Spatial behavior through time can be characterized by an individual’s home range and territory. Home ranges are selected by animals to maximize their survival and reproduction, with territories traditionally defined as a reduced portion of the home range that is actively defended or exclusively used1,2,3,4. Spatial overlap of individuals influences intraspecific interactions, which can drive population processes such as disease transmission5, survival6,7, competition8, and reproduction9. The relationship between the spatial organization of a species and dynamic interaction among individuals is expected to vary based on both behavioral and environmental factors.
Variation in spatial interactions among animals can identify the selective pressures that shape spatial behavior. Differences between males and females in spatial behavior are related to differing reproductive strategies and spatial requirements allometrically scaling with body size 10,11,12, which can influence home range size13 and habitat use14. Seasonal changes in behavioral and environmental influences can also modify spatial behavior and influence overlap and association of conspecifics13,15,16,17. Isolating factors that influence heterogeneity in spatial overlap can improve management and conservation decisions by identifying important time periods for disease transmission19, intraspecific competition8, and management actions19. Despite the growing evidence that spatial relationships affect a diverse range of socio-ecological processes20, variation in spatial overlap patterns between sexes and seasons is poorly understood for many species and is dependent on the individual’s spatial organization.
Territorial behavior can increase an individual’s fitness by excluding conspecifics from areas containing limiting resources, including food, cover, and mates. There is considerable variation, however, in how territories are defined and interpreted3,4 and they are often approximated by estimating core areas (i.e., areas of high probability of use within an animal’s home range21,22,23. Although the identification of core areas can be subjective, territoriality can be identified by examining overlap between adjacent individual’s home range and core areas24,25,26. Non-territorial species are expected to overlap with conspecifics at random which would result in overlap with a similar number of individuals in home ranges and core areas (Fig. 1). Conversely, territorial behavior creates spatial heterogeneity in the number and strength of conspecific interactions, leading to reduced overlap of core areas compared to home ranges (Fig. 1). In the paradigm proposed by Burt (1943)1 and Schoener (1968)2, territorial species overlap in their home ranges but have exclusive core areas and thus home ranges do not overlap core areas between neighboring individuals (Fig. 1; Fig. 2). Other studies have defined territoriality as occurring when core areas are mutually exclusive between animals, but home ranges overlap with core areas of adjacent individuals24,27,26,23,28 (Fig. 1; Fig. 2). It is predicted that if animals are territorial that there will be a reduction in spatial overlap when comparing overlap of home ranges (HR-HR), home ranges and core areas (HR-CA), and core areas (CA-CA) (Fig. 1). If animals are not territorial and spatial overlap among individuals is random, then it would be predicted that overlap would be similar when comparing HR-HR, HR-CA, and CA-CA (Fig. 1). Spatial overlap patterns could be used to quantify territoriality in many species and across a range of different social structures, but this approach could be particularly useful for species that exhibit heterogeneity in space use yet lack conspicuous territorial behaviors or exclusive core areas.
Wild pigs, Sus scrofa, are a large-bodied, gregarious species with an expansive native and invasive range, ecological role as an ecosystem engineer, and complex socio-spatial behavior29,30,31,32. Seasonal wild pig home ranges can vary inversely with forage availability33,34,35,36,37,38, which could influence spatial overlap39,40 and territorial patterns8,40. Although territorial patterns have been predicted for wild pigs41, previous studies have provided conflicting evidence. Female groups are argued to be territorial with other female groups where groups overlap in their home ranges, but exhibit mutually exclusive core areas27,42,43. This pattern, however, is not detected in other systems35,44,45, and the role of territoriality in wild pig social structure is unclear. Males are reported to be less territorial than females (but see Boitani et al.199435), with males exhibiting larger home ranges that overlap with both sexes46. How home range size, spatial overlap, and territoriality vary across seasons has not been widely evaluated for wild pigs and it is unclear how different behavioral and environmental factors influence space-use overlap.
Here we examined variation in the spatial behavior of wild pigs and used this information to understand patterns of territoriality. Our first objective was to examine variation in home range size and spatial overlap between sexes within a single season. We expected that males would 1) exhibit larger home ranges, 2) interact with a greater number of conspecifics within their home ranges, and 3) exhibit increased intra-sexual space-use overlap compared to females46,47. Our second objective was to evaluate how home range size and space-use overlap varied seasonally. Assuming forage availability to be lower during the dry season than the wet season48,49, we predicted that home range size and space-use overlap would be greater during dry seasons because animals would use larger areas to acquire sufficient resources38. Conversely, if resources are more concentrated and limiting during the dry season, then wild pigs might exhibit less space-use overlap during this season. Our third objective was to use measures of spatial overlap to evaluate territoriality in wild pigs among years and between wet and dry seasons. If wild pigs exhibited territoriality, we expected that spatial overlap among individuals would be greatest for home range to home range (HR-HR) interactions, reduced for home range to core area (HR-CA) interactions, and lowest for core area to core area (CA-CA) interactions (Fig. 1). We predicted that males and females would differ in their spatial overlap, with females exhibiting more exclusive core areas. Territoriality was expected to be strongest during the dry season as resources are assumed to be more limiting.