Effect of the habitat and tusks on trunk grasping techniques in African savannah elephants

Among tetrapods, grasping is an essential function involved in many vital behaviours. The selective pressures that led to this function were widely investigated in species with prehensile hands and feet. Previous studies namely highlighted a strong effect of item properties but also of the species habitat on manual grasping behaviour. African savannah elephants (Loxodonta africana) are known to display various prehensile abilities and use their trunk in a large diversity of habitats. Composed of muscles and without a rigid structure, the trunk is a muscular hydrostat with great freedom of movement. This multitasking organ is particularly recruited for grasping food items while foraging. Yet, the diet of African savannah elephants varies widely between groups living in different habitats. Moreover, they have tusks alongside the trunk which can assist in grasping behaviours, and their tusk morphologies are known to vary considerably between groups. Therefore, in this study, we investigate the food grasping techniques used by the trunk of two elephant groups that live in different habitats: an arid study site in Etosha National Park in Namibia, and an area with consistent water presence in Kruger National Park in South Africa. We characterised the tusks profiles and compared the grasping techniques and their frequencies of use for different foods. Our results show differences in food-grasping techniques between the two groups. These differences are related to the food item property and tusk profile discrepancies highlighted between the two groups. We suggest that habitat heterogeneity, particularly aridity gaps, may induce these differences. This may reveal an optimisation of grasping types depending on habitat, food size and accessibility, as well as tusk profiles.

Variation in herbivore space use: comparing two savanna ecosystems with different anthrax outbreak patterns in southern Africa

BACKGROUND

The distribution of resources can affect animal range sizes, which in turn may alter infectious disease dynamics in heterogenous environments. The risk of pathogen exposure or the spatial extent of outbreaks may vary with host range size. This study examined the range sizes of herbivorous anthrax host species in two ecosystems and relationships between spatial movement behavior and patterns of disease outbreaks for a multi-host environmentally transmitted pathogen.

METHODS

We examined range sizes for seven host species and the spatial extent of anthrax outbreaks in Etosha National Park, Namibia and Kruger National Park, South Africa, where the main host species and outbreak sizes differ. We evaluated host range sizes using the local convex hull method at different temporal scales, within-individual temporal range overlap, and relationships between ranging behavior and species contributions to anthrax cases in each park. We estimated the spatial extent of annual anthrax mortalities and evaluated whether the extent was correlated with case numbers of a given host species.

RESULTS

Range size differences among species were not linearly related to anthrax case numbers. In Kruger the main host species had small range sizes and high range overlap, which may heighten exposure when outbreaks occur within their ranges. However, different patterns were observed in Etosha, where the main host species had large range sizes and relatively little overlap. The spatial extent of anthrax mortalities was similar between parks but less variable in Etosha than Kruger. In Kruger outbreaks varied from small local clusters to large areas and the spatial extent correlated with case numbers and species affected. Secondary host species contributed relatively few cases to outbreaks; however, for these species with large range sizes, case numbers positively correlated with outbreak extent.

CONCLUSIONS

Our results provide new information on the spatiotemporal structuring of ranging movements of anthrax host species in two ecosystems. The results linking anthrax dynamics to host space use are correlative, yet suggest that, though partial and proximate, host range size and overlap may be contributing factors in outbreak characteristics for environmentally transmitted pathogens.