Leaving by staying: Social dispersal in giraffes

Abstract 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.

initial year of the study that (1) were adults (≥4 yr) the first time they were observed based on 23 visual determination in the field, and (2) were detected ≥6 times over the course of the study to 24 improve accuracy of social network analysis (Davis et al., 2018). This resulted in a subset of 423 25 adult females from a total of 1039 identified during the entire study period. We used the gambit 26 of the group to define associations, whereby individuals present together in the same group 27 formation were considered associated during that survey (Whitehead & Dufault, 1999). We 28 defined a group formation as one or more giraffes that were foraging or moving together, and 29 with >500 m distance to the closest member of another group (Carter, Seddon, et al., 2013;  pairs of adult female giraffes over 6 years (Carter, Brand, et al., 2013). 51 Appendix 3: Estimating age with photogrammetry 52 For each detection in our dataset, we estimated size as a proxy for age with photogrammetry 53 (Schrader et al., 2006). We calculated multiple heights for each individual to confirm our age 54 estimates. We used data from known-age calves born in captivity to estimate age-specific neck 55 length (NL), total height (TH), and the ratio of NL/TH for giraffes aged 0 to 6 months (M. L. 56 Bond, unpublished data). These data and other studies (Dagg, 2014) found no differences in 57 height between male and female giraffe calves during the first two years of life. For images of 58 our wild giraffes, we measured the number of pixels along the length of the giraffe's neck on all 59 photographs. We used algorithms incorporating the focal length of the lens and distance to the 60 subject to convert pixels to cm and estimate NL and thus age when the photograph was taken.

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For additional details on photogrammetry methods for giraffes, see Supporting Information S4 in 62 period), then inferences should be robust to sampling variation in the number of locations. We 70 conducted a simple linear regression to determine whether home range size was correlated with 71 the number of group locations among communities, and found no significant correlation ((F1,10 = 72 0.116, p = 0.74). We consider the estimated home ranges to adequately reflect the approximate 73 use of space by the community over the 7-year study period; we used the home ranges to 74 estimate the approximate amount of food resources available to them and the local giraffe 75 population density during this time. We utilized ground-based measurements that we collected in  To test for anthropogenic influences on dispersal, we computed the average distance (km) 83 from all locations of community members to the nearest boma (a low-impact human settlement) 84 and the nearest town (a high-impact human settlement). We used Google Earth imagery to map 85 all bomas and towns, and to compute distances we used the "Generate Near Table"   We tested for correlations among the socio-ecological covariates and did not use two 91 variables together in the same model if they were highly correlated (Table S1). Models tested 92 and model selection results are presented in Table S2.

Lake Burunge
Lake Manyara

Lake Burunge
Lake Manyara

MR
LGCA TNP with grey squares are four females removed as spatial dispersers and re-classified as non-dispersers that made spatial sorties.