Using an extensive dataset of wild pig GPS data across a heterogeneous landscape in the Southeastern U.S., here we demonstrate the differential resource selection tactics employed by wild pigs at both broad (i.e., home range placement) and fine (i.e., within-home-range, behavior-specific) spatial scales for males and females across two distinct seasons based on forage availability. Movement path characteristics of wild pigs in our study were influenced by a combination of local and landscape-level habitat attributes such as bottomland and upland hardwoods, streams, secondary roads, and shrub/herbaceous vegetation communities. However, while males and females tended to select areas to establish home ranges (population scale) similarly, we found notable differences in the fine-scale use of habitats within home ranges between sexes and seasons, although both males and females consistently selected bottomland hardwood habitats and areas with extensive canopy cover. Further, through the use of step lengths and turn angles to define behavioral-based resource selection patterns, we found that females and males differed in daily movement patterns and that wild pigs exhibited differential selection of landscape attributes among behavioral states.
Based on the results of our HMM analyses, we distinguished three biologically relevant behavioral states generally based on patterns in the movement characteristics of wild pigs (i.e., resting, foraging, traveling). Previous studies have identified similar patterns for other species [9, 32, 48]; however, behavioral states associated with movement characteristics may be assigned differently depending on prior knowledge of different animal species and fix rate at which GPS data were collected. Although wild pigs exhibit several behaviors that correspond to short and intermediate step-lengths and tight turn angles (e.g., resting, wallowing, rubbing, tusking, foraging, etc.), for management purposes of wild pigs classifying behaviors into resting, foraging, and traveling encapsulated the most common and consistent motivations of space use (e.g., forage, cover, thermoregulation; [26]).
Both females and males decreased movements or traveling behavior in the mid-day, most likely due to the association with high temperatures in the southeast during the high-forage season [26, 33], and males maintained a consistent nocturnal pattern between seasons. However, females exhibited seasonal differences in movement patterns that were likely related to reproductive stages of the reproductive cycle throughout the year, as the timing of farrowing is related to the seasonal availability of forage [49, 50]. In the low-forage season, which corresponded with peak farrowing in our study area ([49]; Chinn, unpublished data), females demonstrated a sharp increase in traveling at dusk along with an increase in foraging throughout daytime hours as well as a slight increase in resting mid-day followed by a distinct increase in resting throughout nighttime hours. However, during the high-forage season when farrowing rates are lower and juvenile pigs are more mobile, females demonstrated a more crepuscular movement pattern compared to the low-forage season. Pre-parturition and parturition-associated behaviors in some wildlife species, such as wild pigs, are associated with reduced movements and home range sizes [22, 33]. Irregular and/or reduced movements can continue after parturition causing an abnormal activity pattern in females [50], as we found throughout the low-forage season. Males and females have different reproductive tendencies and responsibilities as a polygamous species [51] in which males breed multiple females and provide no parental care. Therefore, behavioral differences between sexes likely reflect different reproductive obligations [50] and should be a focus for further research.
Wild pigs can demonstrate multiple behaviors in similar vegetation types [52], but there are certain habitat characteristics and vegetation types that facilitate specific behaviors (e.g., relocation using roads; [6]). Although wild pigs are ecological generalists, they exhibit spatio-temporal differences in resource selection that reflect underlying biological needs (e.g., thermoregulation; [26, 34]). At the population and home-range scale, our analyses revealed that while male and female wild pigs exhibited similarities in resource selection in the low-forage season, notable differences between sexes were apparent. Dense cover and areas proximal to water (i.e., bottomland hardwoods) are two key vegetation characteristics that provide resources that pigs require [26], and we found that females and males selected for bottomland hardwoods and areas with high percentages of canopy cover in every behavioral state during the low-forage season. In addition, wild pigs forage on subterranean foods such as roots and tubers when other sources are scarce [26, 53, 54]; therefore, selecting bottomland hardwoods and areas with extensive canopy cover coincide with these forage types and provide access to water and cover.
While foraging, males selected for a variety of vegetation types and structures throughout the low-forage season. For example, at both the population and the home-range scales males demonstrated a change in selection for primary roads between seasons. In the low-forage season, males established home ranges near primary roads and selected for areas closer to primary roads in all three behavioral states. Also, males avoided secondary roads at the population scale, but selected for secondary roads in the foraging and traveling states at the home-range scale. The selection for areas near or along both primary and secondary roads while foraging is likely due to the decrease in resources in adjacent natural areas and the consistent availability of grasses along open roadsides during the low-forage season [54, 55], which coincides with the increase in use of urbanized and anthropogenic areas when natural forage is scarce [56, 57]. Lastly, during the resting state females demonstrated selection for shrub and herbaceous vegetation, which was characterized by a mixture of areas in early successional stages and grasslands that both typically occurred together along secondary roads, power lines, and streams, while males selected for this vegetation type in every behavioral state. Areas dominated by this vegetation type most likely provided forage, cover, and easy access to linear features when transitioning to traveling in the low-forage season.
During the high-forage season, neither males nor females selected for primary or secondary roads at the population scale. However, at the home-range scale males selected for areas closer to secondary roads while traveling but avoided these areas when foraging and resting. Selecting for anthropogenic and natural linear features can help increase an animal’s pace (step-length) and directional movement, which can assist in traversing the landscape quickly when dispersing, searching for a mate, or transitioning between resting and foraging behaviors [26, 58, 59]. Also, males selected primarily for bottomland hardwoods while foraging in the high-forage season, and females selected for upland and bottomland hardwoods during all behavioral states, likely reflecting the availability of food, water, and cover in these habitats [26]. Selection for bottomland hardwoods is most likely associated with mast producing hardwoods (e.g., oak acorns) and productive plants in the understory throughout summer months, as well as dense cover and proximity to water. Lastly, throughout the high-forage season, males and females selected for streams at the population scale but avoided streams at the home-range scale, which is likely due to the extensive stream system throughout the SRS and the ability to access dense cover during times of extreme temperatures. Other studies have demonstrated the insignificance of streams at the home-range scale throughout certain times of the year when water is generally present throughout the landscape [59].
Wild pigs exhibit substantive behavioral plasticity and can adjust their life history strategies such as daily activity patterns to decrease human interaction in populated areas as well as their diet throughout the year and in a variety of climatic conditions to benefit their long-term survival depending on local environmental conditions [26, 56, 60, 61]. Although our study was limited to the Southeast U.S., wild pigs demonstrate consistent selection patterns for vegetation types associated with certain resources (i.e., water, mast, etc.; [22, 26, 62–64]). Therefore, our findings are likely applicable in similar areas throughout this species’ native and introduced range. However, further research should focus on wild pig behavioral state resource selection in other geographic regions to elucidate spatio-temporal differences in wild pig behavior across areas of differing climate and resource base.
While our general findings are consistent with previous literature on wild pig habitat selection, through the investigation of fine-scale movement patterns coupled with behavioral-based resource selection we were able to demonstrate pigs exhibit clear differences in temporal patterns of activity and selection of habitats among behavioral states. For example, male wild pigs strongly selected for secondary roads while traveling during both seasons but avoided or selected them less while in the foraging and resting behavioral states. Similarly, females demonstrated a selection for shrub/herbaceous communities while resting during both seasons but avoided or selected them less while foraging and traveling. Thus, delineating GPS observational data into unique behavioral states provides unique insights into the relative importance of environmental attributes critical to the conservation or management of a species that may otherwise be obscured through more coarse-scale resource selection approaches [3].