141-5 Body Building with Microbes: Protein Metabolism in Pleistocene Megafaunal Herbivores

Session: New Advances in Geobiology

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Large herbivores build massive, protein-rich bodies despite consuming plant diets that are poor in nitrogen. Moreover, that nitrogen occurs in a mix of amino acids different from that in animal tissues. It is well established that gut microbes break down dietary fiber into short-chained fatty acids that the host can use for fuel; less is known about microbial contributions to body protein. Recent studies using carbon isotope (δ¹³C) analysis of essential amino acids (AA_EES) in small mammals have shown that symbiotic gut microbes convert carbon from dietary molecules (e.g., carbohydrates) into amino acids that are used by the host to build structural proteins (e.g., red blood cells), and our ongoing work is extending this approach to extant large African mammals. Here, we apply δ¹³C analysis of AA_ESS to bone and tooth collagen from North American Pleistocene megafaunal herbivores to quantify whether their tissues were built from dietary (i.e., plant) or gut microbe AA_ESS. We examine specimens from Alaska, Iowa, and the Great Lakes region, and include taxa that survived the Pleistocene extinction in North America (bison, tundra muskox, caribou), and several that did not (Jefferson’s ground sloth, mastodon, mammoth, horse, helmeted muskox, stag-moose). These animals lived in a variety of habitats dominated by C₃ plants and had diverse feeding ecologies (grazers, browsers, mixed feeders) and digestive physiologies (hindgut and foregut fermenters). Gut microbe contributions to ancient herbivore AA_ESS do not appear to differ with location or diet type but instead vary with digestive physiology–hindgut fermenters (mastodon, mammoth, horse) chiefly derived AA_EES from plants, whereas most foregut fermenters (bison, muskox, caribou) and ground sloths had greater contributions of AA_EES from gut microbes. Stag-moose fell into both clusters. These results for Pleistocene herbivores differ from those for extant African herbivores, where all animals consuming C₄ grass (with its low nitrogen concentration) had substantial gut microbe contributions to their AA_EES, regardless of digestive physiology. Together, these studies reveal how large mammal communities, past and present, meet the critical ecophysiological challenge of body building on relatively low-quality plant resources. Further, the sometimes-strong differences in gut microbiome amino acid contributions in foregut versus hindgut fermenting herbivores may have implications for understanding the decline of hindgut fermenters over the Neogene, especially after the rise of C₄ grasslands.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9775

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