Factors influencing pregnancy, litter size, and reproductive parameters of invasive wild pigs

Abstract

Reproduction is the most energetically expensive life stage with the demands of productivity representing a balance between physiological requirements and environmental conditions. Wild pigs (Sus scrofa) throughout most of North America are genetic hybrids of feral domestic pigs and wild boar and have the highest reproductive potential of any wild ungulate. The phenology of reproduction, extent of multiple reproductive events per year, how individual and extrinsic factors contribute to variability in productivity, and impact of genetic lineage on these parameters is not well understood in wild pigs. We quantified reproductive parameters in wild pigs relative to a suite of individual and environmental attributes across seasons and multiple years in South Carolina, USA, from March 2017 and May 2020. We hypothesized that individual attributes (mass, age class, number of teats, rump fat, relative genetic association to wild boar vs. domestic pigs) and extrinsic factors (mast availability) would influence probability of pregnancy and fetal litter size. Wild pigs produced offspring throughout all months with peaks in conception corresponding to a seasonal pulse in food availability. The likelihood of pregnancy was influenced by female mass and nutritional condition and was greatest during years with abundant resources. Similarly, litter size increased with female mass and age, implying larger and older females represent the most important group for population recruitment. In evaluating the relationship between reproductive output and ancestral associations to domestic pigs versus wild boar, the proportion of wild boar ancestry was not an important influence on productivity in our population. We determined juveniles reach a physiological threshold of sexual maturity at approximately 30 kg. Average litter size was comparable to other populations, and wild pigs maintain an average fetal litter size of 5.43 offspring despite 13.6% embryonic mortality. A thorough understanding of biotic and extrinsic factors influencing reproduction are important for realistic population models, which are necessary for identifying areas to focus management needs and implementation.

Reproduction drives changes in space use and habitat selection in a highly adaptable invasive mammal

“…The wild pig population on the SRS descended from free-ranging domestic pigs released or abandoned when private land was converted to a government facility in 1950 (Mayer et al 2020). Later introduction of wild boar and wild boar × feral pig hybrids led to wild boar introgression into the SRS wild pig population (Mayer et al 2020; Smyser et al 2020; Chinn et al 2022). Since 1956, the wild pig population has been lethally managed to control abundance and reduce damage, removing 24,980 individuals through 2018 (Mayer et al 2020); nonetheless, the population has continued to increase over the past several decades to >5,000 individuals (Keiter et al 2017a).…”

Variance in offspring sex ratio and maternal allocation in a highly invasive mammal

“…A subsequent introduction of wild boar hybrids occurred on the SRS in the 1980s, which have since expanded throughout the site and hybridized with established feral domestic pigs. The contemporary wild pig population represents hybrids of Western heritage breeds of domestic pigs and wild boar with considerable variation in wild boar (6–60%) versus heritage breed ancestry among individuals (Chinn et al, 2022; Smyser et al, 2020). Despite being lethally managed since 1956, the wild pig population on the SRS has continued to increase over the last several decades (Keiter et al, 2017; Mayer, Beasley, et al, 2020).…”

“…We included maternal age class, maternal condition, resource availability (i.e., rump fat, a measure of environmental variation on food availability), litter size, percent wild boar ancestry, and select interactions. Age and litter size within our study site have been shown to be correlated (Chinn et al, 2022), therefore, we did not include both variables within the same model. We tested if maternal age and condition were correlated by performing an ANOVA.…”

“…Wild pigs were primarily sampled from ongoing wild pig management on the SRS, although additional individuals were culled as part of other research activities. In wild pigs, age at first reproduction is typically driven by a minimum body size (~30 kg, which may be dependent on available forage) such that juveniles <6 months old may be sexually mature (Chinn et al, 2022;Comer & Mayer, 2009). We collected fetal data, tissue for genetic analysis, and morphological F I G U R E 1 Wild pig (Sus scrofa) mature female, juveniles, and piglets on the Savannah River Site, Aiken, SC, USA.…”

“…Relative genetic influences from wild boar versus domestic/feral pig vary within and among populations, which may have ramifications for litter size given that domestic pigs have been under artificial selective pressures to, in part, maximize reproductive output (Taylor et al, 1998). Studies have documented increased litter size and earlier sexual maturity of wild boar in their native range (Briedermann, 1986; Mauget, 1982; Servanty et al, 2009) and wild pigs in North America (Baber & Coblentz, 1986; Chinn et al, 2022; Johnson et al, 1982; Matschke, 1964) in response to increased mast availability as well as to pulses in other seasonal crops (wild boar: Massei et al, 1996; Frauendorf et al, 2016; wild pig: Barrett, 1978; Warren & Ford, 1997). Abundant resources may increase maternal condition and in turn, females in superior condition may have the physiologic resources to increase offspring size, number, and adjust sex ratio as a means to increase lifetime reproductive success.…”