A feral organism is one that has escaped from domestication and returned, partly or wholly, to its wild state. The introduction of feral animals or plants, like any introduced species, can disrupt ecosystems and may, in some cases, contribute to extinction of indigenous species.
A feral animal is one that has reverted from the domesticated state to a stable condition more or less resembling the wild. Some common examples are goats, cats, and camels.
"An animal that has reverted to the wild from domestication. 'Feral' should never be used to describe the naturalisation of a wild (i.e. non-domesticated) species.
The goat is one of the oldest domesticated creatures, yet readily goes feral and does quite well on its own.
The dromedary camel, which has been domesticated for well over 3,000 years, will also readily go feral. A substantial population of feral dromedaries, descended from pack animals that escaped in the 19th and early 20th centuries, thrives in the Australian interior today.
The cat returns readily to a feral state if it has not been socialized properly in its young life. (See Feral cats.) These cats, especially if left to proliferate, are frequently considered to be pests in both rural and urban areas, and may be blamed for devastating the bird, reptile and mammal populations. A local population of feral cats living in an urban area and using a common food source is sometimes called a feral cat colony. As feral cats multiply quickly, it is difficult to control their populations. Animal shelters attempt to adopt out feral cats, especially kittens, but often are overwhelmed with sheer numbers and euthanasia is used. In rural areas, excessive numbers of feral cats are often shot. More recently, the "Trap-Neuter-Return" method has been used in many locations as an alternate means of managing the feral cat population.
Sheep are close contemporaries and cohorts of goats in the history of domestication, but the domestic sheep is quite vulnerable to predation and injury, and thus rarely if ever is seen in a feral state. However, in places where there are little other predators, they get on well, for example in the case of the Soay sheep.
Cattle have been domesticated since the neolithic era, but can do well enough on open range for months or even years with little or no supervision. Their ancestors, the Aurochs, were quite fierce, on par with the modern Cape Buffalo. Modern cattle, especially those raised on open range, are generally more docile, but when threatened can display aggression. Cattle, particularly those raised for beef, are often allowed to roam quite freely and have established long term independence in Australia, New Zealand and several Pacific Islands along with small populations of semi-feral animals roaming the southwestern United States and northern Mexico. Such cattle are variously called Mavericks, Scrubbers or Cleanskins. Most free roaming cattle, however untamed, are generally too valuable not to be eventually rounded up and recovered in closely settled regions.
Horses and donkeys, domesticated about 5000 BCE, are feral in open grasslands worldwide (see feral horse). In Portugal, feral horses are called Sorraia; in Australia, they are called Brumbies; in the American west, they are called Mustangs. Other isolated feral populations exist, including the Chincoteague Pony and the Banker Horse. They are often referred to as "wild horses," but this is a misnomer. There are truly "wild" horses that have never been tamed, most notably Przewalski's Horse. While the horse was originally indigenous to North America, the wild ancestor died out at the end of the last Ice Age. In both Australia and the Americas, modern "wild" horses descended from domesticated horses brought by European explorers and settlers that escaped, spread, and thrived.
The pig (hog) has established feral populations worldwide, most notably in Australia, New Zealand, New Guinea and the Pacific Islands with small populations in the Midwest and South of the United States. Pigs were introduced to the Melanesian and Polynesian regions by humans from several thousand to five hundred years ago, and to Australia and the Americas within the past 500 years. While pigs were doubtlessly brought to New Zealand by the original Polynesian settlers, this population had become extinct by the time of European colonization, and all feral pigs in New Zealand today are descendants of European stock. Many European wild boar populations are also partially descended from escaped domestic pigs and are thus technically feral animals within the native range of the ancestral species.
Pigeons were formerly kept for their meat or more commonly as racing animals and have established feral populations in cities worldwide.
Dogs can revert to wildness, becoming predators little less effective than the big cats of like size. The Dingo is the oldest verifiable feral dog population, with a history of over 5,000 years since original escape from domestication, although the pariah dogs of Asia may well be older feral populations. The Carolina Dog is the oldest feral dog population known in the Americas.
Colonies of honey bees often escape into the wild from managed apiaries when they swarm; their behavior, however, is no different from their behavior "in captivity", until and unless they breed with other feral honey bees of a different genetic stock, which may lead them to become more docile or more aggressive (see Africanized bees).
Populations of feral parrots descended from escaped pets/zoo specimens have established themselves in various areas of Europe, North America and Australia. Rose-ringed Parakeets and Monk Parakeets have been particularly successful in this regard.