REPRODUCTION OF FERAL PIGS IN SOUTHERN TEXAS

REPRODUCTION OF FERAL PIGS IN SOUTHERN TEXAS RICHARD B. TAYLOR, ERIC C. HELLGREN, TIMOTHY M. GABOR, AND LINDA M. ILSE Texas Parks and Wildlife Department, p. 0. Box 5207, Uvalde, TX 78802 (RBT) Department of Zoology, Oklahoma State University, Stillwater, OK 74078 (ECH, 1MI) Caesar Kleberg Wildlife Research institute, Texas A&M University-Kingsville, Kingsville, TX 78363 and Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843-2258 (TMG) The feral pig (Sus scrofa) is an abundant introduced species with pernicious effects on native species and ecosystems. Its potential reproductive rate is the highest of any ungulate, but data on reproductive rates of feral pigs are limited. We studied reproduction of feral pigs in two regions of southern Texas: the Gulf Coast Prairies and the western South Texas Plains. Pregnancy rates of adults (>21 months) ranged from 78% during winter (December February) in the Gulf Coast Prairie to 6% in summer (June-August) in the western study area. Fetal litter sizes in adults tended to be greater (P = 0.11) than those of yearlings. Fecundity ranged from 1.1 female young/year for juvenile females to 4.5 female young/ year in adult females. Sex ratio of fetuses (n = 298) was male-biased (P < 0.05) when data from both study areas were combined. Two seasonal peaks of births were observed (January-March and June-July). Fecundity of pigs in southern Texas was more than four times higher than native ungulates, raising serious questions about dynamics of the ungulate community in this region. Key words: Sus serofa, feral pigs, reproduction, breeding, fecundity, Gulf Coast Prairies. South Texas Plains Suidae has the highest reproductive rate of any ungulate family (Read and Harvey, 1989). Allometrically, suids have large litter sizes, short gestation periods, and early sexual maturity for their body mass (Eisenberg, 1981; Read and Harvey, 1989). These characteristics have been magnified by animal scientists into breeds of domestic pigs (S. s. domesticus) that produce litters of >12 young (Asdel!, 1964). Feral pigs are free-ranging swine of varied domestic origin that retain these r-selected traits. For example, average litter sizes of feral pigs in several states range from 4.8 to 7.5 for adults (Baber and Coblentz, 1987; Barrett, 1978; Belden and Frankenberger, 1990; Sweeney et ai., 1979). These values compare with litter sizes of 4.5-6.3 for Eurasian wild boar (Ahmad et ai., 1995; Mauget, 1991), which represent the original genetic stock for domestic swine. In Texas, Springer (1977) reported that the main season of birth in the Gulf Coast Prairie Ecoregion was January-May (17 of 19 observed pregnancies). Average litter size was 4.2, with two instances of two litters in 1 year being reported. This population had a history of Eurasian wild boar introductions to the local free-ranging population of pigs. Data on population dynamics are critical to development of management strategies for feral pigs and to address conservation concerns regarding interactions between pigs and native ungulates. Although there are considerable data on reproductive characteristics of pigs, especially litter size, meaningful data on reproductive (i.e., fecundity) and survival rates for populations of feral pigs are rare. Estimates of reproductive rates cannot be made without collecting data on litter size, fetal sex ratio, litter frequency, and age at sexual maturity. Information concerning annual litter frequency is particularly critical but heretofore Journal of Mammalogy, 79(4);1325-1331,1998 1325

1326 JOURNAL OF MAMMALOGY Vol. 79. No.4 has been limited (Baber and Coblentz, 1987; Coblentz and Baber, 1987). Our objectives were to describe reproductive performance in two populations of feral pigs from southern Texas and compare reproductive rates of these pigs to native ungulates. MATERIALS AND METHODS Two study areas representing different ecological regions were selected. The western area was located in the South Texas Plains ecological region (Gould, 1975) and included seven counties: Uvalde, Kinney, Maverick, Webb, Zavala, Dimmit, and LaSalle. Climate was characterized by short moderate winters and long hot summers. Freezing temperatures were brief and infrequent, and growing seasons often exceeded 325 days. Soil types varied from sandy loam to heavy clay, occasionally being alkaline or calcareous, but seldom acidic. Vegetation was predominantly diverse brush dominated by mesquite (Prosopis glandulosa), prickly pear (Opuntia lindheimeri), and acacias (Acacia-Taylor and Hellgren, 1997). The eastern area was situated in the Gulf Coast Prairie ecological region on the Rob and Bessie Welder Wildlife Refuge in San Patricio Co., Texas. The refuge was 3,178 ha of mixed mesquite and live oak (Quercus virginiana) brushland adjacent to the Aransas River (Ilse and Hellgren, 19950). Annual precipitation ranged from 45 to 64 cm in the western site (Hellgren et a1.. 