Species Description: Feral pigs (Sus scrofa) are large terrestrial mammals with rounded bodies and short legs that show a marked degree of variation in terms of size, coat, color, tail shape, and other physical traits. They possess well-developed upper incisors and upper canines that often project outward and upward from the mouth. Their skulls are identifiable by the lack of a bony ring around the sockets and by the tall, steep form of the cranium (Lowery 1974, Whitaker 1988).
Potentially Misidentified Species: The only wild pig native to North America is the collared peccary (Pecary tajacu). In the US, this species is restricted to desert and thorn scrub habitats of Arizona, New Mexico, and south Texas.
Elsewhere in the United States, the feral pig should be unmistakable.
Regional Occurrence: Feral pigs occur within all US Gulf states, including Florida (Whitaker 1988).
IRL Distribution: Feral pigs occur in terrestrial habitats of all six counties within the IRL watershed, and in all 67 Florida counties (Belden 1993).
Age, Size, Lifespan: Nowak (1991) reports that domestic hogs can reach 450 kg, and their feral counterparts are on the same order.
Abundance: Belden (1993) states that in the United States, Florida's feral pig population is second only to Texas. They occur in every county in Florida and occupy a variety of habitat types, though urbanized areas and areas with major agricultural operations lack established populations. Trapping, hunting and agricultural depredation control measures have been undertaken in much of the state to control wild pig populations (Belden 1993).
Reproduction: In temperate regions breeding in S. scrofa is confined to the spring, whereas in subtropical climates the breeding season is protracted. In the tropics, breeding can occur throughout the year. Regardless of location, peak breeding coincides with the rainy season (Nowak 1991).
Both sexes usually reach sexual maturity in the first year of life—between 8-12 months in males and as early as 5-8 months in females (Johnson et al. 1982). Despite early onset of maturity, female feral pigs usually do not breed prior to 18 months of age, while males tend not to achieve reproductive success until they are fully grown at approximately age five. The estrous cycle of female pigs is approximately 21 days (Ingles 1965).
Adult males are solitary outside of the breeding season while females and juveniles are gregarious (Gingerich 1994). Females leave the group to nest and give birth. This nesting behavior is atypical of ungulate (hoofed) mammals. Litters usually consist of between 3 and 12 young, and females generally produce one or two litters each season throughout their reproductive lives (Ingles 1965, Gingerich 1994).
Embryology: Gestation varies between 100 and 140 days. Young are weaned in 3-4 months and often leave their mother before the next litter if the mother produces multiple litters in a breeding season (Nowack 1991). Early mortality rates can be high. Baber and Coblentz (1986) reported that 58% of piglets died before weaning.
Temperature: Feral pigs occur from temperate climates to the tropics and their activity patterns are tied to the temperature regimes of the various climates in which they are found. These mammals lack sweat glands and rely greatly on behavioral means of body temperature regulation (Gingerich 1994). In hot, tropical climates, peak activity occurs in the early morning and late afternoon (Diong 1982), or during the night (Sekhar 1998). In temperate regions with pronounced seasonal variation, peak activity times and foraging areas change to take advantage of biologically accommodating temperatures (Belden and Pelton 1975). For example, feral pigs on California's Santa Cruz Island are most active at mid-day during wet winter months, and at night during the warm, dry summer (Van Vuren 1984).
Other Physical Tolerances: Feral pigs will only become established in hot climates if water supplies are adequate to allow survival (Gingerich 1994).
Trophic Mode: Feral pigs are omnivorous. They use their tusks to root through the ground in search of roots, tubers, bulbs, worms, insects, slugs and snails, and other dietary items. Additionally they will consume fallen acorns and other nuts, frogs, lizards and snakes, rodents and other vulnerable mammals, and bird eggs (Lowery 1974, Bratton et al. 1982, Laycock 1984, Baber and Coblentz 1987, Gingerich 1994). Feral pig feeding activity can impact population desities of preferred prey types (Meads et al. 1984).
Feral pigs are highly adaptable and opportunistic in terms of diet, and seasonal dietary shifts occur as food items become either scarce or more abundant. For example, Wood and Roark (1980) note that in South Carolina feral pig populations, acorns and other nuts and fruits make up the bulk of the diet in the fall and winter when they are abundant. In the spring, pigs shift to foliage and herbaceous vegetation, and to tubers and roots in the summer. As a result of these dietary shifts, the degree of destructiveness caused by rooting can also vary by season.
Associated Species: Although adult feral pigs are safe from most predators other than humans, young animals are reportedly vulnerable to eagles and hawks, owls, foxes, and bobcats (Laycock 1984, Gingerich 1994). In south Florida, panthers are capable of taking adult pigs as prey (Gingerich 1994).
Invasion History: The historic native range of S. scrofa encompassed Europe and extended through continental Asia south and east into Malaysia, and into the islands of Sumatra and Java (Ickes et al. 2005). Sus scrofa is now extinct across much of this historic range (Tisdell 1982).
Pigs were among the first mammals to be domesticated by humans, beginning in China some 7,000 years ago and possibly dating further back to 10,000 B.C., in the region that is now Thailand (Nowack 1991). Several millennia of selective breeding have yielded a domesticated animal that is morphologically quite distinct from the wild type from which they derived.
