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Wild pig (Sus scrofa) reproduction and diet in the rolling plains of Texas.

Wild pigs (Sus scrofa) are a species of economic and ecological concern in North America where they are nonnative and invasive. Their negative impacts on native ecosystems are well documented (Bratton, 1975; Singer et al., 1984; Synatzske, 1993; Massei and Genov, 2004). The economic impact of wild pig damage is estimated to be in the billions of dollars annually in the United States (Pimentel, 2007). In spite of being able to occupy and exploit a wide variety of habitats (Eisenburg, 1981), wild pigs have only recently expanded their range into the more arid regions of Texas (Adkins and Harveson, 2006). Their impacts are of particular concern in these arid regions because of the effects that their rooting, wallowing, and defecating have on the limited wetland and aquatic habitats found in those areas (Mapson, 2004; Chavarria et al., 2007; Zengel and Connor, 2008). Wild pig activity can also negatively impact the quality of water at stock ponds and water tanks provided for livestock (Tisdell, 1982; Mapson, 2004). It is therefore important to document population characteristics of wild pigs in arid environments. Two parameters are particularly responsible for the invasiveness of wild pigs. their dietary breadth and their reproductive potential (Comer and Mayer, 2009; Ditchkoff and Mayer, 2009).

Wild pigs have the most diverse diets of any ungulate, which has contributed to their success as an invasive species. The wild pig diet includes algae, fungi, grasses, forbs, roots or tubers, browse, soft mast (fruits and berries), hard mast (seeds and nuts), vertebrates, and invertebrates (Ditchkoff and Mayer, 2009). The proportion of plant material varies, but it typically makes up the majority of wild pig diets (approximately 88%; synthesized in Ditchkoff and Mayer, 2009). Subsurface herbaceous material (roots and tubers) are staples in wild pig diets. During periods of low food availability, roots and tubers are especially important and only accessible through rooting behavior. Mast species and agricultural crops are preferred (Barrett, 1978; Sweeney et al., 2003) and generally consumed opportunistically when available. Mast and agricultural crops can constitute 70-90% of seasonal diets by occurrence (Fournier-Chambrillon et al., 1995). Vertebrate and invertebrates are consumed year-round (Scott and Pelton, 1975), and although the percent composition by volume is generally low, the frequency of occurrence in stomachs can be as high as 89.2% (Diong, 1982). A preliminary research report indicated that the wild pig diet in the Rolling Plains differed from diets in other areas in Texas (Lucia et al., 1999).

Suids have the highest reproductive rate of any ungulate due to their short interbirth intervals, large litter size, and a young age at first reproduction (Eisenberg, 1981; Read and Harvey, 1989). Reproductive parameters, however, vary between populations due to genetic lineage, food availability, and latitudinal gradients (Comer and Mayer, 2009; Bywater et al., 2010). Seasonality of reproduction is highly variable, depending on environmental factors. In areas with fluxes in resource availability, conception tends to follow periods with high food availability (Baber and Coblentz, 1987; Taylor et al., 1998). Areas where food is available year-round typically have year-round breeding (Fernandez-Llario and Mateos-Quesada, 1998; Fonseca et al., 2004). Reproductive output also correlates positively with sow age and weight (Barrett, 1978; Fonseca et al., 2004). Fetal sex ratios tend to show parity or have a slight male bias (Comer and Mayer, 2009).

The objectives of this study were to investigate the following diet and reproductive parameters of wild pigs in northwestern Texas: 1) annual and seasonal diet; 2) seasonal changes in body condition; 3) sexual differences in body condition; 4) productivity of sows; 5) the effects of sow age, weight, and condition on productivity; 6) seasonality of breeding; and 7) fetal sex ratio and litter size.

MATERIALS AND METHODS-Study Site-Wild pigs were collected on eight ranches in five counties (Cottle, Dickens, Foard, King, and Motley) in the Rolling Plains Ecoregion of Texas (U.S. Environmental Protection Agency, 2003). This region is typified by mesquite-buffalo grass (Prosopis glandulosa-Bouteloua dactyloides) rangelands varying in elevation between 500 and 900 m. Temperatures average 14-18[degrees]C, with 45-60 cm of rainfall annually (McNab and Avers, 1994).

Capture Methods-Wild pigs were collected by Texas Parks and Wildlife staff from June 1996 to October 1998 via aerial gunning, trapping, and ground shooting. Age was estimated by tooth eruption and tooth wear (Matschke, 1967; Barrett, 1978). Individuals older than 2 months of age were sexed, weighed, and measured for morphological metrics, including rump fat. Rump fat thickness was measured by cutting through the animal's back adjacent to the spine between the hips and measuring the fat depth.

