Should elephants graze or browse? The nutritional and functional consequences of dietary variation in a mixed-feeding megaherbivore.

Unlike specialist browsers and grazers, the diets of mixed-feeding megaherbivores are broad and complex, comprising numerous plant species of variable nutritional quality. Understanding key axes of nutritional variation in the diets of mixed-feeding megaherbivores is challenging but is crucial to understand their impacts on vegetation. Here, we revisit a long-standing debate on whether browse is more nutritious than grasses for elephants, as browse is thought to contain higher crude protein (CP). We quantified diet composition using carbon isotope analyses and analysed forage quality in 102 Asian elephant faecal samples from southern India, and found that high-browsing and low-browsing diets had similar forage quality, as indexed by nitrogen and carbon-to-nitrogen ratio. To explore the generality of this finding, we analysed nutritional differences between browse and grass across 141 plant species consumed by Asian elephants across their distribution range. We show that woody tissues and non-legume plants, which dominate elephant browse, do not have higher forage quality or CP than grasses, a trend which may be common in Asia's mixed-feeding large herbivores. Finally, based on the observed habitat-wide variation in browsing, we provide a new framework to assess the impacts of Asian elephants on woody vegetation, with important implications for carbon cycling.