Date of this Version
Baldvins, T., 2015, Climate effects on mammalian body size and grassland composition inferred from late Quaternary grazers in the Great Plains of North America [M.S. thesis]:Lincoln, University of Nebraska, 66 p.
Several mammalian taxa exhibited a diminution during the Pleistocene-Holocene transition. Two hypotheses have been proposed to explain this phenomenon: (1) human hunting and (2) climate change along with its effects on grasslands. I use stable isotopes from a variety of Quaternary grazing mammals to measure grassland changes following the Last Glacial Maximum (LGM) in Nebraska and to develop a climate proxy to compare with mammalian body size. Results indicate that latest Pleistocene equids and mammoths have δ13C values indicative of a C3 or mixed C3/C4 diet. However, Holocene bison have greatly elevated δ13C values, indicating a diet dominated by C4 vegetation, suggesting a northerly expansion of C4 grasses with the warm conditions following the LGM. High δ13C values from Bison bison sampled in this study suggest a greater abundance of C4 grasses in Nebraska than today, and may indicate that these bison lived during the Holocene thermal maximum. Bison bison sampled in this study display a significant but weak correlation (r2=0.20, p=0.004, n=39) between δ13C and first molar occlusal area, which acts as a proxy for body size, indicating that size decreased as dietary C4 increased and climate warmed; however, weakness of the correlation does not support previous hypotheses that attribute bison diminution solely to increased dietary C4. These data indicate that other factors, such as temperature, aridity, and/or human hunting, are needed to explain the decrease in bison body size. On the other hand, a strong significant relationship (r2=0.83, p=0.011, n=6) between δ13C and equid metacarpal length of middle Pleistocene equids spanning both warm and cold intervals suggests that an increase in body size occurred with warming. I propose that both of these trends in body size were driven by changes in productivity. C4 dominated habitats of the Holocene provided less primary productivity than mixed C3/C4 habitats contributing to bison diminution, while increased productivity during warmer intervals of the Pleistocene allowed equids to increase in size.
Advisor: Ross Secord