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More than 30 species of exotic freeranging mammals have become established in the United States since European colonization (De Vos et al., 1956; McKnight, 1964; Roots, 1976). These species often become serious economic pests and can have grave consequences on their host environments (Cottam, 1956; De Vos et al., 1956; Mayer and Brisbin, 1991). True wild pigs (Suidae) are not native to the United States. Only the collared peccary (Tayassu tajacu; Tayassuidae) that inhabits the southwestern and south-central parts of the United States is native (Mayer and Brandt, 1982; Mayer and Wetzel, 1986). Feral swine (Sus scrofa) in the United States have originated from varieties of domestic swine, Eurasian wild boar, and their hybrids (Jones, 1959; Wood and Lynn, 1977; Rary et al., 1968; Mayer and Brisbin, 1991). Domestic swine were introduced to the United States as early as 750-1000 A.D. during the settlement of the Hawaiian Islands (Towne and Wentworth, 1950; Joesting, 1972; Smith and Diong, 1977). Christopher Columbus introduced domestic swine to the West Indies during the 1400s, where they proliferated and became pests. In the 1500s, Spanish explorers, such as DeSoto and Cortez, were the first to bring domestic swine to the United States mainland (Towne and Wentworth, 1950; Beldon and Frankenberger, 1977). By the 1960s, domestic swine and Eurasian wild boar were established in >20 states (McKnight, 1964). Swine introductions have intentionally or accidentally occurred by a variety of means, including: 1) translocation to establish populations for hunting, 2) escapees from shooting preserves or confinement operations, 3) avoidance of capture by domestic pigs in free-ranging livestock operations, 4) abandonment by their owners, and 5) dispersal from established feral populations (Gipson et al., 1997; Witmer et al., 2004).
Feral swine are the most abundant free-ranging, exotic ungulate in the United States (McKnight, 1964; Decker, 1978) and have become widespread because of their reproductive potential and adaptability to a wide range of habitats. Like domestic swine, litter size depends on the sow’s age, nutrition, and time of year. Feral swine are capable of producing two litters per year with average litter size varying from 4.2 to 7.5 piglets (Taylor et al., 1998), but up to 10 piglets can be born during ideal conditions (Conquenot et al., 1996). Mayer and Brisbin (1991) and Mackey (1992) report feral swine populations in 23 states. A Southeastern Cooperative Disease Study (1994) and Nettles (1997) point out an additional 16 states with feral swine populations. An estimated population of 4 million feral swine currently occur in the United States (Pimentel et al., 2000) with the largest populations inhabiting Texas (1 to 1.5 million; Pimentel et al., 2000), Florida (>500,000; Layne, 1997), Hawaii (80,000; Mayer and Brisbin, 1991), and California (70,000; Barrett, 1993). Since 1965, feral swine have expanded their range from 15 (26%) to 45 (78%) of the 58 California counties (Frederick, 1998). Feral swine populations continue to increase (Gipson et al., 1997) because they possess the greatest reproductive potential of all free-ranging, large mammals in the United States (Wood and Barrett, 1979; Hellgren, 1999) and because of the absence of large native predators (e.g., mountain lion (Felis concolor) and wolves (Canis lupus) over much of the area occupied by feral swine. In southwest Florida where feral swine and a large predator coexist, feral swine is the most common food item (42%) in Florida panther (F. c. coryi) scats (Maehr et al., 1990), which may suggest that the presence of a large predator helps regulate feral swine density and associated damage.