1995) to 89 cm in the eastern site (Ilse and Hellgren, 1995a). In the western area, 119 female feral pigs were collected singly or in groups «12 individuals) by ground shooting and aerial gunning during each season (autumn, September-November; winter, December-February; spring. March-May; summer, June-August) from September 1989 through November 1991. In addition, 28 females were sampled from public hunts conducted on the Chaparral Wildlife Management Area in Dimmit and LaSalle counties (in the western area) in February 1994, January-February 1995, and November-February 1995-1996. In the eastern site, 54 females were collected from December 1992 to February 1993 by shooting. All pigs were weighed (kg) and aged by tooth eruption, replacement. and wear (Barrett, 1978; Matschke, 1967). Pigs <12 months were classified as juveniles, pigs 12-21 months of age were classified as yearlings, and pigs >21 months of age were classified as adults. We removed ovaries from each reproductive tract and preserved them in 10% formalin. We sliced ovaries in 1-2 mm sections and recorded number of corpora lutea (Barrett, 1978). Fetuses were counted and sexed. Implantation rates were defined as the number of fetuses divided by the number of corpora lutea of pregnancy observed in the same tract. Fetal crown-rump length was measured to the nearest nun to estimate age (Henry, 1968a). Assuming a gestation period of 115 days (Henry, 1968b), we backdated aged fetuses to conception and fore dated them to birth. Mean number of litters per adult female was determined by the method of Keith et a!. (1993) for the western area. For each month of collection, we calculated average rate of pregnancy. We assumed that pregnancy rates in months when no collections were made (March and September) were average and assumed a 90-day period (of the 115-day gestation) when pregnancy could be detected by macroscopic examination. Pregnancy rates only for months with a sample size of :2:4 were included in that calculation. Gross fecundity, defined as number of female young per female per year, was calculated by multiplying age-specific litter frequency by agespecific litter size and proportion of females in the sex ratio of fetuses. We compared ovulation rates (e.g., number of corpora lutea) and litter sizes of pigs by age and area with a two-way analysis of variance (SAS Institute, Inc., 1990). RESULTS Reproductive tracts from 134 pigs (n = 42 for the eastern area; 92 for the western area) were examined for corpora lutea. On the western study area, tracts from 5 of 25 juveniles, 11 of 25 yearlings, and 32 of 42 adults contained corpora lutea. When only winter tracts from the western area were tallied, 1 of 2 juveniles, 6 of 6 yearlings, and 16 of 17 adults had ovulated. These proportions were very similar to the eastern sample. which was restricted to winter, where 1 of 14 juvenile tracts contained corpora lutea, but all yearling (n = 5) and adult (n == 23) tracts had evidence of ovulation. Adults had greater (P = 0.01) ovulation rates than yearlings, and the western area

November 1998 TAYLOR ET AL.-REPRODUCfION OF FERAL PIGS 1327 TABLE 1.-Age-specijic reproductive perfonnallce offemale feral pigs collected in 1989-1991 and 1994-1996 in the western SouIh Texas Plains (STP) alld 1992-/993 in the Gu/fCoasl Prairies (GCP) of Texas. Gross Ovulation rale b Liner fecundity Liuer size frequcn- (F young! A ge class (month)' A rea n n ii SE n ii SE cy/ycar F)< Juvenile «12) STP 50 5 SA 0.5 6 4.8 0.7 0.49 1.07 GCP 14 5.0 I 5.0 Yearling (12-2t) STP 33 11 6.5 0.8 7 4.4 0.9 0.85 1.72 GCP 6 5 5.4 0.7 3 4.7 0.3 Adult (> 21) STP 58 32 8.0 0.4 2 1 6.3 OA 1.57 4.55 GCP 23 23 7.1 0.2 18 5.6 0.4 A se c1:mes ba!scd on M:ltschke (1967) and Barrell (t 978). Im:lude~ only animals for which data on corpora lulea were collected. <.A:;suming in utero R~ ralio of I Itl males: 100 female s. tended to have greater (P = 0.06) rates than the eastern area when all data (n = 92) were included (Table 1). However, neither facror (age or area) varied (P > 0.32) when only winter samples (n = 51) were compared. Implantation rates were 80.7% (n = 26) on the eastern area and 68.4% (n = 24) on the western area. Annual rates of pregnancy in the western area were 0.12 (6/50) in juveniles, 0.21 (7/ _ 08 o o ~ e 0.6 ~ o i? 04 & e a. 0.2..,,. o ~~:-:-:1La J F M A M J J A SON D, 1_.c...Ill.L FlO. i.