The African lungfish has a special swim bladder whose inner walls are covered with a dense network of blood vessels, making it structurally similar to the lungs of land animals — it can take oxygen directly from the air. When the water starts to dry up, the lungfish digs a hole about 30 centimeters deep into the mud, curls up inside, and secretes a large amount of mucus. As the mucus mixes with the mud and dries, it forms a hard mud cocoon that wraps the fish up tightly. This shell locks in moisture and keeps the body from drying out, with only a small breathing hole left open to the surface. Once it enters estivation (its dry-season dormancy), the lungfish's heartbeat drops from its normal rate to just 2 to 3 beats per minute, and its overall metabolic rate falls to less than 2% of normal. It doesn't eat, drink, or excrete; metabolic wastes are stored in the body as urea and dealt with all at once when water returns. As for energy, it can even break down its own tail muscle to stay alive. A normal dry-season dormancy in the wild usually lasts seven or eight months, but in extreme cases, the African lungfish can survive without food or water for four to five years. Once the rains return and soften the mud cocoon, it breaks out and is back to normal activity within minutes.
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