Diet-After collection, stomach contents were removed and frozen for later dietary analysis. Wild pig diets were determined following methods by Chamrad and Box (1964). Thawed samples were spread and washed over a sieve (0.5 x 0.5 cm) and air-dried. After drying, contents were spread evenly in a pan (17 x 28 cm) with a moveable frame holding 10 pins at a 45[degrees] angle. The frame was moved 10 times and the pins were pushed into the stomach contents. This procedure resulted in 100 points per individual. The first diet component that each pin hit was recorded as grass, forb, root or tuber, browse, soft mast, hard mast, crop, vertebrate, or invertebrate. For those samples that did not cover the entire pan, contents were spread over half the pan and the results were extrapolated. Samples were described seasonally (spring, March-May; summer, June-August; fall, September-November; winter, December-February) and annually.

Body Condition-Body condition, estimated by rump fat thickness, was compared seasonally for all pigs. Sexual differences in rump fat thickness were tested via an independent sample t-test in SPSS (IBM Corporation, Armonk, New York).

Reproductive Parameters-Upon capture, pigs were examined and necropsies were performed on sows. The total number of teats and the number of functioning teats in lactating females were recorded. Functioning teats were determined by visible distention and palpation. During field necropsy, reproductive tracts were removed from adult females and their pregnancy status was determined. Ovaries and fetuses were removed and stored in 10% formalin. Ovaries were sectioned at 1.5-mm intervals and examined under a dissecting microscope to count corpora lutea and corpora albicantia. Fetuses were counted, sexed (when possible), measured, and aged based on crown rump length (Henry, 1968a). Once fetuses were aged, conception and farrowing dates were estimated using an average gestation length of 115 days (Henry, 1968b). We tested for differences in body condition (estimated by rump fat thickness) associated with reproductive condition (pregnant, lactating, or neither). Analyses of rump fat thickness were performed using a one-way analysis of variance in SPSS 22.0 (IBM Corporation).

RESULTS-One hundred and forty-eight wild pigs >2 months of age were collected, with a bias in collection toward females (males, 64; females, 84). Complete dietary and reproductive data were not collected on all individuals; therefore, sample sizes are indicated for each analysis.

Diet-The contents of 89 wild pig stomachs were removed and examined. Males (n = 36) and females (n = 53) were analyzed together. Wild pig diets changed seasonally, with food types varying in importance (Table 1). Grasses were found in highest abundance during the spring and summer and then dropped in percentage in fall and winter. Forbs were of comparatively less importance in wild pig diets. Pigs used forbs most during the green-up in spring, but little during the remainder of the year. Roots and tubers were found in high abundance during every season, especially during the winter and early spring. Browse was of highest importance in the spring and used relatively less during the remainder of the year. Soft mast was highest during the summer, but low throughout the year. Fall and winter diets had the greatest percentages of hard mast. Agricultural crops, mainly corn (Zea mays) and sorghum (Sorghum bicolor), were found in high percentages throughout all seasons and constituted 63.59% of fall diets (Table 1); this value was the highest percentage of any food type in any season. Vertebrate matter was found at low percentages in fall and winter. Similarly, invertebrate matter was found at low levels, but it was present in all seasons.

Annual wild pig diet displayed the feeding plasticity for which wild pigs are well known. Agricultural crops were found in highest abundance (42.21%) followed by roots and tubers (25.83%) and grasses (12.64%). Browse and hard mast had similar annual percentages at 6.10 and 6.66%, respectively. Forbs, soft mast, vertebrate matter, and invertebrate matter were each <4% of annual wild pig diets (Table 1). Other identifiable materials included feathers and fungi.

Body Condition-Wild pig body condition varied with season for all individuals (n = 147). Rump fat thickness was higher (P [less than or equal to] 0.001) in fall than in the other seasons. Body condition steadily deteriorated from 16.10 [+ or -] 9.75 mm in fall to 3.78 [+ or -] 5.187 mm in summer (Table 1). Rump fat was not different (P = 0.08) between males (7.97 [+ or -] 7.59; n = 63) and females (10.11 [+ or -] 10.31; n = 84). Rump fat was highly variable between individuals.


