Distinguishing Feral Hogs from Introduced Wild Boar

and Their Hybrids: A Review of Past and Present Efforts

JOHN J. MAYER, Westinghouse Savannah River Company,
P.O. Box 616, South Carolina, 29802

I. LEHR BRISBIN, JR., Savannah River Ecology Laboratory,
P.O. Drawer E, Aiken, South Carolina 29802


Abstract: Three types of introduced wild swine have been found in the United States: feral hogs, Eurasian wild boar, and hybrids between these two types. Attempts have been made over the past 50 years to determine identifying characteristics that will reliably distinguish among the three types. Such attempts have been based upon morphological, cytological, electrophoretic, molecular genetic, and behavioral characters, and are reviewed here. Identifications based on morphology have been partially successful. Cytological studies have had only limited success. Electrophoretic analyses have had some success in discriminating between localized populations of the three forms. Molecular genetic differentiation of the three forms based on DNA may have some potential, but research in this area is only beginning. Behavioral differences have been anecdotal and unsubstantiated. Based on what is now known, the identification of completely reliable defining characteristics for the three forms has yet to be achieved.


Introduction

Wild swine (Sus scrofa) have occurred in the United States in three non-native introduced forms: Eurasian wild boar (often referred to as “Russian boar”); feral hogs (i.e., wild swine solely of domestic ancestry); and wild boar/feral hog hybrids or crosses (Wood and Barrett 1979, Tisdell 1982, Mayer and Brisbin 1991). From legal and scientific perspectives, there is both an interest and a need to be able to differentiate among these three wild forms of Sus scrofa. However, because these three forms represent a complex hybrid situation, clear means of identification do not always exist (Mayer and Brisbin 1991). In part because of the perpetuation of early misinformation, the identification of precisely defining characteristics for the three forms has yet to be achieved. This paper is a review of where such efforts currently stand. Within the different categories of characteristics, the following are presented: (1) what is currently known; (2) some new information; and (3) what other methods are being explored at present to resolve this identification issue.
History of Wild Swine in the United States

Historically, wild populations of swine in the United States were derived from two types of founding stock: (1) Eurasian wild boar; and (2) domestic/captive-feral swine (Wood and Barrett 1979, Mayer and Brisbin 1991). The introduced wild boar were released in specific areas to provide huntable game. Domestic/captive-feral swine either were released under open range conditions to fend for themselves or escaped confinement and became wild-living. In places where the two forms were sympatric, hybridization occurred. Sportsman interest in wild swine as game animals has led to countless subsequent releases and relocations of various combinations of these forms with the intent of either establishing new populations or “improving” existing populations (Mayer and Brisbin 1991). Prior to the mid 1980s, all of the wild boar populations in this country had been hybridized to some extent with the feral hog genotype. Beginning in 1986, there have been at least two importations of pure Eurasian wild boar into the United States. Although none of these animals has been documented as having been used to start a wild population, given the interest in pure wild boar as big game and the history of such stocking efforts in this country, the potential for such an occurrence is almost certain. Therefore, it currently is possible to encounter wild populations that might vary from pure feral hog to pure Eurasian wild boar in composition.

Review of Identifying Characteristics

Early Descriptive Efforts

Aside from brief subjective descriptions and anecdotal accounts, few of the early attempts to distinguish between the types of wild swine in the United States were empirically based. In the late 1930s, Stegeman (1938) described both the physical and behavioral characteristics of the introduced wild boar in the Cherokee National Forest in Tennessee. He provided some comparisons with the local free-ranging domestic or feral hogs. In general, these comparisons were subjective (e.g., “narrower than…” and “taller than…”) and would necessitate knowledgeable assessments to identify a type of wild Sus scrofa. Shaw (1941) reviewed Stegeman’s list of descriptive characteristics and stated that the longitudinal striping in the young was the determining feature in distinguishing wild boar from free-ranging domestics. Jones (1959) elaborated on Stegeman’s (1938) descriptions, but the comparisons remained qualitative and subjective. Henry (1969) reported three characteristics as being indicative of at least partial wild boar ancestry in a wild swine population. These were a striped pattern in the juvenile pelage, split tips on the bristles, and a diploid chromosome number of 36. Upon further review, however, these characteristics were shown either to be incorrect or not very accurate (Marchinton et al. 1974, Mayer and Brisbin 1991). Conley et al. (1972) restated the identifying characteristics presented by both Stegeman (1938) and Henry (1969), citing the karyotype and juvenile striping as being the most useful. To further complicate an already confusing situation, popular sport hunting periodicals have added further subjective or incorrect information regarding wild swine identification. For example, in wild swine hunting articles during the 1960s and 1970s it was often stated that the coat coloration of the pure wild boar was solid black.

