Vibrissae, more commonly known as whiskers, are sensory hair that generally have a tactile sense, which is equivalent to skin. They grow on most mammals except monotremes (duck-billed platypus and echidnas) and humans, though humans still have remnants of the muscles once associated with vibrissae in their upper lips. Vibrissae serve a number of purposes: such as sense of touch; detection of danger, air and water currents, and movements of other animals; and the locating of food.