Flipper (anatomy)

In animals with two flippers, such as whales, the flipper refers solely to the forelimbs. In animals with four flippers, such as pinnipeds and sea turtles, one may distinguish fore- and hind-flippers, or pectoral flippers and pelvic flippers.

Animals with flippers include penguins (whose flippers are also called wings), cetaceans (e.g., dolphins and whales), pinnipeds (e.g., walruses, earless and eared seals), sirenians (e.g., manatees and dugongs), and marine reptiles such as the sea turtles and the now-extinct plesiosaurs, mosasaurs, ichthyosaurs, and metriorhynchids.

Usage of the terms "fin" and "flipper" is sometimes inconsistent, even in the scientific literature. However, the hydrodynamic control surfaces of fish are always referred to as "fins" and never "flippers". Tetrapod limbs which have evolved into fin-like structures are usually (but not always) called "flippers" rather than fins. The dorsal structure on cetaceans is called the "dorsal fin" and the large cetacean tails are referred to primarily as flukes but occasionally as "caudal fins"; neither of these structures are flippers.

Some flippers are very efficient hydrofoils, analogous to wings (airfoils), used to propel and maneuver through the water with great speed and maneuverability (see Foil). Swimming appendages with the digits still apparent, as in the webbed forefeet of amphibious turtles and platypus, are considered paddles rather than flippers.

Locomotion

For all species of aquatic vertebrates, swimming performance depends upon the animal's control surfaces, which include flippers, flukes and fins. Flippers are used for different types of propulsion, control, and rotation. In cetaceans, they are primarily used for control while the fluke is used for propulsion.

The evolution of flippers in penguins was at the expense of their flying capabilities, in spite of evolving from an auk-like ancestor that could 'fly' underwater as well in the air. No penguins capable of flight have been found in the fossil record, so we know very little about what birds preceded the flightless penguin. Form constrains function, and the wings of diving flying species, such as the murre or cormorant have not developed into flippers. However, species such as these that can both fly and dive support the hypothesis that penguins would have evolved from a bird that could fly and dive before losing flight completely. In fact, murres and cormorants have extremely high flight costs and higher dive costs than penguins, showing that there is a tradeoff between efficiency in the air and efficiency in the water.

The flippers of penguins became flatter, denser, and smaller while being modified for hydrodynamic properties. The functionality of the penguin hydrofoil comes from the wing bones themselves as opposed to feathers in a volant bird. Their bones have the thickest cortical bone of any bird, a very small medullary cavity, and are considered largely osteosclerotic. There is also fusing of digits and joints within the penguin wing that prevent most flexion and extension.

Hydrodynamics

Cetacean flippers may be viewed as being analogous to modern engineered hydrofoils, which have hydrodynamic properties: lift coefficient, drag coefficient and efficiency. Flippers are one of the principal control surfaces of cetaceans (whales, dolphins and porpoises) due to their position in front of the center of mass, and their mobility which provides three degrees of freedom.

Flippers on humpback whales (Megaptera novaeangliae) have non-smooth leading edges, yet demonstrate superior fluid dynamics to the characteristically smooth leading edges of artificial wings, turbines and other kinds of blades. The whale's surprising dexterity is due primarily to its non-conventional flippers, which have large, irregular looking bumps called tubercles across their leading edges. The tubercles break up the passage of water, maintaining even channels of the fast-moving water, limiting turbulence and providing greater maneuverability.

The foreflippers used by the pinnipeds act as oscillatory hydrofoils. Both fore and hind flippers are used for turning. A 2007 study of Steller's sea lion found that a majority of thrust was produced during the drive phase of the fore flipper stroke cycle. Although previous findings on eared seals suggested that thrust was generated by the initial outward movement of the fore flippers or the terminal drag-based paddling phase, the 2007 study found that little or no thrust was generated during those phases. Swimming performance in sea lions is modulated by changes in the duration and intensity of movements without changing their sequence. Using criteria based on velocity and the minimum radius of turns, pinnipeds' maneuverability is superior to cetaceans but inferior to many fish.