Estivation is a reaction to diverse environmental conditions such as drought. Frogs like the African bullfrog dig deep into the soil just like during hibernation and become dormant. They then shed several layers of skin. This forms a waterproof cocoon that leaves only the nostrils exposed to the air. The frog can survive in this manner for a period as long as seven years if necessary. The frogs free themselves of the skin layers when moisture returns. In fact, the cocoon of skin might be their first meal after precipitation returns to the area.