Quality is the key: Seasonal home range shifts of white rhinos are driven by food quality not availability

Background/Question/Methods

As the dry season progresses across southern Africa, the availability and quality of food declines for large herbivores. Female white rhinos (Ceratotherium simum) compensate for this by expanding their home ranges. This expansion may be to incorporate habitat types that contain more food or high quality food. To determine the factors that drive this expansion, we focused on dry season changes in the availability and quality of grass in habitats utilised by white rhinos in the Ithala Game Reserve, South Africa. We expected that, if food quality was the main driver, white rhinos would incorporate habitat types with the highest nutritional quality into their dry season home ranges. In contrast, due to their large body size and ability to survive on low quality food, they may rather incorporate habitat types with high food availability.

Results/Conclusions

Surprisingly, we found that during the dry season female white rhinos did not increase the size of their home ranges, but rather shifted their home range boundaries. This resulted in individuals increasing the amount of bushveld and decreasing the amount of wooded grasslands within their dry season home ranges. These same adjustments took place in the home range cores, but in addition the white rhinos increased the amount of wooded outcrops and reduced the amount of old land grasslands. When we explored the different factors that could explain these patterns, we found that a change in the crude protein content of grass was the key factor driving the incorporation and exclusion of habitat types in both the core and total home ranges. During the dry season, white rhinos incorporated habitats that experienced the smallest seasonal reduction in crude protein content (i.e. 23-44%), while excluding those with a large decrease in crude protein. Our results indicate that, in contrast to the predictions of the Jarman-Bell principle, white rhinos did not adjust their dry season home ranges based on food availability. Rather, they made large scale adjustments to their home ranges to compensate for seasonal changes in food quality.