Morphological Characters

Gross external and skeletal morphological parameters have been the most common area used by researchers to attempt to define identifying characteristics. This is at least in part because of the relative ease of data collection using these structures compared to that of features requiring the use of more complex laboratory methods. Morphological criteria have included a variety of structures and parameters including cranial and mandibular measurements, external body measurements, coat coloration patterns, and hair measurements and coloration. The utility of these different criteria are summarized in Table 1 and discussed in the following paragraphs.

Skull - Characteristics of the skull, especially size and shape of the cranial bones, have long been recognized by taxonomists as one of the best means for classifying vertebrates. It is not surprising that skull characteristics also are some of the most reliable means for identifying the three types of wild swine. Using canonical variate analyses, Mayer and Brisbin (1991) showed that known groups of pure Eurasian wild boar, pure feral hogs, hybrids, and domestic swine could be separated with a high degree of resolution using seven cranial measurements in adult males and three cranial measurements in adult females (Fig. 1, Table 2, Fig. 2). Group separation was found to decrease as the age of animals being investigated decreased and to be less reliable in females than in males of a comparable age (Mayer and Brisbin 1991). Overall, mandibles were less distinctive among the groups, and therefore, were of less use in identifying the type of Sus scrofa. In an earlier study, Barrett (1971) compared skull measurements from feral hogs from the Dye Creek Ranch in California with those of wild boar from four locations in northeast Poland and found that the best descrimination was produced by the slopes of regressions between the nasal length and zygomatic breadth of the adults. As in Mayer and Brisbin’s (1991) study, Barrett (1971) also found that the ability to distinguish between the two types decreased in younger animals.

External Body Dimensions - External body dimensions of wild Sus scrofa have been presented in a number of published studies (e.g., Nichols 1962, Henry 1970, Sweeney 1970, Barrett 1971, Hell and Paule 1983, Mayer and Brisbin 1991). However, only one study (Brisbin et al. 1977) has presented an analytical comparison of body measurements between two or more types of wild swine. The comparisons in this study were between long-term and short-term feral hog populations, and did not include any data from hybrids or pure wild boar. To date, only subjective differences have been reported for external body dimensions in the three types of wild Sus scrofa.

