ERIC C. HELLGREN, Caesar Kleberg Wildlife Research Institute,
Campus Box 218, Texas A&I University, Kingsville, TX 78363
Abstract: Feral hog (Sus scrofa) abundance and distribution have been increasing in Texas in recent years, yet information on hog biology and population dynamics has not kept pace. I review preliminary research on hogs conducted in post oak savannah, gulf coastal prairie, and south Texas plains ecoregions. Range and habitat use, food habits, nutrition, and disease status of feral swine in Texas appear to follow patterns observed elsewhere in North America. Reproductive and demographic data are urgently needed to understand growth rates and to properly manage hog populations in the state.
Free-ranging hogs, including those of domestic origin, wild origin (European wild boar), and hybrids, heretofore termed feral hogs, of the Old World family Suidae are distributed in 20 states, primarily in the Southeast (Mayer and Brisbin 1991). In Texas, the distribution is primarily in the eastern half of the state (Synatzske 1979). Abundance and distribution of this exotic species have been increasing in Texas in recent years, and concomitantly, scientific and management interest has also increased. However, data on biology and population characteristics of feral hogs in Texas are very scarce. Preliminary research has been conducted in 3 ecoregions of the state (Figure 1): post-oak savannah (Kroll 1986, Yarrow and Kroll 1989), gulf coastal prairie (Springer 1977, Ilse and Hellgren 1991 and unpubl. data), and south Texas Plains (eastern: Everitt and Gonzalez 1980, Mansouri and DeYoung 1987; western: Ellisor 1973, Taylor 1991 and unpubl. data). However, a quick glance at the Literature Cited section of this manuscript will illustrate how few published data on feral hog biology in Texas exist. My objectives in this paper are to review current knowledge on feral hog biology in Texas, compare hog biology among regions within Texas, and review published data from introduced hog populations in ecologically similar habitats in other areas.
Spatial use of feral hogs in Texas has been described by several workers. Though slim, available data suggest small hog home ranges in coastal habitats and larger ranges in the western Rio Grande Plains and the post oak savannah. For instance, in the eastern Rio Grande Plains, Mansouri and DeYoung (1987) reported that annual home range sizes of female hogs averaged 2.80 km2 (n = 6). Sows with supplemental feed within their ranges used less area (2.06 km2; n = 5) than sows without supplementation (2.94 km2; n = 3). However, only two sows were monitored with and without supplementation. Ilse (unpubl. data) reported winter ranges for adult female hogs to average 1.8 km2 (n = 4) in a study site in the gulf coastal prairie. These ranges are similar in size to other North American feral swine range size estimates, which vary from 1 to 5 km2 (Kurz and Marchinton 1972, Wood and Brennemann 1980, Singer et al. 1981, Baber and Coblentz 1986, Sterner 1990)
However, hog ranges may be as large as 50 km2 (Barrett 1978). Two marked hogs (1M, 1F) that were repeatedly observed in the western Rio Grande Plains covered areas of 26.5 and 25.5 km2, respectively (Ellisor 1973). Animals in this study area were observed traveling up to 8 km to cultivated grain fields or supplemental feeders. Similarly, Springer (1977) reported animals travelling 5-7.5 km to cultivated grain fields to feed.
Kroll (1986) reported that range size for hogs from post oak savannah habitat did not vary (P< 0.05) by season, although fall (33.3 km2, n = 4) and winter (30.0 km2, n = 5) ranges of male hogs appeared to be larger than summer ranges (3.2 km2, n = 2) in one year. Kroll (1986) suggested that larger fall and winter ranges resulted from a decrease in food availability, although concomitant food habits data (Yarrow and Kroll 1989), which documented the preponderance of acorns (Quercus spp.) in the diet in these seasons, suggested that food distribution was more clumped in fall and winter and forced individual hogs to travel more in search of food. In addition, fall movements (measured as distance between consecutive radio-locations) were greater (P<0.05) than during other seasons in one year. Fall or winter breeding may also be play a role in seasonal range size variation, with males having larger ranges (P=0.051) than females in winter of one year. Kroll (1986) did not report annual ranges.
Activity patterns of feral swine are influenced by temperature. They are primarily diurnal in the cool months and become more crepuscular or nocturnal in the warmer months (Ellisor and Harwell 1969, Kurz and Marchinton 1972, Barrett 1978, Singer et al. 1981). In south Texas, this annual rhythm in activity patterns is exhibited (Ellisor 1973).
Social organization and group composition of hogs has not been well-described in Texas. Feral hog males are usually solitary (Sweeney and Sweeney 1982), while females may form sounders that exhibit group foraging and predator defense (Eisenberg 1981).