Reproductive Parameters-Seventy-eight female wild pig reproductive tracts were removed and analyzed. We found corpora lutea and lactation evident in females as young as 8-12 months of age. The oldest three females in the study (>60 months old) were either pregnant or had corpora albicans present. After excluding those individuals younger than 8 months of age (n = 12), 24.24% of sows were pregnant at collection (n = 16), 27.27% were lactating (n = 18), and 48.48% were neither (n = 32). The number of teats on sows averaged 10.13 [+ or -] 1.03 and on lactating individuals averaged 5.45 [+ or -] 2.63 functioning teats. An average of 7.00 [+ or -] 2.39 corpora lutea or corpora albicans were found in 45.33% of all sows [greater than or equal to] 8 months old of age.

Sow age, weight, and body condition were positively correlated with both reproductive condition and output (see Table 1). Rump fat thickness for all sows averaged 10.62 mm (SD [+ or -] 10.48) and varied with reproductive condition (pregnant, 18.88 [+ or -] 7.84; lactating, 5.33 [+ or -] 6.41; nonreproductive, 9.53 [+ or -] 10.56; P = 0.004). Fisher's least significant differences showed pregnant sows had thicker rump fat than both lactating (P = 0.001) and nonreproductive sows (P = 0.012). There was no difference in rump fat thickness (P = 0.13) between lactating and nonreproductive sows. Sow age and weight were positively correlated with fetal litter size in this population. Age and weight were combined into an interaction term and compared to fetal litter size. Larger and older sows tended to have larger fetal litters than their younger and lighter counterparts ([R.sup.2] = 0.594).

Sixteen fetal litters were examined to estimate peak farrowing period and fetal litter parameters. Farrowing occurred year-round, with a peak during winter (n = 11; Fig. 1). In addition, four fetal litters were estimated to farrow in April and May and one litter in August. Using a 115-day estimate of gestation (Henry, 1968b), peak conception occurs August-October. Fetal litters averaged 4.75 [+ or -] 2.67 piglets, with near parity in sexes (38 female:36 male). Embryonic loss, measured by the difference in mean number of corpora lutea and mean number of fetuses, was 32.14%.

DISCUSSION-Wild pig diets in the Rolling Plains followed trends reported in other studies showing seasonal variation with strong preferences for plants with high nutrition levels (Barrett, 1978; Fournier-Chambrillon et al., 1995; Taylor and Hellgren, 1997). Wild pig diets in our study reflected temporal shifts in availability and abundance of forage types. During spring, grasses increase in both digestibility and quality, and this improved forage quality continues through the summer. Wild pigs used grasses in the highest percentage during these seasons until senescence in the fall. Spring diets were highest in grass and browse, which likely compensated for a reduction in available agricultural crops. Roots and tubers seem to be consumed as a fallback food by wild pigs. Although used throughout the year, fallback foods are most important during periods of low food availability. Greater use of roots and tubers during winter and spring corresponds with findings of other studies that reported high use during winter (Pine and Gerdes, 1973; Wood and Roark, 1980) when preferred foods are unavailable. Wild pigs seem to use mast species in proportion to their abundance. Soft mast, consisting mainly of wild plum (Prunus), is available during summer; the percentage of soft mast was also highest during that season. Mesquite (Prosopis) beans and acorns (Quercus) are available during fall and winter in the Rolling Plains. Wild pigs consumed a high percentage of hard mast during this period when its nutritional quality is highest. Agricultural crops had the highest use of any forage type in our study. Although summer diets may reflect use of agricultural fields, reliance on agricultural crops is exacerbated by sport hunting during the fall and winter. Corn and sorghum are used to bait wildlife, and it seems that pigs are heavily reliant on these resources. seven individuals were collected via traps that were baited with corn. These individuals' agricultural crop percentages (average = 20.57%) were low and not expected to inflate the importance of agricultural crops.

Wild pig reproduction in the Rolling Plains is also consistent with other areas throughout the united states. Age at first reproduction varies in wild pigs and is dependent on nutritional status (Barrett, 1978). We documented sows reproducing as early as 8 months of age, an age that is later than in some more productive habitats (Wood and Brenneman, 1977; Johnson et al., 1982) but that mirrors that of other arid regions (Baber and Coblentz, 1987; Taylor et al., 1998). Productivity, regardless of age class, was within the range of reported values for wild pigs. Published reproductive data on pigs in Texas reported an average of 5.0-8.0 corpora lutea and fetal litter sizes of 4.4-5.6 (Taylor et al., 1998). However, embryonic loss was higher in our study than in a study of the coastal and southern prairies (25%; Taylor et al., 1998). Farrowing was concentrated during the winter and spring, with only one recorded farrowing date during the summer. Estimated conception dates are mainly during the fall, coinciding with an increase in hard mast and agricultural crop use. We acknowledge that collection of sows was not even throughout the year and that sample sizes were small. However, given a 115-day gestation period, we believe that sampling was frequent enough to identify a peak in farrowing in December-February.

