Niche partitioning reduces competition between species and facilitates coexistence. However, species can coexist within the same niche if they have sufficiently similar fitness. Here, we study the interplay between competition and fitness equivalence in diverse large mammal herbivore communities in the framework of modern coexistence theory. We used stable isotope analysis to quantify niche overlaps and compared them to similarities in species’ intrinsic growth rates. We then combined these data with long-term census data to parameterize multispecies models of population dynamics. Isotopic niche partitioning was clumped, with less overlap between, and more overlap within, feeding guilds. This partitioning was in excess of what is mathematically necessary to overcome fitness differences over a wide range of body sizes (~40 to 600 kg). Although partitioning increases population growth rates, coexistence in mammal herbivore communities is maintained primarily by life history traits that ensure some level of fitness equivalence among species.