Synatzske (1979) stated that, in Texas, feral hogs prefer moist habitats when available, with hog distribution limited primarily to bottomland areas. He also stated that hogs will concentrate in areas of mast-producing trees. Data from telemetry studies support these contentions. Kroll (1986) reported that hogs used shrub marsh, sphagnum bog and bottomland hardwood sites greater than their availability throughout the year in post oak savannah. Open areas and upland hardwood habitats were generally used less than their availability. Bottomland hardwood mast production was greater than that of uplands. Mansouri and DeYoung (1987) suggested that hogs seemed to prefer dense stands of oak in the Rio Grande Plains. In the Rio Grande Plains, hogs also use mesquite-blackbrush, mesquite-liveoak, and coastal prairie vegetative communities (Synatzske 1979). As mentioned above, hogs will move large distance to agricultural grain fields.
Patterns in habitat use of feral swine in Texas appear to be very similar to other North American populations. In the southeastern United States, feral hogs use marshes, river swamps, and associated bottomlands, with seasonal movements to uplands linked to food availability (Kurz and Marchinton 1972, Wood and Brennemann 1980). In western habitats, dense thickets of oak (Quercus spp.) and brush are preferred (Barrett 1978).
Food Habitats and Nutrition
Feral hog food habits have been studied in all ecoregions of Texas mentioned above. These data show some basic similarities in food use between hogs in Texas and those in other regions of North America. Briefly, feral hogs feed on grasses and forbs in the spring, fruits in summer and fall, and roots, tubers, and invertebrates throughout the year (Springer 1977, Wood and Roark 1980, Sweeney and Sweeney 1982, Baber and Coblentz 1987). Composition changes seasonally and with food availability. In the western Rio Grande Plains, grasses, forbs, root/tubers, and corn each composed >14% of stomach contents on an annual basis (Hellgren and Holzem 1992). Cactus fruit and hard mast were 10.4 and 9.0% of the annual diet, respectively. Differences between years were related to changes in precipitation and soil moisture (Everitt and Alaniz 1980).
Seasonal changes in hog diets were similar among regions within Texas. Spring diets were dominated by vegetative matter, particularly grasses, forbs, and roots/tubers (Springer 1977, Kroll 1986, Yarrow and Kroll 1989, Hellgren and Holzem 1992). Yarrow and Kroll (1989) reported high use of animal matter over 2 spring seasons in the post oak savannah. Summer diets shift to available soft and hard mast, with species composition varying among regions. In the Rio Grande Plains, major mast species consumed were prickly pear (Opuntia spp.), mesquite (Prosopis glandulosa), and guajillo (Acacia berlandieri) (Ellisor 1973, Hellgren and Holzem 1992). In the gulf coastal prairie and post oak savannah, grapes (Vitis spp.) were the major fruit consumed (Springer 1977, Kroll 1986, Yarrow and Kroll 1989). These results were consistent across years. Other food categories of importance included roots/tubers and animal matter (both vertebrate and invertebrate).
Fall diets of hogs in Texas were dominated by grapes, acorns (Quercus spp.) and invertebrates in the gulf coast and post oak savannah regions (Springer 1977, Kroll 1986, Yarrow and Kroll 1989); and by roots/tubers, corn, and herbaceous material in the western Rio Grande Plains, where oak trees are less abundant (Everitt and Alaniz 1980, Hellgren and Hozem 1992). Mast composed nearly 25% of the fall diet in 1 of 3 years in the Rio Grande Plains, with acorns and fruits of persimmon (Diospyros texana) and huisache (Acacia farnesiana) being the major species consumed (Hellgren and Holzem 1992). Winter hog diets in Texas were also dominated by herbaceous material (grasses, forbs) and agricultural grains in the Rio Grande Plains (Ellisor 1973, Everitt and Alaniz 1980, Hellgren and Holzem 1992). Springer (1977) reported winter use of primarily acorns and herbaceous material, while Kroll (1986) and Yarrow and Kroll (1989) documented the use of animal matter and forbs.
Animal matter generally was a small (<5%) to moderate (15-25%) but seasonally variable component of hog diets, except in the post oak savannah study (Kroll 1986, Yarrow and Kroll 1989), where animal matter composed >25% of the diet in 4 of 8 study seasons. The post oak savannah data are extremely biased, however. For example, in winter 1983, hog represented 30% of the diet by volume, but only occurred in 1 of 20 stomachs (5% frequency of occurrence). Similarly, armadillo (Dasypus novemcinctus) occurred in only 1 of 4 stomachs (25% frequency) in spring 1983, yet represented 38% of the overall diet. The authors determined seasonal composition of diets based on total gross stomach material examined (Kroll 1986), rather than weighting each stomach equally. Major invertebrate foods of Texan hogs were lepidopteran (Hellgren and Holzem 1992) and march fly (Springer 1977) larvae, and earthworms (Lumbricus spp.; Kroll 1986). Major vertebrate foods consumed were hog, armadillo, white-tailed deer (Odocoileus virginianus; including fawns), and some birds, reptiles, and amphibians.