- Proportion of female feral pigs (~ 1 year old) lhal were pregnant on a Gulf Coast Prairie sile (open bars) and a western South Texas Plains site (hatched bars) in southern Texas from 1989 to 1996. Numbers above bars represent sample sizes collected in each month. Animals were collected in the Gulf Coast Prairie site only in December, January, and February. 33) in yearlings, and 0.38 (22/58) in adults. Five of the six pregnant juveniles conceived in October-December and ranged in age fcom Ihe 20-33-week class (Il = 2) to the 30-51-week class (n = 4). Body mass of these pregnant juveniles ranged from 30 to 60 kg. For all females 2: 1 year old, pregnancy rate averaged 0.29 (Fig. 1) by month, and mean number of litters was 1.22/year. Average monthly pregnancy rate for adults only was 0.39, and litter frequency was estimated to be 1.57. Winter pregnancy rates on the eastern area were 0.07 (1/14) in juveniles. 0.50 (3/6) in yearlings, and 0.78 (18123) in adults. Lactation was noted for 20 of 58 adults. 5 of 32 yearlings. and I of 48 juverules in the western area. One adult was both lactating and pregnant. Litter sizes (Table 1) did not vary by age (F ~ 2.66; d.f ~ 1.46; p ~ 0.11) Ot study area (F ~ 0.03. d.f ~ 1.46; P = 0.87). [n the western area, highest productivity was observed for adult females (Table 1). Fecundity could not be calculated for females from the eastern area because sampling was limited to winter and estimates of annual litter frequency could not be made. Sex ratios of fetuses for both study areas were male-biased, but the sex ratio did not differ from 50:50 for either the eastern (58 males: 42 females. x' ~ 3.81. n ~ 139.0.10 > P > 0.05) or the western area (54 males:

1328 JOURN A L OF M AMMALOGY Vol. 79, No.4 10 8 i;' 6 c, e 4.. Conception 0 Birth 11 n r1l l F M A M J J A SON 0 Month FIG. 2.-Frequency of estimated conception and birthing events for feral pigs collected in (he western South Texas Plains from 1989 to 199 I. Sample included 28 (of 119) pregnant females. 46 females, X' ~ 1.06, = 159, P > 0.25). However, when data were combined, the sex ratio was male-biased (56 males: 44 females, X' = 4.35, = 298, P < 0.05). Seasonal reproduction was evident in the western study area. Conception dates, estimated by backdating fetuses, were concentrated in autumn and early winter (September- December) and spring (March-April), with subsequent births in late winter-early spring (January- March) a nd summer (June-July; Fig. 2). DISCUSSION Feral pigs in southern Texas exhibited reproductive characterislics similar to the species in o ther parts of its native and introduced range. Feral pigs have larger (1 = 2.21, d.f. = II, P = 0.05) litter sizes (X ± S = 5.7 :!: 0.4, n = 9) than Eurasian wild boar (4.8 ± 0.2, n = 10; Table 2). Average size of litters has a higher variance (F = 4,74, P = 0.03) for feral pigs than Eurasian wild boar. This resu1t is consistent with the diversity of stock origins and breeds Ihat produced these populations of feral pigs. Seasonality of reproduction of feral pigs in the western area also was consistent with other introduced and native populations. Most breeding in wild populations occurs in autumn, with a secondary peak in spring (Mauget, 1991 ; cf. Barrett. 1978; Coblentz and Baber, 1987). Several factors, notably nutrition and phoioperiod, have been proposed as driving reproduction in populations of feral pigs in other parts of the species' distribution (Claus and Weiler, 1985; Mauget. 1991). In our study area, quality of TABLE 2.-Comparison of litter sizes between /eral pig and Eurasian wild boars. TyP' Litter size Reference Location Feral pig 4.7 Coblentz and Baber, J 987 Galapagos Islands, Ecuador 5.0 Baber and Coblentz. 1986 Ca lifornia, Uniled States 7.0 Pavlov. 1991 Australia 5.1 Pavlov, 1991 Australia 6.6 Saunders, 1993 Australia 7.5 Sweeney et ai., 1979 South Carolina. United States 5.6 Barrett, 1978 California, United Statcs 5.9 Wood et ai.. 1992 South Carolina. United States 4.2 Spri nger, 1977 Texas, United States Wi ld boar 4.2 Garzon-Heydl, 199 1 Spain 4.9 Henry. 1966 Tennessee, Unitcd Stales 4.3 Saez-Royuela and Telleria, 1987 Spain 4.6 Mauget. 1982 France 5.9 Ahmad cl ai., 1995 Pakistan 4.4 Saez-Royuela and Telleria, 1987 France 4.' Saez-Royuela and Tellcria, 1987 France 5.0 Saez-Royuda and Telleria, 1987 Germany 5.0 Saez-Royuda and Telleri a, 1987 Germany 5.3 Saez-Royuela and Tel1cri a, 1987 Gennany

