Extreme ecological specialization in a rainforest mammal, the Bornean tufted ground squirrel, Rheithrosciurus macrotis

The endemic Bornean tufted ground squirrel, Rheithrosciurus macrotis, has attracted great interest among biologists and the public recently. Nevertheless, we lack information on the most basic aspects of its biology. Here we present the first empirical data on the feeding ecology of tufted ground squirrels, and use data from 81 sympatric mammalian and avian vertebrates to place it within a broad comparative context. R. macrotis is a dedicated seed predator and shows much more extreme ecological specialization than any other vertebrate, feeding on a far smaller subset of available plant foods and demonstrating a greater reliance on a single plant species- Canarium decumanum-than any other vertebrate taxon. Our results suggest that R. macrotis plays an important, previously unknown role in the ecology of Bornean lowland forests, and highlight how much we have yet to learn about the fauna inhabiting some of the most diverse, and most severely threatened, ecosystems on the planet.