Effect of relatedness on spatial and social structure of the wild boar Sus scrofa population in Białowieża Primeval Forest

Fitness benefits of sociality increase individual survival and reproductive success through a complex network of social interactions. Kin selection theory predicts that individuals increase their inclusive fitness through altruistic behaviours directed towards kin. Thus, preferential social interactions with relatives lead to the emergence of kin structures in the social system. Cohesive social groups, female philopatry, and high reproductive output of wild boar creates conditions for cooperation through kin selection and make the species a good biological model for studying kin structures. Yet, the role of kinship in shaping the social structure of wild boar populations is still poorly understood. The main goal of this study, conducted in Białowieża Primeval Forest (BPF) in 2007-2011, was to explore spatial and temporal patterns of social interactions in the wild boar population, and to determine the role of relatedness in shaping the emergent social structure of the species. A combination of behavioural (telemetry) and genetic (microsatellite DNA) data were used to investigate group structure and composition, dispersal patterns, and population structure of wild boar. Network techniques, association analyses, and estimates of lagged association rates were used to determine temporal and spatial structure of the social interactions in the studied population. A total of 141 wild boar were captured, 114 of which belonged to 16 marked groups and the rest were either solitary or belonged to unmarked groups. Telemetry-marked animals (n = 75) included all solitary wild boar and, on average, 66% of the groups’ members. Genetic analyses included all captured animals and samples collected by hunters in the Polish and Belarusian parts of the BPF (n = 411). All individuals were genotyped with a panel of 16 microsatellite loci. The average family group size estimated from the trapping data was 7 individuals. This result was consistent with analyses based on social interactions alone (association rates and network analysis). The vast majority of adult females and young animals were associated in groups, whereas adult males were mostly solitary. Individuals associated in groups were significantly more genetically related to each other than non-associating, solitary individuals. On average, the intra-group level of relatedness corresponded to second-degree relatives. Relatedness within a group was negatively correlated with group size. Members of the family groups showed extensive spatial overlap, whereas animals from different family groups shared little space. Wild boar in the study population formed non-random, preferential associations which were largely stable and long-lasting. The temporal stability of associations was particularly strong among adult females and animals forming family groups. Conversely, adult males formed short-lived associations disintegrating within a few days. High temporal stability of associations observed among animals forming groups implies strong group cohesion. There was a positive correlation between strength of social bond and genetic relatedness but kin-targeted interactions extending beyond spatial proximity of individuals were observed only among females. On the population level, there was a negative relationship between geographic distance and genetic relatedness. Spatial genetic structure was detected at small spatial scale corresponding to few home ranges. Genetic structure was maintained at the same distance in males and females. However, females showed a stronger relationship between genetic and spatial distance at short distances (0-1 km), whereas males showed stronger structuring in the distance class of 1-5 km. Assignment index provided little evidence for sex-biased dispersal, yet indicated a tendency towards greater dispersal in males. Telemetry data on dispersal provided results consistent with genetic data. Sex bias was manifested in dispersal distance, but not dispersal frequency, with females usually settling in the direct vicinity of the maternal home range and males few home ranges away from the natal site. Dispersal occurred at the age of 18-21 months. The social network, derived solely from association data, was spatially structured with well defined and cohesive social units. The units were also genetically distinct, with high intra-group and low inter-group genetic relatedness. Space utilised by the members of a given social unit overlapped extensively while little overlap was observed across social units. The genetic structure in the network can be interpreted as an emergent property of philopatry and spatial segregation of social groups. However, spatial distribution and overlap of individuals could not fully explain the association patterns and network structure. Association and genetic data indicated that active social preferences and targeted interactions played important roles in shaping the social structure of the population. The close match of the social structure revealed in social units and family groups (obtained from trapping data) indicated that they mirror the same level of social organisation. Sociality of wild boar, estimated by three network centrality measures, generally decreased with age of the individuals, and magnitude of age effect was modified by sex. Young wild boars showed the strongest grouping tendency and were the most diversely connected within the networks. The onset of dispersal marked a decrease in sociality, which was manifested stronger in males than in females. These results highlight the role of young wild boar in maintaining cohesion of the social network. Changes in sociality mirrored major transitions in wild boar ontogeny.