November 1998 TAYLOR ET AL.-REPRODUCTION OF FERAL PIGS 1329 forage during breeding periods was high. DUring late summer, pigs consumed primarily hard mast (beans of mesquite and acacias) and cactus fruit, but they concentrated on succulent forbs and grasses in spring (Taylor and Hellgren, 1997). We suggest that heat plays a role in seasonality of breeding of feral pigs in southern Texas. Peak breeding coincided with peaks in bimodal precipitation and moderate temperatures in the region. Summers in southern Texas are hot, with daily mean and maximum temperatures averaging >30 C and > 38 C, respectively, in the western area. In several mammals, high temperatures have been reported to depress reproductive performance (Bronson, 1989). Exposure of domestic pigs to elevated ambient temperature is associated with decreases in reproductive efficiency (Cameron and Blackshaw, 1980; Wettemann and Bazer, 1985; Wettemann et ai., 1979). Behavioral thermoregulation, such as wallowing, may ameliorate effects of high ambient temperature on reproductive processes. Age-related changes in reproductive productivity were similar to previous reports (Baber and Coblentz, 1986; Pavlov, 1991; Wood et ai., 1992). In female wild boar in Pakistan, productivity of females >2 years of age was 250% higher than younger females (Ahmad et al., 1995). Litter sizes and number of litters per year were larger in mature adults. Determination of litter frequency is problematic with feral pigs because of their capability to breed year-round. Although we collected animals in all seasons in the western area, sample sizes were adequate for inclusion in the calculation of litter frequency during only 7 months. If pregnancy rates were dramatically different during the other 5 months, our estimates would be biased. Although capable of two litters per year, feral pigs generally produce only one litter per year (Diong, 1982; Mauget, 1982; Singer and Ackerman, 1981). Double litters may increase when nutritious forage is available year round or if entire litters are lost soon after birth (Barrett, 1978). They also are more likely in mature adults because of larger body mass and nutrient reserves. High productivity of pigs relative to other ungulates in the region is of ecological, conservation, and management interest. Feral pigs in southern Texas may compete with native ungulates, fill an empty niche, or use a new niche created by changes in land-use patterns. Data on demography and resource use are needed to address these concerns. Annual gross fecundity in collared peccaries (Tayassu tajacu), an ecologically-similar species, was estimated to be 1.0 female young/female in the same habitat (Hellgren et ai., 1995). White-tailed deer (Odocoileus virginianus) in the region only produce one litter per year, litter size averages 1.8, and fecundity is <1.0 (Ruthven et al., 1994). Relative to native ungulates, the four-fold higher reproductive output of feral pigs could affect ungulate community structure in southern Texas. However, reproductive data alone provide an incomplete view. More information is needed on survival rates of cohorts of feral pigs, especially juveniles and adult females. Preliminary estimates of annual survival of female feral pigs in the South Texas Plains range from 0.40 to 0.88 and average 0.59 (Gabor, 1997), with most mortality being anthropogenic. Survival estimates from life-table analyses are similar (Barrett, 1978; Jezierski, 1977). Without simultaneous estimates of survival and reproductive rates or repeated population estimates, population growth rates for feral pigs cannot be calculated. Competitive relationships between feral pigs, peccaries, and deer need to be explored more fully. lise and Hellgren (l995a) demonstrated different habitat and diet selection between feral pigs and peccaries in sympatry in the Gulf Coastal Prairie. In the same study area, Ilse and Hellgren (1995b) found that ecological densities of pigs and peccaries were related inversely. They opined that low densities of peccaries in the presence of feral pigs were a function

1330 JOURNAL OF MAMMALOGY Vol. 79, No.4 of poor quality of resources for peccaries. Additionally, diets of feral pigs did not overlap substantially with those of either peccaries or deer in the South Texas Plains (Taylor and Hellgren, 1997). Landscape-related analyses of resource use by feral pigs should lead to a better understanding of the future composition of the ungulate community in this region. ACKNOWLEDGMENTS We gratefully thank Texas Parks and Wildlife Department (TPWD), especially D. Frels and 1. Williams for their guidance. and the District 8 staff for many field-hours in the intense climate of southern Texas. We thank D. R. Synatszke and his staff at the Chaparral Wildlife Management Area (TPWD) for their assistance in this project. This study could not have been accomplished without the cooperation and assistance of the United States Department of Agriculture Animal and Plant Health Inspection Service, Animal Damage Control, in animal collection. 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