The difference between hearing in humans and animals is often visible externally. For example some animals (e.g. birds) lack external ears/pinnas, but maintain similar internal structures to the human ear. Although birds have no pinnas they have middle ears and inner ears similar to humans, and like humans, hear best at the frequencies around 2,000 to 4,000 Hz. All mammals (the animals most closely related to the human) have outer ears/pinnas. Many mammals have the ability to move the pinna to help with localization of sounds. Foxes, for example, have large bowl shaped pinnas which can be moved to help locate distant or faint sounds. In addition to sound localization, some animals are able to manipulate their pinnas to regulate body temperature. Elephants do this by using their huge pinnas as fans and for heat exchange.