section_path: Other animals
section_name: Other animals
paragraph_text: Other animals
  
  The pinna helps direct sound through the ear canal to the eardrum. The complex geometry of ridges on the inner surface of some mammalian ears helps to sharply focus sounds produced by prey, using echolocation signals. These ridges can be regarded as the acoustic equivalent of a Fresnel lens, and may be seen in a wide range of animals, including the bat, aye-aye, lesser galago, bat-eared fox, mouse lemur and others.
  
  Some large primates such as gorillas and orangutans (and also humans) have undeveloped ear muscles that are non-functional vestigial structures, yet are still large enough to be easily identified. An ear muscle that cannot move the ear, for whatever reason, has lost that biological function. This serves as evidence of homology between related species. In humans, there is variability in these muscles, such that some people are able to move their ears in various directions, and it has been said that it may be possible for others to gain such movement by repeated trials. In such primates, the inability to move the ear is compensated for mainly by the ability to easily turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.
  
  In some animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help localise the direction of the sound source.
  
  The ear, with its blood vessels close to the surface, is an essential thermoregulator in some land mammals, including the elephant, the fox, and the rabbit. There are five types of ear carriage in domestic rabbits, some of which have been bred for exaggerated ear length —a potential health risk that is controlled in some countries. Abnormalities in the skull of a half-lop rabbit were studied by Charles Darwin in 1868. In marine mammals, earless seals are one of three groups of Pinnipedia.