Greater sirens live in shallow, seasonally unreliable water. As a pond dries, a siren burrows into the substrate and secretes mucus that hardens with shed skin layers into a parchment-like cocoon around the body, with an opening at the mouth. Metabolic rate falls sharply and the animal enters aestivation, drawing on stored reserves and losing substantial body mass.
Documented aestivation periods run to many months, and sirens have been dug out of dried pond beds and revived when water returned. During this state they neither feed nor move; the cocoon's function is chiefly to reduce water loss through the permeable amphibian skin.
Sirens are frequently mistaken for eels or for legless amphibians. They are salamanders, with small forelimbs, external gills retained for life, and lungs they use to gulp air. Unusually among salamanders, they lack maxillary teeth and instead have a keratinised beak, and their diet includes a substantial amount of plant material.