The feeding niches and trophic ecology of two South African grazers, blue wildebeest Connochaetes taurinus and black wildebeest Connochaetes gnou, are compared using stable carbon and nitrogen isotope data from feces and tooth dentine collagen. As sympatric, closely related taxa predicted to occupy similar trophic positions, the blue and black wildebeest provide a good model for studying the mechanisms of coexistence and macroevolution in mammals. Data from feces collected from a single reserve in the Free State Province reveal different trophic behaviors between two herds of blue wildebeest and between both compared with a single herd of black wildebeest. These data suggest that sympatric coexistence of blue and black wildebeest is facilitated by differential niche occupation at family group or herd levels, rather than between species. However, such separation does not occur over longer time scales: results from dentine collagen support the hypothesis that the two species are indistinct in terms of trophic behavior, although blue wildebeest show more feeding flexibility, probably because of their wider habitat tolerance range. Similarities in premaxillary width of males and females of both species also suggest that both species are adapted to similar feeding styles. Thus, it is unlikely that changes in trophic behavior provided the trigger for divergence of the black from the blue wildebeest lineage in the Middle Pleistocene. We argue that the case of these two species represents an example of speciation that was not driven by resource competition, as is often assumed for many turnover events in mammalian evolution. We briefly discuss a previous suggestion that links black wildebeest evolution to their more territorial breeding behavior associated with Middleto-Late Pleistocene landscape changes in southern Africa.