The evolution of ecological specialization in southern African ungulates: competition- or physical environmental turnover?

Using long-term diet reconstructions spanning the past one million years, we contrast hypotheses that biotic interactions versus physical environmental changes are primary drivers of evolutionary turnover in mammals. We use stable carbon (δ₁₃C) and oxygen (δ₁₈O) isotope ratios in tooth enamel carbonate to trace herbivore niche shifts through the Late Quaternary Land Mammal Ages (LMAs) of grassland savannas in the South African interior (Cornelian-1.0 to 0.6 Ma; Florisian-500 to 10 ka; and Holocene/modern). Data reveal niche separation amongst closely related coeval taxa, and dispersals through time into empty niche spaces following extinctions. This suggests a primary role of competitive exclusion and niche displacement for speciation and extinctions in these early grassland environments. However, niche changes through time show a similar trend in many taxa, entailing increased δ₁₃C (elevated C₄ grass consumption) from the Cornelian to the Florisian, and from the Florisian to the Holocene/modern, and elevated δ₁₈O in Holocene/modern taxa that reflect global aridification around the terminal Pleistocene. Commonality in isotopic trends implies universal environmental forcing of ecological, and ultimately macroevolutionary, turnover. Yet some taxa shift from a mixed C₃/C₄ diet in the Florisian to a near-pure C₃ diet today. Indeed, we find that while δ₁₃C data are normally distributed for Cornelian fossils, non-normal distributions characterize more recent time intervals. Such distributions are in line with the bimodal distribution of δ₁₃C and diet in contemporary African ungulates. Thus, while environmental forcing did not, by necessity, lead to increases in C₄ intake, the results show changes from mixed to more specialized diets. We propose that this niche specialization was a function of long-term exposure to C₄ grasslands, consistent with predictions that relatively high metabolic demands of C₄ grazing in subtropical environments forced the differentiation of herbivores into one of two highly specialized feeding niches, i. e. C₃ browsing or C₃ grazing.