The late Pleistocene was a climatically dynamic period, with abrupt shifts between cool-wet and warm-dry conditions. Increased effective precipitation supported large pluvial lakes and long-lived spring ecosystems in valleys and basins throughout the western and southwestern U.S., but the source and seasonality of the precipitation are debated. Here we present stable carbon and oxygen isotope data from tooth enamel of late Pleistocene herbivores recovered from paleowetland deposits at Tule Spring Fossil Beds National Monument in the Las Vegas Valley of southern Nevada, as well as modern herbivores from the surrounding area, to investigate whether winter or summer precipitation was responsible for driving the wet hydroclimate conditions that prevailed in the region during the late Pleistocene. Tooth enamel δ18O values for Equus, Bison, and Mammuthus are generally low (average 22.2±0.7‰, 2 s.e., VSMOW) compared to modern equids (26.1±1.0‰), and imply lower water δ18O values (–16.5 ±0.8‰) than what is observed in active springs and wells in the Las Vegas Valley (–12.9‰) or implied by modern equids and a local calibration of equid and water compositions (-12.2±1.1‰). Notably, Camelops generally yielded higher δ18O values (23.7±1.1‰), possibly suggesting drought tolerance. Mean δ13C values for the Pleistocene grazers (–6.4±0.8‰, 2 s.e., VPDB) are considerably higher than for modern equids (-10.4±0.4‰) and indicate more consumption of C4 grass (18±6%) than today 0±3%). However, calculated C4 grass consumption in the late Pleistocene is strikingly lower than the amount of C4 grass taxa currently present in the valley (55-60%) which is unexpected in the context of widespread C4 grassy patches. δ13C values in Camelops tooth enamel (-7.4±1.2‰) are interpreted as reflecting moderate consumption (16±9%) of Atriplex (saltbush), a C4 shrub that flourishes in regions with hot, dry summers. Lower water δ18O values, lower abundance of C4 grasses, and the inferred presence of Atriplex are all consistent with general circulation models that show the enhanced winter moisture delivered into the interior western U.S. during the late Pleistocene was sourced from the north Pacific, but do not support alternative models that infer enhanced summer precipitation was sourced from the tropics. In addition, we hypothesize that dietary competition between the diverse and abundant Pleistocene fauna may have driven the grazers analyzed here to feed preferentially on C4 grasses. Dietary partitoning, especially when combined with decreased pCO2 levels during the late Pleistocene, could readily explain the relatively high δ13C values observed in late Pleistocene grazers in the Las Vegas Valley and elsewhere in the southwestern U.S. without the input of additional summer precipitation as it has been interpreted previously. This suggests that Pleistocene hydroclimate parameters derived from floral records may need to be reevaluated in the context of the potential effects of dietary preferences and lower pCO2 levels on the stability of C3 vs. C4 plants.