Many frogs contain mild toxins that make them distasteful to potential predators. For example, all toads have large poison glands. The parotid glands, located behind the eyes on the top of the head. Some frogs, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors (which narrow the blood vessels). Many predators of frogs have adapted to tolerate high levels of these poisons. Others, including humans, may be severely affected. Try to Listen for the "Ribbits"