Dietary Ecology of Extant Artiodactyls: Biogeography, Ecomorphology, and Isotope Ecology, with Implications for Paleoecology

Abstract

The order Artiodactyla is a diverse group of terrestrial mammals that has been an important component of most terrestrial ecosystems since the Eocene. Understanding artiodactyl dietary ecology in relation to environmental gradients, morphological traits, and isotope ecology provides useful tools for making ecological inferences in the fossil record. I analyzed these topics for a wide range of extant species, utilizing a classification of herbivorous diets based on six rather than the usual three categories of dietary habits. The six-category scheme includes frugivores, browsers, browser-grazer intermediates, generalists, variable grazers, and obligate grazers. My dissertation research has three components. First, I analyzed the ecological diversity represented in the diet and body size of artiodactyls in relation to climate and topography, using a global dataset of locality-based occurrence data for 161 extant species. Results show that higher species richness is associated with greater ecological diversity. The highest artiodactyl richness occurs in Africa and is achieved by accommodation of more ecological traits as well as more coexisting species with the same traits. A range of ecological diversity levels, however, can occur in different areas with comparable species richness. Among climatic variables, seasonal extremes of temperature and rainfall are important predictors of artiodactyl richness. The dietary extremes (frugivores and obligate grazers) occur in the most restricted climatic conditions. The occurrence of these dietary categories in the ungulate fossil record, as well as the ecological structure of fossil ungulate faunas, can be useful for inferring paleoclimatic conditions. Next, I conducted an ecomorphological analysis of bovid mandibles with landmark-based geometric morphometrics. Mandibular morphology proves to be useful for differentiating dietary categories. Frugivores differ from browsers and obligate grazers from variable grazers. Notably, frugivorous bovids have a mandibular shape that is readily distinguished from all other dietary groups. The main differences in mandibular shape among dietary groups are related to the functional needs of species during forage prehension and mastication. Compared to browsers, both frugivores and grazers have mandibles that are adapted for higher biomechanical demand of chewing. Additionally, frugivore mandibles are adapted for selective cropping. These results offer an approach for reconstructing the diet of extinct bovids with mandibular morphology. In the third study, I compiled a global dataset of carbon-isotope composition of artiodactyl tooth enamel to evaluate the isotopic composition of ingested forage (δ13Cdiet). The herbivore dietary spectrum is expressed through increasing mean δ13Cdiet values from frugivores to obligate grazers, although the most depleted values occur in browsers that live under dense forest canopy. Grazing taxa generally have a wider range of δ13Cdiet values than browsing taxa. Variation in δ13Cdiet values also occurs among ecoregions, taxonomic groups, and geographic regions. Notably, variable grazers exhibit a bimodal distribution of δ13Cdiet values, with North American taxa consuming C3 vegetation and African taxa consuming C4 vegetation, reflecting the different amounts of C4 biomass available in these regions today. Clarifying the contributing factors to variation in δ13Cdiet will refine paleoecological reconstructions. These findings support use of the more detailed dietary classification in the study of artiodactyls. Identifying frugivory and obligate grazing, especially, will inform paleoenvironmental reconstructions. In addition, this work highlights the importance of integrating different data types in (paleo)ecological research. For example, stable-isotopic data showed strong separation of enriched and depleted values within morphospace for bovid mandibles, demonstrating the complementarity of different kinds of ecological data.