Dynamics of a Socially and Spatially Structured Giraffe Population in a Human-Natural Landscape

Sociality involves a constant trade-off between fitness benefits and costs of living in groups, and this trade-off can be influenced by the social and ecological environment in which individuals live. In this PhD I explored socioecological factors underlying the social and spatial population structure and dynamics of a large tropical herbivore with a highly fission-fusion social system, the giraffe (Giraffa camelopardalis). Using a dataset of more than 3,000 uniquely identified individuals collected over a period of 8 years in the coupled human-natural Tarangire Ecosystem of northern Tanzania, I (1) investigated natural and anthropogenic factors as mechanisms of giraffe grouping dynamics, social structure, space use, and vital rates; (2) quantified fitness consequences of social behaviours of adult female giraffes in relation to the influence of their physical environment; and (3) compared social versus spatial dispersal of subadult female and male giraffes. I used capture-mark-recapture techniques to estimate survival rates while accounting for imperfect detection, and social network analysis to derive network- and individual level social metrics and to delineate discrete communities of socially associated adult female giraffes within a larger contiguous metapopulation. Natural and anthropogenic factors included vegetation types and
preferred plant forage species, natural predation, and distance to traditional (bomas) and modern human settlements (towns). I found that grouping patterns of giraffes were influenced by food availability, predation risk, and presence of humans, with particular requirements for mothers with calves (chapter 1). I parsed the metapopulation into 14 distinct, modular yet overlapping communities of socially associated adult female giraffes, with 11 communities large enough to test hypotheses explaining variation in social structure (chapter 2). Adult females in communities closer to bomas had weaker relationship strengths among all members of the community and more exclusive relationships with fewer other females, suggesting that the presence of humans disrupted their social structure. In an examination of social versus ecological drivers of variation in reproduction and survival among 10 of the communities, I showed demographic rates were correlated with vegetation and proximity to humans, as communities with more dense bushlands had lower calf survival while those closer to human settlements had higher reproductive rates (chapter 3). Adult female survival did not differ among communities (chapter 3), but more gregarious females (being in larger groups) and females with higher betweenness (associated with more groups) had higher survival (chapter 4). Survival of adult females is improved by being well-integrated into their larger social community through having weaker bonds with many others rather than by forming stronger and highly stable bonds with just a few individuals. This suggests that the disruption of social structure close to bomas as evidenced in chapter 2 could have demographic consequences, although proximity to
bomas did not influence adult female survival as much as their level of sociability (chapter 4). In chapter 5, I investigated patterns of natal dispersal, and found that while most young males dispersed into new social communities far from where they were first detected as calves, many shifted into new communities that were close to their natal areas. In contrast, few young females dispersed, but those that did disperse rarely shifted into a new social community. Instead females moved spatially while remaining within their natal community, further demonstrating the importance of maintaining social ties, from calf to adulthood, across their community of associates. Human presence influenced space use of adults, as adult females living closer to densely populated towns had significantly larger home ranges, but no such relationship was evident with bomas, indicating a difference in anthropogenic impact on movements of giraffes between traditional versus modern human lifestyles (chapter 6). My research indicates that social associations among individuals in addition to ecological conditions are likely to be important for population persistence, and should be considered when developing and implementing conservation measures for giraffes such as land-use plans and translocations.

Publish DateMay 28, 2021
Last UpdatedMay 28, 2021
Size0.00 KB