 The middle finger is unique in that it possesses a ball-and-socket metacarpophalangeal joint.The aye-aye has also evolved a sixth digit, a pseudothumb, to aid in gripping.The complex geometry of ridges on the inner surface of aye-aye ears helps to sharply focus not only echolocation signals from the tapping of its finger, but also to passively listen for any other sound produced by the prey.