Table 1: Comparison of the identifying morphological characters used to determine the three primary types of wild swine ound in the United States. This comparison is for adult animals only.
Morphological Character Eurasian Wild Boar Wild Boar/Feral Hog Hybrids Feral Hog
Skull Determined by Multivariate Analysis of Cranial Measurements
External Body Dimensions Determined by Multivariate Analysis of External Body Measurements
Coat Coloration Patterns Light-tipped/brown-base bristles over most of the body with dark brown to black solid-colored pointsa; white-tipped facial pattern (saddle or mouth streak) Include wild boar and feral hog colorations in pure form or combinations of the parental stock patterns Combinations of black, white and red/brown; can include solid or mottled patterns; white points and belting also observed
Bristle Coloration Light to Dark Brown bristles with white to cream/buff tips Solid-colored and light-tipped/dark-based bristles Solid-colored bristles
Underfur Coloration Color variable (e.g., cream to smoke gray) but typically different from the base coloration of the bristles Color variable; color can be the same or different from the bristles in the same area of the pelage Color variable but same as the bristles in the same area of the pelage
Other Morphological Structures No neck wattles or syndactylous digits Neck wattles or syndactylous digits can be present Neck wattles or syndactylous digits can be present
a The “points” include the distal portion of the snout, the distal half of the legs, the distal portion of the tail, and the entire pinnae of the ears.
Table 2. Cranial measurements and the coefficients and constants used in calculating the first and second canonical variables for plotting adult specimens of wild swine. Data taken from Mayer and Brsibin (1991).
Cranial Measurement Canonical Coefficient Product and Canonical
Variable Calculation
Males
Occipitonasal Length times 0.10438 = (Product)
Zygomatic Width times -0.03494 = (Product)
Rostral Length times -0.04312 = (Product)
Palatal Length times -0.02899 = (Product)
Premaxillary Rostral Width times -0.01726 = (Product)
Palatal Constriction times -0.00719 = (Product)
Supraoccipital Constriction times -0.03911 = (Product)
(Sum of Products)
Constant = -10.63739
= Canonical Variable I
Occipitonasal times -0.07436 = (Product)
Zygomatic Width times 0.04571 = (Product)
Rostral Length times 0.20678 = (Product)
Palatal Length times -0.14397 = (Product)
Premaxillary Rostral Width times -0.12071 = (Product)
Palatal Constriction times 0.04681 = (Product)
Supraoccipital Constriction times -0.02446 = (Product)
(Sum of Products)
Constant = 9.55382
= Canonical Variable II
Females
Occipitonasal Length times 0.05769 = (Product)
Zygomatic Width times -0.06474 = (Product)
Supraoccipital Constriction times -0.04942 = (Product)
(Sum of Products)
Constant = -4.60695
= Canonical Variable I
Occipitonasal Length times 0.02618 = (Product)
Zygomatic Width times 0.03918 = (Product)
Supraoccipital Constriction times 0.01721 = (Product)
(Sum of Products)
Constant = -14.78038
= Canonical Variable II

As a component of this review, six linear external body measurements (Fig. 3) from 211 adult specimens (i.e., completely erupted dentition) of known ancestry (i.e., pure Eurasian wild boar, pure feral hogs or hybrids) were compared using discriminant function analyses (SAS Institute Inc. 1985). These data were obtained from either specimens collected in the field or specimens in private or institutional collections. In these analyses, the sexes were compared separately. The ability to discriminate among specimens was significant (Table 3). The basic statistics of the body measurements that were used are presented in Table 4. Graphs of the two most useful measurements (i.e., hind foot length and snout length) for separating the forms are presented in Fig. 4. The most useful measurements in order of importance in discriminating among the three types of wild swine were hind foot length, snout length, and head-body length. In general, Eurasian wild boar tended to have longer hind foot and snout lengths than either hybrids or feral hogs.

Table 3. Percentage of specimens correctly classified into the type of wild swine to which it was originally assigned. The sample size of each group is in parentheses.
Percent Correctly Classified
Sex Eurasian Wild Boar Wild Boar /
Feral Hog Hybrids
Feral Hog Total
Male 84.0 (25) 69.2 (14) 97.2 (71) 110
Female 95.2 (21) 52.9 (17) 98.4 (63) 101

Coat Coloration Patterns - Detailed comparisons of coat coloration in both adult and juvenile wild Sus scrofa were presented by Mayer and Brisbin (1991). The coloration in pure Eurasian wild boar consists of light brown to black bristles with cream to tan distal tips. The lateral portions of the head and medial dorsal portion of the rostrum are covered with brown to black bristles with white tips. The undersides are lighter. The distal portions of the rostrum, limbs, ears and tail are darker than the rest of the coat, usually dark brown or black, with no light-colored tips. Feral hogs are variable in coat coloration pattern; common patterns include all black, all red/brown, all white, spotted (various combinations of black, white and red/brown), belted (black or red/brown with a white band across the shoulder and forelimbs), and miscellaneous rare domestic patterns (e.g., blue or gray roans). Hybrids can exhibit all of the aforementioned coat coloration patterns and also combinations of the wild boar pattern with the various feral patterns (Mayer and Brisbin 1991).

