed by a controlled process of cell death, and its material is recycled to fuel the rest of the change. A froglet with a stump of tail still showing is ready to leave the water, often in large numbers on a wet evening in early summer.

What triggers metamorphosis?

The signal comes from the thyroid gland. As a tadpole reaches the right size and stage, its thyroid releases more thyroid hormone, which tells different tissues to grow, change or break down. The same hormone that makes the legs grow tells the tail cells to self-destruct.

This was discovered in 1912, when the physiologist J. F. Gudernatsch, working in Prague, fed tadpoles on thyroid tissue and found that they metamorphosed early into tiny frogs. Since then, scientists have shown that tadpoles without a working thyroid never change at all. Environmental conditions also matter: tadpoles in a drying pond or a crowded one can speed up metamorphosis and leave the water at a smaller size, while plenty of food and stable water let them grow larger first.

A famous relative shows what happens when the switch is not flipped. The axolotl, a Mexican salamander, normally keeps its gills and stays aquatic for life because its body does not trigger full metamorphosis.

How the frog life cycle varies between species

Not all frogs follow the common frog’s schedule. Spadefoot toads in North American deserts can go from egg to toadlet in about two weeks, racing the drying of temporary rain pools. At the other extreme, American bullfrog tadpoles in the north spend one to three years in the water and grow to about 15 cm (6 in) long.

The paradoxical frog (Pseudis paradoxa) of South America is named for its giant tadpoles, which can be over 20 cm (8 in) long, several times longer than the adult frog. Many other frogs, such as the coqui of Puerto Rico, skip the free-swimming tadpole stage entirely: they lay eggs on land, and the young develop inside the egg and hatch as miniature frogs.

Why so few tadpoles become frogs

A female common frog may lay well over a thousand eggs, but in a typical pond only a few percent survive to leave the water, and fewer still reach adulthood. Fish, newts, dragonfly larvae, diving beetles, water bugs and birds all eat tadpoles, and ponds that dry out too early can kill an entire year’s brood. Laying huge numbers of eggs is how frogs make up for these losses, and it is why a pond full of tadpoles in spring produces only a handful of adult frogs a few years later.