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Mammals are members of class Mammalia ( /məˈmeɪli.ə/), air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth, and the largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, the latter featuring red blood cells lacking nuclei and a large, four-chambered heart maintaining the very high metabolism rate they have. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) bumblebee bat to the 33-meter (108-foot) blue whale.

A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, (in other words, a foil (fluid mechanics)). The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices. Fins, as with other foils, operate in fluids such as water or air.

Fins are seen both in nature and in manmade iterations.

Swimming water animals such as fish and cetaceans actively use pectoral fins for maneuvering, and dorsal fins contribute stability as the animal swims, propelling and maneuvering with its tail, itself recognizable as a fin.

The fin on fixed-wing aircraft is known as a vertical stabilizer. Fins are also seen used as e.g., fletching on arrows and at the rear of some bombs, missiles, rockets, and self-propelled torpedoes. These are typically "planar" (shaped like small wings), although grid fins are sometimes used in specialized cases.

Examples of fins include:

In surfing, there are two major types of (typically stationary) surfboard fins (hydrofoils), and a host of illustrative issues.

The humpback whale (Megaptera novaeangliae) is a species of baleen whale. One of the larger rorqual species, adults range in length from 12–16 metres (39–52 ft) and weigh approximately 36,000 kilograms (79,000 lb). The humpback has a distinctive body shape, with unusually long pectoral fins and a knobbly head. It is an acrobatic animal, often breaching and slapping the water. Males produce a complex song, which lasts for 10 to 20 minutes and is repeated for hours at a time. The purpose of the song is not yet clear, although it appears to have a role in mating.

Found in oceans and seas around the world, humpback whales typically migrate up to 25,000 kilometres (16,000 mi) each year. Humpbacks feed only in summer, in polar waters, and migrate to tropical or sub-tropical waters to breed and give birth in the winter. During the winter, humpbacks fast and live off their fat reserves. The species' diet consists mostly of krill and small fish. Humpbacks have a diverse repertoire of feeding methods, including the bubble net feeding technique.

Like other large whales, the humpback was and is a target for the whaling industry. Due to over-hunting, its population fell by an estimated 90% before a whaling moratorium was introduced in 1966. Stocks have since partially recovered; however, entanglement in fishing gear, collisions with ships, and noise pollution also remain concerns. There are at least 80,000 humpback whales worldwide. Once hunted to the brink of extinction, humpbacks are now sought by whale-watchers, particularly off parts of Australia, New Zealand, South America, Canada, and the United States.