The ferret is a domestic mammal of the type Mustela putorius furo. Ferrets are sexually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur, have an average length of 20 inches (51 cm) including a 5 inch (13 cm) tail, weigh about 1.5–4 pounds (0.7–2 kg), and have a natural lifespan of 7 to 10 years.
Several other small, elongated carnivorous mammals belonging to the family Mustelidae (weasels) also have the word ferret in their common names, including an endangered species, the Black-footed Ferret. The ferret is a very close relative of the polecat, but it is as yet unclear whether it is a domesticated form of the European Polecat, the Steppe Polecat, or some hybrid of the two.
The history of the ferret's domestication is uncertain, like that of most other domestic animals. It is very likely that ferrets have been domesticated for at least 2,500 years. They are still used for hunting rabbits in some parts of the world today, but increasingly they are being kept simply as pets.
Being so closely related to polecats, ferrets are quite easily able to hybridize with them, and this has occasionally resulted in feral colonies of ferret polecat hybrids that have been perceived to have caused damage to native fauna, perhaps most notably in New Zealand. As a result, some parts of the world have imposed restrictions on the keeping of ferrets.
In common with most domestic animals, the original reason for ferrets' domestication by human beings is uncertain but it may have involved hunting. It was most likely domesticated from the European polecat (Mustela putorius), though it is also possible that ferrets are descendants of the Steppe polecat (Mustela eversmannii), or some hybridization thereof. Analysis of mitochondrial DNA suggests that ferrets were domesticated around 2,500 years ago, although what appear to be ferret remains have been dated to 1500 BC. It has been claimed that the ancient Egyptians were the first to domesticate ferrets, but as no mummified remains of a ferret have yet been found, or any hieroglyph of a ferret, and no polecat now occurs wild in the area, that idea seems unlikely.
The Greek word ictis occurs in a play written by Aristophanes, The Acharnians, in 425 BC. Whether this was a reference to ferrets, polecats, or the similar Egyptian Mongoose is uncertain. The name "ferret" is derived from the Latin furittus, meaning "little thief", a likely reference to the common ferret penchant for secreting away small items. Ferrets were probably used by the Romans for hunting.
Colonies of feral ferrets have established themselves in areas where there is no competition from similarly sized predators, such as in the Shetland Islands and in remote regions in New Zealand. Where ferrets coexist with polecats, hybridization is common. It has been claimed that New Zealand has the world's largest feral population of ferret-polecat hybrids. In 1877, farmers in New Zealand demanded that ferrets be introduced into the country to control the rabbit population, which was also introduced by humans. Five ferrets were imported in 1879, and in 1882-1883, 32 shipments of ferrets were made from London, totaling 1,217 animals. Only 678 landed, and 198 were sent from Melbourne, Australia. On the voyage, the ferrets were mated with the European polecat, creating a number of hybrids that were capable of surviving in the wild. In 1884 and 1886, close to 4,000 ferrets and ferret hybrids, 3,099 weasels and 137 stoats were turned loose. Concern was raised that these animals would eventually prey on indigenous wildlife once rabbit populations dropped, and this is exactly what happened to New Zealand bird species which previously had no mammalian predators.