Dolphins evolved from hoofed land mammals that took to the sea 50 million years ago. Their ancestors’ hooves changed into fins that allowed dolphins to become efficient swimmers. The fins of different species of dolphins are adapted to how they move: Long, narrow fins are good for fast swimmers, while broad, short fins have evolved in species that need to maneuver in small spaces and make tight turns.
Frogs have evolved into thousands of species with hands that have changed to accommodate different activities. Some of the species that live in water grow webbed hands so that they can swim. Tree frogs use long fingers with expanded fingertips for climbing; they have even evolved tiny adhesive disks on their fingers that help them stick to smooth surfaces such as leaves.
Cat hands grow narrow, curved claws. Most of the time ligaments running over the top of a cat’s hands keep the claws retracted inside sheaths. To catch prey, a cat pushes off its hind legs and stretches out its arms. Muscles along the top and bottom of its hands contract, which draws the claws out like switchblades. The claws sink into the prey, the cat shifts its weight to its hind legs, and the hands draw the prey to the cat’s mouth.
A bat wing may look like a sheet of skin, but within its flesh are five fingers. The bones act something like tent poles, stretching out the membranous wing so that it can catch the air and lift the bat’s body. By adjusting each finger, a bat can dodge through a forest, hover in front of a flower to feed, or slip into the mouth of a cave.
The elephant hand has adapted to withstand tremendous weight. The stout digit bones—recent research shows a sixth “faux” digit that begins as cartilage but grows into bone in some older elephants—function like the base of a pillar. A pad of fatty, fibrous tissue absorbs the shock of each step. Tendons and ligaments store some of the energy as the hand hits the ground and release it as the hand rises. There is literally a spring in the elephant’s step.
The aye-aye’s long fingers allow it to get food that’s beyond the reach of other animals in Madagascan forests. One of this lemur’s favorite meals is insect larvae hidden inside trees. It taps one specially designed finger on the bark, listening and feeling for vibrations, and even uses smell to detect the presence of larvae. Then it gnaws a hole. A ball joint at the base of the digit allows the aye-aye to push it through the hole at any angle; a claw hooks the larvae to pull them out.