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Geometric tortoises (Psammobates geometricus) are endangered as a result of many human influences, but primarily because 97% of their habitat, West Coast Renosterveld, has been converted to agricultural fields during the past 400 years. 1–3 Several small fragments of renosterveld remain, in part because these fragments are less desirable for agriculture but also because of concerted efforts to understand and conserve geometric tortoises 2, 4–7 and renosterveld. 8 Yet the basis for the critical link of these animals to West Coast Renosterveld is not known. In 2002, we began a project aimed at identifying the resources used by geometric tortoises in West Coast Renosterveld. In one study we collected data and samples for evaluating the tortoises’ diet selectivity and its determinants, such as plant nutrients and biomechanics. Biomechanical results suggested that the new leaves of several common, non-food plant species are too tough for the tortoises to pull, break and consume. 9 In contrast, the leaves of several food plant species required little force to consume. 9 To understand food choice better, it is important to evaluate the costs and benefits of the biomechanics, secondary compounds and nutrients of potential food items. Variations in plant nutrients may affect the survival, growth and reproductive success of geometric tortoises. Yet, we do not know the nutritional requirements of these animals or the nutritive value of their food plants. We thus measured the nutrient concentrations of several food and non-food plant species found in renosterveld at the largest geometric tortoise reserve (Elandsberg Private Nature Reserve, Western Cape …