The first introduction to the present-day United States may have been intentional introduction of domesticated hogs to the Hawaiian islands by Polynesians perhaps 1,000 years ago (Nowack 1991).
The first introduction of domestic hogs to the continental US is historically documented. A vessel captained by the Spanish explorer Hernando De Soto and carrying domestic hogs destined for the New World landed on the Gulf Coast in 1539 (Lowery 1974, Gingerich 1994). Intentional or accidental release of animals derived from these stocks likely represent the source of the first feral pig populations in the continental U.S. and in the Gulf and southeast regions.
Feral pigs currently found within the United States represent a combination of descendant lines of European wild boars originally released for sport hunting purposes and feral animals derived from escaped domestic pigs. These readily interbreed where they co-occur (Whitaker 1988). The greater the percentage of wild boar a feral pig contains, the more it will resemble the wild type in appearance, typically bearing a bristly coat and mane, a straight tail, and impressive tusks (Whitaker 1988). Gingerich (1994) suggests that Florida's wild hogs may represent an amalgam of lines derived from Spanish and Russian wild boars, European hunting stock, and escaped domestic hogs.
Potential to Compete with Natives: The feeding activities of feral pigs may preempt dietary resources from co-occurring animal populations. More importantly, the omnivorous nature and, particularly, the destructive rooting habits of feral pigs make them particularly troublesome invaders.
Rooting digs up and overturns sizable patches of earth, destroys vegetation and seed banks, and exposes tree roots. Soil nutrient leaching is accelerated (Kotanen 1994, Singer et al. 1984, Arrington et al. 1999). Ground-nesting birds and other species may be negatively impacted. Elsewhere in the United States where feral pigs occur, species such as northern short-tailed shrews (Blarina brevicauda), southern red-backed voles (Cleithronomys gapperi), and red-cheeked salamander (Plethodon jordani) are deemed at risk (Laylock 1984, Singer et al. 1984). Endemic herbaceous vegetation such as Clingman's hedgenettle (Stachys clingmanii) and Virginia chain fern (Woodwardia virginiana) may also be impacted (Bratton et al. 1982).
In Hawaii, feral pigs kill several native tree species (by felling or debarking them) in pursuit of native tree ferns that are a dietary staple (Diong 1982). On Santiago Island in Ecuador, egg predation by feral pigs has reduced giant tortoise and sea turtle population numbers (MacFarland et al. 1974, Green and Ortiz 1982, Coblentz and Baber 1987).
Where feral pigs occur in association with wetlands and coastal marshes, pig foraging may add to the loss of these already imperiled habitats. There is at least some indication, however, that plant diversity in some instances may actually increase on localized scales in response to disturbance by pigs, e.g., if pioneering species move into areas that have been upturned by rooting pigs (Arrinton et al. 1999, Ford and Grace 1998).
Baber and Coblentz (1987) indicate that feral pigs represent the most successful non-native large mammal in the United States.
Possible Economic Consequences of Invasion: Landowners and farmers regularly report damage and loss due to feral pig activity. Delicate food crops like corn, oats, wheat and soybeans are vulnerable, as are young trees planted in silviculture operations. Home gardens often suffer damage from these animals. A 1998 study by Frederick that surveyed 40 California counties estimated economic loss resulting from pig rooting at $1.73 million (Frederick 1998).
Natural habitats are also susceptible to damage from feral pig populations. A study by Singer et al. (1984) monitoring feral pigs within Great Smoky Mountains National Park reports that the destructive foraging of these invaders exposed several thousand tree roots per hectare, reduced plant cover by as much as 80%, and increased bare ground by nearly 90%. Forest litter and soil bulk density were also greatly reduced while erosion and nutrient loss from the forest floor to receiving river waters was doubled (see also Peine and Farmer 1990). Habitat alteration may include loss of native vegetation and spread of opportunistic weeds into newly disturbed areas.
Feral pigs represent a potential source of disease. They carry pseudorabies, (Aujeszky's disease, porcine herpesvirus 1), a viral swine disease of considerable economic importance to the hog industry. Cattle are susceptible as secondary hosts, and infection results in the cattle disease known as mad itch. Rats, dogs, and horses are also known secondary hosts, as are populations of wild animals such as panthers (Fenner et al. 1993, Gingerich 1994).
Feral pigs are also a source of trichinosis and swine brucellosis, which is potentially fatal in humans (Gingerich 1994). Leptospirosis, foot-and-mouth disease, Japanese encephalitis and the parasite Toxoplasma gondii are other disease agents harbored by feral pigs (Tolleson et al. 1995, Hampton et al. 2004, Gauss et al. 2005).
Newly emerging evidence has also implicated feral pigs in a recent (2006) outbreak of E. coli spinach contamination in California that killed at least three people and caused illness in at least 200. The proposed infection pathway suggests that feral pigs transmitted the pathogenic E. coli strain to spinach fields from adjacent cattle pastures. Samples from cow manure in the pastures tested positive for the same bacterial strain responsible for the disease outbreak. The World Conservation Union's Invasive Species Specialist Group (ISSG) lists feral pigs as among "100 of the world's worst invasive alien species" and recognizes them as potentially major drivers of extinction and ecosystem change.
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