Wild pig body condition varied throughout the year. Fall rump fat measures were larger than those in the other seasons. Rump fat thickness decreased, reaching a low during the summer when pigs are likely nutrient stressed. The increased use of hard mast and agricultural crops is likely to provide the nutrition required for reproduction and is responsible for the farrowing peak. Rump fat thickness was also different in postpubescent sows, depending on reproductive status. Pregnant sows had higher rump fat thickness than nonreproductive females, suggesting that nutrition is an important factor in determining reproductive status. Lactation is the most energetically costly period of a pig's reproductive cycle, and it is expected that lactating females will be in poorer body condition.

Diet and reproductive data can have important implications for managing wild pigs in the Rolling Plains. Their reliance on agricultural crops is of particular concern. Protecting crop fields during the summer may not only increase yield but also deprive pigs of a valuable forage resource. Spotlight shooting or trapping near agricultural crops could produce this effect. Wild pigs use high levels of agricultural crops during the fall and winter. This is likely due to the use of corn and sorghum for baiting or feeding preferred wildlife species. If eradication or population suppression is desired, stakeholders may look to use exclusionary fencing around wildlife feeders or feeders modified to more selectively feed desired species. Year-round access to a preferred food resource helps maintain body condition and ensure adequate nutrition for reproduction. Proper timing of control events could also improve wild pig population management. Baiting wild pigs into large corral traps should be most effective during summer when pigs are nutrient stressed. In addition, the majority of sows gave birth during the late winter or early spring. We suggest that lethal management actions should occur directly preceding this farrowing peak. This ensures removal of a greater percentage of pregnant sows that would help slow population growth. Management should focus on removal of all sows >8 months of age and especially on large mature sows as they have the greatest reproductive output.

We thank R. Cranford, D. Dvorak, S. Gray, T. Hinkle, J. Hughes, C. Ibarra, J. Lionberger, D. Lucia, G. Pleasant, X. Rodriguez, B. Simpson, S. Sudkamp, and D. Wright for data collection. We thank M. Nadal-Mayer for translating the abstract of this article into Spanish. A special thanks to D. Lucia and B. Simpson, who supervised much of the data collection.


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Submitted 27 March 2015.

Acceptance recommended by Associate Editor, Troy Ladine, 15 June 2015.


Department of Natural Resources Management, Texas Tech University, Lubbock, TX 79409 (PES, BC, PSG, CBD)

Texas Parks and Wildlife Department, Canyon, TX 79015 (CLR)

Savannah River National Laboratory, Savannah River Site, Aiken, SC 29808 (JJM)

* Correspondent.
TABLE 1--Seasonal changes in diet and body composition in wild pigs
(Sus scrofa) in the Rolling Plains, Texas.

Season   n Diet   Grass   Forb    R/T    Brow    Soft   Hard    Crop
          (a)                                    mast   mast

Spring     29     23.28   7.34   41.66   16.14   0.83   0.03    10.59
Summer     4      22.25   0.25   24.50   3.00    3.00   0.00    46.25
Fall       44     7.25    1.64   9.98    1.16    1.16   11.57   63.59
Winter     12     3.50    2.75   46.17   1.00    1.00   6.92    38.92
Annual     89     12.64   3.58   25.83   6.10    1.34   6.66    42.21

Season   Vert   Invert   n BC      Rump fat (mm)

Spring   0.00    0.21     47    5.36 [+ or -] 6.85
Summer   0.00    0.75     31    3.78 [+ or -] 5.19
Fall     1.39    1.57     52    16.10 [+ or -] 9.75
Winter   0.42    0.33     17    8.53 [+ or -] 6.35
Annual   0.74    0.92    147    9.19 [+ or -] 9.27

(a) Diet is presented as percentages of grass, forb, roots and tubers
(R/T), browse (Brow), soft mast, hard mast, agricultural crop (Crop),
vertebrate matter (Vert), and invertebrate matter (Invert). Body
condition scores are displayed seasonally by average rump fat
thickness (Rump fat [+ or -] SD). Samples sizes are provided for
dietary measures (n Diet) and body condition (n BC).
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Author:Schlichting, Peter E.; Richardson, Calvin L.; Chandler, Brian; Gipson, Philip S.; Mayer, John J.; Da
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2015
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