Going to extremes for sodium acquisition: use of community land and high‐altitude areas by mountain gorillas Gorilla beringei in Rwanda

Space use in mammals may be influenced not only by their primary foods, but also by localized sources of physiologically critical resources such as sodium‐rich plants. We examined how sodium acquisition influences habitat use in mountain gorillas (Gorilla beringei) in Rwanda which have increased the amount of time they forage on community land outside of Volcanoes National Park (VNP), where eucalyptus (Eucalyptus spp.) tree bark is their most frequently eaten food. We measured sodium content in samples from 34 main dietary items and quantified sodium intake by 22 gorillas in three social groups over one year. On a dry weight basis, eucalyptus bark contains 3100 mg Na/kg. In contrast, the four herbs most frequently exploited for food inside the park are relatively sodium‐poor (<70 mg/kg each). Further, sodium intake rates were highest when the gorillas were on community land. Of the two groups that fed outside of the park, one obtained 73% and the other one 45% of their sodium in that habitat despite only feeding for minimal amounts of time there. However, one group that did not feed on community land acquired 78% of its sodium in the subalpine and alpine zones through the consumption of pith of giant lobelias and groundsels. Obtaining sodium thus likely creates an incentive for the gorillas to leave the park and make forays into high‐altitude habitat. Both strategies are not without risks: exiting their natural habitat and feeding on crops may increase human‐wildlife conflict and visiting high‐altitude areas may increase the risk of hypothermia.