Keratin in a frog's skin: how Chytrid fungus spreads

Stage 1: tadpoles only have keratin (shown in red) in their mouth area, so they can scrape algae and decaying vegetation from the bottom of a pond or stream. Chytridfungus only affects their mouthparts>

Stages 2 and 3: as the tadpole develops legs, it grows keratin on its feet and hands, to make them tougher. Chytrid fungus in the water can infect these new areas of keratin.

Stage 4: the tadpole develops into a froglet, and keratin grows on other parts of its body that need to be protected from wear and tear. Chytrid fungus can now spread to many parts of the frog's body.

What is keratin?

Keratin is a type of protein. It's in your hair, and in your skin. Other animals have it in their feathers, fur and scales.

Keratin forms a tough, impervious, waterproof layer that protects animals, stopping unwanted substances in the outside world from getting into their bodies. It also builds up in areas of the body that get exposed to lots of wear and tear. Have you ever had a callous on your hands or feet? That's keratin.

As keratin builds up in cells, the cells die and eventually peel away. People shed cells gradually, losing small amounts of skin and hair all the time. Snakes and other reptiles do it more quickly, shedding their skins - and their keratin - in one go.

Keratin in frogs

A frog needs its skin to be thin, moist and pervious to things from the outside because it breathes through its skin. Frogs have simple lungs, which do not give them all the oxygen they need. They have to absorb oxygen from the air or water through their skin. If their skin was full of keratin, they would not be able to do this.

Frogs only have keratin in the parts of their body that get exposed to wear and tear - such as their hands and feet, and the places where their legs rub against their bodies. Tadpoles only have keratin around their mouths, to help them scrape food from the bottom of their pond or stream. As they develop into frogs, they grow keratin in other parts of their bodies (see the pictures on the right).

How the frog Chytrid fungus spreads

The Chytrid fungus only attacks the parts of a frog's skin that have keratin in them. Tadpoles can be infected around their mouths, but this is not enough to kill them. It's only when they start turning into frogs, and grow keratin in other areas, that the Chytrid fungus can spread throughout their bodies.

They may then die from the disease - but not before they've hopped and swum around, spreading the fungal spores to other ponds and streams. Once a pond has become infected with Chytrid fungus, the fungus may stay in the water forever. This means it is very important not to move frogs from one area to another.