Life cycle of a frog

A frog begins life with entirely the wrong body for the life it will lead. It starts out like a fish, breathing water through gills, and then rebuilds itself into a lunged animal that walks on land.

Frogspawn

A common frog can lay up to two thousand eggs in a single night. The jelly around them is the whole defence. It is slippery, it is barely worth eating, and it traps warmth, so the middle of a big clump sits warmer than the pond around it.

Embryo develops

Inside each black dot a head and a tail take shape over roughly two weeks, fuelled entirely by the egg's own yolk. Pond temperature sets the pace, and a cold spring can easily double the wait before anything hatches.

Hatchling with outside gills

The first tadpoles wriggle out wearing feathery gills on the outside of the head, like tiny plumes. They cannot swim properly yet, so they hang from pondweed using a sticky patch just under the mouth.

Free swimming tadpole

Within a week skin grows over those gills, giving the tadpole a smooth head and a gill chamber tucked inside. Now it grazes algae with rows of horny teeth, and it carries one of the longest coiled guts of any animal its size.

Back legs bud

Around week six to eight the back legs bud first, useless little stumps that slowly grow knees and webbed toes. For now the tadpole ignores them completely and still steers with nothing but its tail.

Front legs break out

Here is the surprise. The front legs have been growing hidden inside the gill chamber for weeks. The left one slips out through the breathing hole and the right one tears through the skin, so both appear almost overnight.

Lungs take over

The lungs finish developing and the young frog starts swimming up to gulp air. It still lives in the water, but from this moment drowning is possible, and one that cannot reach the surface is in real trouble.

Tail is absorbed

The tail is not dropped, it is eaten. Special cells digest it from the tip inwards and the body reuses every scrap as food, while the mouth is rebuilt from a grazing beak into a wide hunting jaw.

Froglet

Now a froglet the size of your thumbnail hauls out at the water's edge. In a few short weeks it has gone from a plant eating grazer to a hunter that will only strike at prey that moves.

Young frog on land

On land it takes insects, slugs and worms with a tongue that is hinged at the front and flicks out backwards, coated in a spit that turns runny on impact then sticky again. Full size takes two to three years.

Adult frog

An adult frog breathes two ways, with lungs and straight through its damp skin. That is why a frog must stay moist, and it is how one can sit at the bottom of a cold pond all winter without taking a single breath.

Returns to its home pond

In spring the adults walk back to the exact pond they grew up in, sometimes more than a kilometre, steering by smell. The males arrive first and call all night, and a big chorus carries for hundreds of metres.

Amplexus and spawning

The male grips the female's back in a hold called amplexus and fertilises the eggs outside her body as she releases them. Within hours the jelly has swollen into fresh spawn and the whole journey starts again.