RABBITS RESPOND TO INCREASED C4 ABUNDANCE DURING THE POST-LGM

The evolutionary history of Leporidae (rabbits and hares) has been driven in part by the spread of C₄ grasslands from the late Miocene onward. As grasslands expanded, leporid diversity, body size, and distribution increased, with several genera appearing in the late Miocene and Pliocene. Today in North American prairies, C₄ grasses are often an important component of rabbit and hare diet. Understanding the local evolutionary response of fauna, such as leporids, to environmental change in deep time has proven difficult because of temporal uncertainty. Fossil deposits with high temporal resolution can be used as case studies to investigate local faunal response to environmental change.

In order to test for leporid response to changes in C₄ grass abundance, we collected rabbit fecal pellets (n=18) from a late Pleistocene rockshelter in south-central Montana, for δ¹³C analysis and reconstructed C₄ contribution to diet. We compared our results with climate-vegetation model %C₄ estimates for the site, δ¹³C values of sedimentary organic matter (n=27) deposited within Last Canyon Cave, and pollen records for the site. Today, the C₄ grass Bouteloua gracilis is a small but significant (~8-10%) portion of the steppe flora below Last Canyon Cave. During the Last Glacial Maximum (LGM), the climate-vegetation model and sediment δ¹³C values suggested an absence of C₄ grass and reduced (~36%±11%) seasonal C₄ abundance, respectively. In addition, grass pollen abundance was low. Rabbits were absent from the rockshelter during this LGM period with low C₄ abundance to absence of C₄ grass. In contrast, post-LGM (~15-12 ka) climate-vegetation model results (~24%±0%), sediments (~65%±8%), and a peak in grass pollen abundance all suggest a post-LGM C₄ pulse. Rabbits were present during this pulse, and rabbit pellet δ¹³C values suggest high C₄ grass consumption (~45%±6%). Rabbit fecal pellets and sediments detect the post-LGM C₄ pulse simultaneously, and provide evidence of rapid late-glacial occupation of the area as %C₄ increased. In addition, rabbit fecal pellets and sediments provide evidence of a high temporal resolution local leporid response to an increase in C₄ grass abundance.