A feral pig is a domestic pig that has escaped or been released into the wild, and is living more or less as a wild animal; or one that is descended from such animals. Zoologists generally exclude from the feral category animals that, although captive, were genuinely wild before they escaped. Accordingly, Eurasian wild boar, released or escaped into habitats where they are not native, such as in North America, are not generally considered feral, although they may interbreed with feral pigs. Likewise, reintroduced wild boars in Western Europe are also not considered feral, despite the fact that they were raised in captivity prior to their release.
The natural habitat of wild boar are woodlands, however, feral populations root and forage in areas where they conflict with human activities, such as in picnic areas, on golf courses, football pitches, village greens, etc. In the UK, wild boar can be farmed under licence. However, it is illegal to release them into the wild.
There are established populations of feral wild boar in the Forest of Dean, Gloucestershire. These are often active during the daytime (diurnal behavior) and are less wary of people. This is in contrast to populations in East Sussex which are nocturnal and wary of people.
Groups of feral wild boar have been reported in the Scottish Highlands including Invermoriston, near Loch Ness, and between Newtonmore and Laggan. A group believed to be a mix of wild boar and domestic pig that escaped from a farm, have been seen in the Strathnairn area near Inverness; there were worries they would dig up potato crops. Feral wild boar occur elsewhere in the UK according to the Department for Environment, Food and Rural Affairs (Defra). It said between one- and two-hundred were estimated to be in Kent and East Sussex and about twenty to thirty in West Dorset.
The practice of introducing domestic pigs into the New World persisted throughout the exploration periods of the sixteenth and seventeenth centuries. The Eurasian wild boar (S. s. scrofa), which originally ranged from Great Britain to European Russia may have also been introduced. By the nineteenth century, their numbers were sufficient in some areas such as the Southern United States to become a common game animal.
The first recorded release of pigs in Australia was made by Captain James Cook at Adventure Bay, Bruny Island in 1777. This was part of his policy of introducing animals and plants to newly discovered countries. He "carried them (a boar and sow) about a mile within the woods at the head of the bay and there left them by the side of a fresh water brook". The deliberate introduction of pigs into previously pig-free areas seems to have been common. As recently as the early 1970s, pigs were introduced to Babel Island, off the east coast of Flinders Island. These pigs were eradicated by Department of Agriculture staff with local assistance.
One common story about the feral pig population on Flinders Island is that pigs were released when the ship City of Foo Chow went ashore on the northeast coast of the Island in March 1877. On Flinders Island, feral pigs usually invade agricultural areas adjacent to the National Park and east coast swamps. Farmers consider damage caused by the pigs to be minor as it is restricted to rooting in pasture adjacent to scrub-land edges. The total pasture area damaged each year is estimated to be less than 50 ha. Feral pigs are reported to visit paddocks where ewes are lambing but there are no reports of lambs being killed. However, pigs being omnivores scavenge any carcasses left near the scrub-land. In the Strzelecki National Park on the island, the ecosystem has been severely damaged; extensive rooting in the gullies led to water erosion and loss of regenerating forest plants. Bracken fern (Pteridium esculentum) flourishes in this damaged environment and dominates large areas forming dense stands to about 4 m which prevent light reaching the forest floor.
As of 1987, feral pigs are considered to be the most important mammalian pest of Australian agriculture.
Feral pigs are a growing problem on the southern prairies in Canada and in the United States. As of 2013, the estimated population of six million feral pigs causes billions of dollars in property damage every year in the United States, both in wild and agricultural lands. Because pigs forage by rooting for their food under the ground with their snout and tusks, a sounder (group) of feral pigs can damage acres of planted fields in just a few nights. Because of the feral pig's omnivorous nature, it is a danger to both plants and animals endemic to the area it is invading. Game animals such as deer and turkeys and, more specifically, flora such as the Opuntia plant have been especially affected by the feral hog's aggressive competition for resources. For commercial pig farmers, great concern exists that some of the hogs could be a vector for swine fever to return to the U.S., which has been extinct in America since 1978. Feral pigs could also present an immediate threat to "non-biosecure" domestic pig facilities because of their likeliness to harbor and spread pathogens, particularly the protozoa Sarcocystis.
In the early 2000s, the range of feral pigs includes all of the U.S. south of the 36°N. The range begins in the mountains surrounding California and crosses over the mountains, continuing consistently much farther east towards the Louisiana bayous and forests, terminating in the entire Florida peninsula. In the East, the range expands northward to include most of the forested areas and swamps of the Southeast, and from there goes north along the Appalachian Mountains as far as upstate New York, with a growing presence in states bordering West Virginia and Kentucky. Texas has the largest estimated population of 2.6 million feral pigs existing in 253 of its 254 counties. Outside mainland U.S., Hawaii also has feral pigs introduced to Oahu soon after Captain Cook's discovery of Hawaii in 1778, where they prey on or eat endangered birds and plants. The population of feral pigs has increased from two million pigs ranging over twenty states in 1990, to triple that number twenty-five years later, ranging over 38 states with new territories expanding north into Oregon, Pennsylvania, Ohio, and New Hampshire. Some of these feral pigs have mixed with escaped Russian boar that has been introduced for hunters from the early 1990s.
