Significance of diet type and diet quality for ecological diversity of African ungulates

Abstract

1. We test two nutritional hypotheses for the ecological diversity of ungulates, the browser/grazer (diet type) and diet quality models, among free-ranging herbivores in a South African savanna, the Kruger National Park. Tests are based on assessment of relationships between diet type and diet quality with body mass and hypsodonty, two morphological features that have been associated with both elements. 2. We use stable carbon isotope ratios of faeces to reconstruct diet in terms of proportions of C(3) plants (browse) and C(4) plants (grass) consumed by different species in different seasons. These data are combined with proxies for diet quality (per cent nitrogen, neutral detergent fibre, acid detergent fibre, and acid detergent lignin) from faeces to track changes in diet quality. 3. Two statistical approaches are used in model selection, i.e. tests of significant correlations based on linear regression analyses, and an information-theory approach (Akaike's Information Criterion) providing insight into strength of evidence for models. 4. Results of both methods show that, contrary to many predictions, body mass and diet type are not related, but these data confirm predictions that diet quality decreases with increasing body size, especially during the dry season. Hypsodonty, as expected, varies with diet type, increasing with increased grass intake. 5. These findings support both a diet type and diet quality model, implying some degree of exclusivity. We propose that congruence between models may be achieved through addition of diet quality proxies not included here, because hypsodonty is more likely a reflection of the abrasive properties of consumed foods, i.e. related to food quality, rather than food type. This implies that adaptation to diets of varying quality, through changes in body size and dental features, has been the primary mechanism for diversification in ungulates. 6. Our interpretation contrasts with several recent studies advocating diet type as the primary factor, exemplifying that further reconciliation between the two models is needed. We discuss the implications of this study for future approaches to achieve a more cohesive understanding of the evolutionary outcomes of herbivore nutrition.