Effects of density and kinship on the spatial organization of an asocial ground squirrel

Abstract Among mammals, spatial organization (home range size and overlap) may be influenced by kinship, and tolerance among kin is believed to play a role in the evolution of sociality. In solitary species, adults—including close kin—are presumed to have home ranges that are distinct and non-overlapping. We characterized spatial organization and behavioral interactions, including the role of kinship in a free-living population of golden-mantled ground squirrels (Callospermophilus lateralis), a presumably asocial and solitary species. We also capitalized on changes in population size over 25 years (1995 to 2019) to quantify the influence of density on that spatial organization. We found extensive overlap between female home ranges (30%) and between male and female home ranges (28%) after the breeding season. However, the core areas of adult female home ranges overlapped minimally (7%), suggesting that squirrels were territorial, but only for the inner part of their home range. Contrary to expectation for solitary ground squirrels, kinship played a role in the spatial organization: related females shared space more than unrelated females. We did not find an effect of overall population density on home range or core area size, but increased local density was associated with an increase in home range size. While we did not find an effect of density alone on spatial overlap, there was a density-mediated increase in space-sharing among kin; space-sharing by female kin was expressed primarily at high densities, suggesting that familiarity and inclusive fitness benefits may offset costs of competition and promote philopatry under certain conditions. We also found behavioral evidence to support this interaction: greater spatial overlap of home range core areas reliably increased the number of chases between squirrels, unless those squirrels were highly related. Overall, we provide evidence of a facultative transition in golden-mantled ground squirrels from asociality to the formation of single-family kin clusters at high densities, the first step toward developing sociality. Our study highlights the importance of considering behavioral interactions in asocial species, as well as the interactive effects of environmental conditions and kinship when assessing the spatial and social organization of mammals.