Amphibians are ectothermic (cold-blooded) tetrapod vertebrates that do not maintain their body temperature through internal physiological processes. Their metabolic rate is low and as a result, their food and energy requirements are limited. In the adult state, they have tear ducts and movable eyelids, and most species have ears that can detect airborne or ground vibrations. Amphibians have muscular tongues, which in many species can be protruded. Their skin contains many mucous glands and in some species, poison glands. Most amphibians go through metamorphosis, a process of significant morphological change after birth. Amphibians include frogs, toads, and salamanders.