Leaving by staying: Social dispersal in giraffes

Dispersal is a critical process that shapes the structure of wild animal populations. In species that form multi-level societies, natal dispersal might be social (associating with a different social community while remaining near the natal area), spatial (moving away from the natal area while continuing to associate with the same community) or both social and spatial (associating with a different community and moving away from the natal area). For such species, classical spatial measures of dispersal, such as distance moved, might not capture social dispersal. We examined dispersal outcomes for 67 male and 70 female giraffe calves over 7 years in a large, unfenced, ecologically heterogeneous landscape. We tested predictions about the influence of sex, food availability, low- and high-impact human settlements, and local giraffe population density on social or spatial dispersal, dispersal distance, and age of dispersal. We found that dispersal is sex-specific, with females being predominately philopatric. When dispersing, both sexes did so at a mean of 4 years of age. Most (69% of total) young males dispersed, with 84% of male dispersers associating with a different adult female social community than that of their mother, but one in four of these dispersers remained spatially near to their natal area. For adolescent males that dispersed socially but not spatially, overlapping female social communities may represent a potential pool of unrelated mating partners without the risks of travelling to unfamiliar areas. Just 26% of young females dispersed and half of these continued to associate with the adult female social community into which they were born, confirming the importance of maintaining ties among females from calf to adulthood. Furthermore, individuals born farther from high-impact human settlements were more likely to spatially or socially-and-spatially disperse, move greater distances from their natal areas, and disperse at a younger age. Our study highlights the potential importance of social structure in dispersal decisions, and of tracking social structure when studying dispersal in multi-level societies, as effective dispersal can be attained without large-scale spatial displacements.