The striped pattern of juveniles is found in all geographic races of Eurasian wild boar (Martys 1991, Mayer and Brisbin 1991, Mayer 1992). The presence of striped piglets frequently has been cited as means of verifying the existence of wild boar ancestry within a wild swine population (Stegeman 1938, Shaw 1941, Jones 1959, Henry 1969, Conley et al. 1972). Contrary to this widely-held belief, this color pattern is not exclusive to wild boar and can be present in both hybrid and pure feral hog populations (Fig. 5). Because of the potential presence of striping in juveniles of all three types of wild swine, this is not a reliable character for identification (Mayer and Brisbin 1991, Mayer 1992).

Bristles - Differences in bristle or guard hair size, shape and color among the three types of wild swine have been studied extensively (Hansen et al 1972, Feder 1978, Mayer and Brisbin 1991). Bristles of the Eurasian wild boar are the longest and thickest of the three types. Feral hogs have the shortest mean bristle length, while hybrids have the smallest average bristle shaft diameter. Substantial overlap in these measurements exists among the three types (Mayer and Brisbin 1991). Henry (1969) had stated that the bristles of wild boar had split tips while those of domestic (or feral) swine did not. Subsequent studies have shown that all three wild types of Sus scrofa have split-tipped bristles (Marchinton et al. 1974, Feder 1978, Mayer and Brisbin 1991). Bristle coloration in wild boar is primarily brown/black with light tips. However, pure wild boar also have some bristles that are all brown/black, and even a few bristles that are brown/black with a white band and a black tip. Feral hogs have solid colored bristles that are black, red/brown or white. Hybrids can exhibit any of the above bristle coloration patterns (Mayer and Brisbin 1991). Hess et al. (1985) reported that bristles of domestic swine could be distinguished from those wild boar on the basis of internal morphology. However, the wild boar sample used in that study consisted of hairs collected from only one animal.

Table 4. External body dimensions (mm) of adult Eurasian wild boar (EWB), feral hogs (FER), and wild boar/feral hog hybrids (HYB) separated by type of wild Sus scrofa and sex.
Measurement Type Sex N Mean Range SD
Head-Body Length EWB M 25 1434.0 1060-1750 164.4
F 21 1283.9 995-1518 147.3
FER M 71 1372.7 877-1630 143.1
F 63 1287.1 1060-1580 96.8
HYB M 14 1415.1 1334-1593 79.9
F 17 1322.1 1027-1505 137.0
Tail Length EWB M 25 234.4 90- 309 50.3
F 21 227.4 130- 318 48.2
FER M 71 320.7 110- 410 57.7
F 63 304.6 153- 395 44.0
HYB M 14 267.6 178- 368 60.6
F 17 261.4 171- 355 45.0
Hind Foot Length EWB M 25 303.3 244 – 380 34.0
F 21 283.7 212- 333 38.6
FER M 71 282.6 234- 340 21.5
F 63 265.9 219- 300 17.7
HYB M 14 280.6 250- 318 21.7
F 17 266.1 235- 305 22.3
Ear Length EWB M 25 126.0 92- 165 17.2
F 21 117.4 86- 160 19.5
FER M 71 168.1 114- 250 23.9
F 63 158.0 126- 204 20.2
HYB M 14 145.3 120- 170 16.8
F 17 141.4 108- 178 15.6
Snout Length EWB M 25 288.4 200- 368 39.8
F 21 262.8 190- 305 38.7
FER M 71 232.5 160- 290 19.2
F 63 219.9 180- 260 16.5
HYB M 14 265.2 240- 298 15.4
F 17 233.9 190- 270 23.4
Shoulder Height EWB M 25 858.1 570-1060 160.5
F 21 747.2 537-1000 145.5
FER M 71 762.1 600- 975 87.2
F 63 683.4 600- 770 36.6
HYB M 14 726.7 680- 822 46.4
F 17 677.0 570- 765 64.8

Underfur - Curly, wool-like underfur can be found in any of the three types of wild swine (Mayer and Brisbin 1991). In wild boar, the underfur is variable in color, ranging from smoke gray to dark brown; in almost all cases, it is lighter than the base color of the overlying bristles. Feral hogs have underfur that is the color of the bristles found in the same area of the pelage. The underfur in hybrids varies from white/smoke gray to black, and can be the same or different in color from the overlying bristles (Mayer and Brisbin 1991).

