Bat
Reference: Bat
The only mammal in true powered flight, and the clearest demonstration of what a vertebrate wing costs: a bat flies on its hands.
Scored on every axis
The hand is the wing
Four fingers are elongated into spars carrying the membrane. The thumb stays short, free and clawed, and is used for climbing and handling food. There is no fifth pair of limbs, so flight is paid for out of the forelimbs — which is why bats hang by their feet: the hands are unavailable.
Compare the eagle, which solved the same problem with feathers on a fused forelimb, and the dragonfly, which did not have to pay at all because insect wings are not limbs.
Hearing as a lamp
Most bats emit calls and read the returns, resolving position, size, texture and movement in complete darkness. The common belief that bats are blind is false — they see perfectly well — but in the dark, sound carries the load.
Toothed whales built the same capability separately, and the two lineages converged down to the same changes in the same gene.