Predators such as bobcats and coyotes may occasionally take feral piglets or weakened animals, but are not large enough to challenge a full-grown boar that can grow to three times their weight. In Florida, feral pigs make up a significant portion of the Florida panther's diet. Because feral pigs are omnivorous, their feeding behaviour disrupts the entire food-chain. Plants have difficulties regenerating from their wallowing as North American flora did not evolve to withstand the destruction caused by rooting pigs, unlike Europe or Asia. Feral pigs in the U.S. eat small animals such as wild turkey poults, toads, turtles and the eggs of reptiles and birds. This can deprive other wildlife that normally would prey upon these important food sources. In some case, other wildlife are out-competed by the feral pigs' higher reproductive rate; a sow can become pregnant as early as six months old and give birth to multiple litters of piglets yearly. In the autumn, other animals such as the American black bear, compete directly with feral pigs as both forage for tree mast (the fruit of forest trees).
In the U.S., the problems caused by feral pigs are exacerbated by the small number of species which prey on them. In North America, these large predators would include the gray wolf, cougar, jaguar, red wolf, black bear and the grizzly bear. Unfortunately, each keystone predator presents problems: the jaguar is extirpated from California and the Southwest. The grizzly, while native to most of the American West, is gone from the states that have large feral pig populations, namely Texas, Arizona, California and New Mexico; and the species has a very slow reproductive rate. Wolf numbers are small and expected to remain so as they slowly repopulate their range; only one has thus far been recorded as inhabiting California in spite of thousands of square miles of good habitat. The cougar is present in most of the West, but is gone from the East, with no known populations east of Minnesota in the north, and very thin numbers east of Houston in the South. The black bear is both predator and competitor. Programs do exist to protect the weakened numbers of large predators in the US, but it is expected to take a very long time for these animals to naturally repopulate former habitat.
To control feral pig numbers, American hunters have taken to trapping and/or killing as many individuals as they can. Some, in Texas, have even turned the trapping and killing of razorbacks into small businesses. Legal restrictions on methods of hunting are lax, as most state departments of wildlife openly acknowledge feral pigs as an ecological threat and some classify them as vermin. For example, in Wisconsin, the DNR considers them unprotected wild animals with no closed season or harvest limit, and promotes aggressive removal.
Hunting with dogs is permitted and very common; it has been practiced in the Southeast for generations. Competitions for producing the fastest bay dogs are prevalent in the South, with Uncle Earl's Hog Dog Trials in Louisiana a popular example, held every summer since 1995. Preferred scent dogs for catching feral pigs mostly are native breeds, and include the Louisiana Catahoula Leopard Dog, the Blue Lacy, all members of the Coonhound family, the Plott Hound, and the Blackmouth Cur; catch dogs typically are American Pit Bull Terriers and their crosses, and American Bulldogs. The method of hunting has little variation and usually the hunter will send out bay dogs trained to chase the pig until it tires and then corner it, then a bigger catch dog is sent out to catch and hold down the pig, which may get aggressive, until the hunter arrives to kill it.
No single management technique alone can be totally effective at controlling feral pig populations. Harvesting 66% of the total population per year is required to keep the Texas feral pig populations stable. Best management practices suggest the use of corral traps which have the ability to capture the entire sounder of feral pigs. The federal government spends $20 million on feral pig management.
In February 2017, Texas Agriculture Commissioner Sid Miller approved the use of a pesticide called Kaput Feral Hog Lure, which is bait food laced with warfarin (a rodenticide used to kill rodents). It is estimated that 2.5 million feral hogs are in Texas and they cause about $50 million in agriculture damage per year.
Domestic pigs were first introduced to the Americas in the sixteenth century. Christopher Columbus intentionally released domestic swine in the West Indies during his second voyage to provide future expeditions with a freely available food supply. Hernando de Soto is known to have introduced Eurasian domestic swine to Florida in 1539, although Juan Ponce de León may have introduced the first pigs into mainland Florida in 1521.
In South America, during the early twentieth century, free-ranging boars were introduced in Uruguay for hunting purposes and eventually crossed the border into Brazil in the 1990s, quickly becoming an invasive species. Licensed private hunting of both feral boars and their hybrids with domestic pigs was authorized from August 2005 on in the southern Brazilian state of Rio Grande do Sul, although their presence as a pest had been already noticed by the press as early as 1994. Releases and escapes from unlicensed farms (established because of increased demand for boar meat as an alternative to pork), however, continued to bolster feral populations, and by mid-2008, licensed hunts had to be expanded to the states of Santa Catarina and São Paulo.
Recently established Brazilian boar populations are not to be confused with long-established populations of feral domestic pigs, which have existed mainly in the Pantanal for more than a hundred years, along with native peccaries. The demographic dynamics of the interaction between feral pig populations and those of the two native species of peccaries (collared peccary and white-lipped peccary) is obscure and is being studied presently. The existence of feral pigs could somewhat ease jaguar predation on peccary populations, as jaguars would show a preference for hunting pigs, when they are available.