Nutritional analyses of hog stomachs in the post oak savannah revealed that crude protein content of stomachs varied between seasons from 7.3% in summer to 12.6% in spring. Acid detergent fiber (ADF) content of stomachs ranged from 51.9% in summer to 36.6% in spring (Yarrow and Kroll 1989). The authors suggested that nutrition was poorest in summer and best in spring. However, metabolizable energy of diets was not estimated and may not correspond with the above assessment. Additionally, animal matter reportedly composed >50% of the diet in a winter and spring season, yet stomach protein content did not exceed 12.6 % (Yarrow and Kroll 1989). Protein content of stomachs containing that much animal matter should have been much greater during these seasons. As discussed above, however, the food habits data are extremely biased, and the relatively low crude protein content of stomachs is a function of large amounts of low protein fruits, mast, and roots. Finally, treatment of the stomach samples between collection and nutritional analyses was not described. Nevertheless, protein and ADF data were similar to those reported by Baber and Coblentz (1987) for feral hogs on Sanata Catalina Island.
Diseases and Parasites
Springer (1977) reviewed literature on parasites of feral hogs in the coastal population on Aransas National Wildlife Refuge. This population was heavily parasitized, with 100% of animals infected with swine kidney worm (Stephanura dentatus), with cases of necrosis of the lungs and liver and associated bacterial infections. Other internal parasites found included lungworms (Metastrongylus spp.), roundworms (Ascaris suum), hookworms (Globocephalus urosubulatus), and various stomach worms.
A serologic survey of 10 feral hog populations in Texas revealed that pseudorabies virus was found in swine in 7 populations, antibodies to leptospirosis were discovered in all 10 populations, and swine brucellosis (Brucella suis) was isolated from 4 individuals from 2 populations (Corn et al. 1986). Swine were negative for a wide variety of other viruses and for incidence of Trichinella spiralis. The authors concluded that feral hogs may act as reservoirs of pseudorabies virus and swine brucellosis, and potentially could infect domestic swine.
Competitive interactions and niche overlap between feral swine (Sus scrofa) and other vertebrate herbivores have not been adequately researched. Sweeney and Sweeney (1982) emphasized the need to document the impact of feral swine on native flora and fauna. Previous work has focused on dietary overlap between feral swine and white-tailed deer (Odocoileus virginanus; Springer 1975, Wood and Barrett 1979, Yarrow and Kroll 1989), but has failed to indicate whether competition for a limiting resource is occurring. In Texas, Yarrow and Kroll (1989) suggested that during years of low mast availability, deer populations may be seriously impacted by competition with hogs for scarce food. Empirical data on this topic are lacking.
Potential competition of feral hogs with cattle and collared peccaries (Tayassu tajacu; Everitt et al. 1981) also has been inferred from dietary overlap. Competition between hogs and peccaries, a natural area of study because of their ecological similarities, has not been described. Ellisor (1973) and Ellisor and Harwell (1979) stated that competition for space between collared peccary and feral hogs is intense, based on observations of interspecific aggression. However, they provide no data on the degree of range overlap or separation between the two species.
Reproduction and Population Characteristics
Demographic characteristics are arguably the most important data needed to develop management strategies concerning wild species. Population and reproductive data on feral hogs in Texas is extremely limited. Springer (1977) reported that the main farrowing season in the Gulf Coast Prairies was January-May (17 of 19 observed pregnancies). Average litter size in the Gulf Coast was 4.2, with a maximum of 8. Two instances of 2 litters in one year were reported. Reproduction in sows younger than 14 months was uncommon (<10%). Springer (1977) suggested that kidney worm infestation may negatively impact reproduction. In the western Rio Grande Plains, January-March (14/28; 50%) and June/July (8/28; 28%) were the main birthing periods (Taylor, unpubl. data). In utero litter sizes averaged 6.6 (n=16) for hogs >2 yr-old, 5.0 (n=5) for hogs 1-2 yr-old, and 4.8 (n=6) for hogs < 1 yr-old; and prenatal losses averaged 23% (Taylor, unpubl. data). In the same region, eleven observed litters had a mean size of 4.6, with only 55% survival to age 3 months (Ellisor 1973). This figure is probably an underestimate because of mortality to piglets prior to observation of individual litters.
No reports on density or survival of feral hogs in Texas could be found. Such data are essential to understand dynamics of and to manage for hog populations. Density estimates in semiarid regions of California (30-65 cm annual precipitation), which are comparable to the Rio Grande Plains and the lower coastal prairie of Texas, have ranged from 3.2-6 hogs/km2 in mainland oak woodland in central California (Barrett 1978, Schauss et al. 1990) to 14-34 hogs/km2 in Mediterranean-climate islands west of southern California (Baber and Coblentz 1986, Sterner 1990). In the southern Appalachian Mountains, densities have been estimated at 2-9 animals/km2 (Tate 1984). Hone (1990), in a tropical area (140 cm annual precipitation) in northern Australia, reported seasonal changes in densities from 0-10.9 hogs/km2 between seasons within habitats. These changes occurred in response to seasonal flooding and food availability, and emphasized the inclusion of scale considerations (both time and space) on density estimation.
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