Other Morphological Structures - Two unusual morphological features that have been found in Sus scrofa are neck wattles (or “waddles”) and syndactylous (or “mule-footed”) digits. However, only hybrids and feral/domestic hogs have been reported as having these structures (Mayer and Brisbin 1991).

Cytological Characters

The ancestral or basic diploid number of chromosomes (2n) within the genus Sus is 38 (Bosma et al. 1991). This number is variable within Sus scrofa, ranging from 36 to the ancestral 38 (Bosma et al. 1991). This polymorphism in the diploid number is the result of Robertsonian translocations (McFee et al. 1966). These translocations reportedly can be either of two types, one involving chromosomes 16 and 17, or another involving chromosomes 15 and 17 (Bosma et al. 1991). Rary et al. (1968) reported cytogenetic differences to be more useful than morphological characters for identifying the three forms of wild swine. However, within the geographic range of Eurasian wild boar, there appears to be a shift from 2n=38 in eastern Asia to 2n=36 in the western Europe (Bosma et al. 1991), and all three diploid numbers have been found in some populations (Tikhonov and Troshina 1974, Bosma 1976, Arroyo Nombela et al. 1990). The diploid number of chromosomes in both domestic and feral swine is 38 (Barrett 1971, Bosma et al. 1991, Mayer and Brisbin 1991). Wild boar/feral hog hybrids can have 36, 37 or 38 chromosomes (Mayer and Brisbin 1991). An animal with 2n=38 could therefore be any one of the three forms, and one having 2n=37 or 36 could be either a wild boar or a hybrid, and differences in karyotype are thus not nearly as useful as were initially reported.

Staining for G-, R- and C-bands and nucleolar organizer regions (NOR) has been conducted on domestic/feral swine, Eurasian wild boar, and hybrids (Bosma 1976, Popescu et al. 1980, Arroyo Nombela et al. 1990, Bosma et al. 1991). To date, the large degree of similarity found in these banding patterns and the karyotypic locations of NORs has been of little value in distinguishing the three forms.

Molecular Characters

There have been three comparative studies of biochemical genetic variation within components of the wild Sus scrofa hybrid complex. Using horizontal starch gel electrophoresis to determine levels of genetic variability, Smith et al. (1980) compared the allozyme patterns of 20 proteins from feral hogs from the Savannah River Site (SRS), South Carolina, feral hogs from Ossabaw Island, Georgia, hybrids from the Great Smoky Mountains National Park, Tennessee, and local slaughter house domestic swine from Jackson, South Carolina. Although the four samples differed in the amount of genetic variability that was observed, no attempt was made to provide a means of identifying the type or population of Sus Scrofa used in that study. Hartl and Csaikl (1987), also using starch gel electrophoresis, found a high percentage of polymorphic loci within their wild boar sample. The average heterozygosity found by Hartl and Csaikl (1987) also was higher than the values reported by Smith et al. (1980). Some potentially diagnostic alleles were reported between the wild boar and domestic swine samples (Hartl and Csaikl 1987). However, the sample of domestic swine consisted of only seven animals from the same farm. Based on the electrophoretic analysis of 33 loci, Randi, et al. (1989) were able to separate pure wild boar from both domestic swine and hybrids in a dendrogram computed from Nei’s D values. However, neither the domestic nor the hybrid samples were identifiable as such. Although there appears to be a potential for electrophoretic analyses to be useful in the identification of the types of wild swine, current methodologies only operate at the population level and to date have not given consistent differences among the wild types. The use of a standardized suite of allozymes and protocols for their detection may permit a better resolution of the three forms.

