The aye-aye is the largest nocturnal primate. When leaping from tree to tree, it uses its excellent night vision, but when hunting for food it switches to a form of echolocation. Aye-ayes eat insect larvae that burrow in tree trunks. To find them they use their middle finger, which is especially long, as well as thin and bony.
By rapidly tapping on the trunk they send vibrations through the wood. After each tap, the fingernail drags lightly along the bark to sense the return echoes and the aye-aye’s huge ears enfold the target area to listen for changes in the pitch of the drumming sound.
When it finds a hollow channel, the aye-aye uses its forward-slanting teeth to gnaw a hole in the bark and its long finger then does double duty as a specialised winkling tool to extract the juicy grub inside.