Blood samples collected from pure European wild boar (S. s. scrofa), wild boar/feral hog hybrids, and feral hogs currently are being used to assess the differences among the three forms using mitochondrial DNA. The results of these analyses are not complete and the usefulness of this technique as an identification tool remains uncertain. Researchers in Italy also are beginning similar comparative studies using DNA analytical methods (E. M. Randi, pers. commun.).

Behavioral Characters

Several purported behavioral differences have been noted among the three forms (J. W. Reiner pers. commun., Stegeman 1938, Shaw 1941, Jones 1959, Conley et al. 1972). For example, pure wild boar are reported to have a more upright or digitigrade stance compared to hybrids or feral hogs, which supposedly walk flatter on their digits. Wild boar also reportedly differ from feral hogs in their feeding habits by rooting in straighter, shallower, but more scattered locations (Stegeman 1938). Pure wild boar also are supposed to run with their tails held in an upright or vertical orientation (i.e., similar to that documented for wart hogs, Phacochoerus ethiopius), while feral hogs and hybrids are variable in their manner in which their tails are held when running. Wild boar also are reported to make narrower trails than feral hogs, and will traverse steep slopes, obstacles in their path, and logs over streams, which feral hogs supposedly will not do (Stegeman 1938, Shaw 1941, Jones 1959, Conley et al. 1972). At present, however, these differences are all qualitative and remain anecdotal and unsubstantiated. They therefore are presently of little value for the identification of types of wild swine.

Conclusions

The ability to distinguish among the three forms of wild Sus scrofa in the United States has improved over the past 50 years, but still remains somewhat insufficient. Morphological characters in adult animals can enable one to determine if a specimen in question resembles a pure Eurasian wild boar or feral hog, but only hybrids exhibiting a mixture of both wild boar and feral characters can be absolutely identified as such. Based solely on morphological characters, animals which appear to be either pure wild boar or feral hogs could in fact be from the respective parental ends of the hybrid spectrum. Cytological methods of identification are even less reliable than the morphological approach. Genotypic and molecular genetic methods may have promise with regards to being able to resolve identification problems, but both require more work to fully assess the capabilities of these methods.

Literature Cited

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Barrett, R. H. 1971. Ecology of the feral hog in Tehama County, California. Ph.D. Dissertation. Univ. California, Berkeley. 368 pp.

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APPENDIX

The following are descriptions of the cranial and external body measurements illustrated in Figs. 1 and 3, respectively. The descriptions were taken from Mayer and Brisbin (1991).

The cranial measurements depicted in Fig. 1 were:

Occipitonasal Length – from the anteriormost edge of the nasal bones to the posteriormost projection of the occipital condyles.

Zygomatic Width – the greatest distance between the outer margins of the zygomatic arches.

Rostral Length – from the midpoint between the lacrimal notches to the anteriormost edge of the nasal bones.

Palatal Length – from the anteriormost ventral edge of the premaxillae to the posteriormost margin of the palate at the midline.

Premaxillary Rostral Width – the greatest breadth across the premaxillae above the upper canine alveolar buttresses.

Palatal Constriction – the least breadth across the palate at the lateral junction of the premaxillae and maxillae.

Supraoccipital Constriction – the least distance between the lateral edges of the supraoccipital.

The external body measurements depicted in Fig. 3 were:

Head-Body Length – head and body length in a straight line with the animal on its back.

Tail Length – from the base to the distal end of the last vertebrae.

Hind Foot Length – from the proximal end of calcaneus to the distal end of the longest hoof.

Shoulder Height – the straight-line distance from the dorsal midline of the shoulder to the base of the foot with forelimb positioned in a normal upright posture.

Ear Length – from the notch to the farthest point of the distal edge.

Snout Length – from the distalmost dorsal point of the rhinarial pad to a point equidistant between the eyes.