SEASONAL DIET ECOLOGY OF RANCHO LA BREA DIRE WOLVES

Dire wolves were large-bodied hypercarnivores that lived during the Pleistocene epoch. We investigated whether Rancho La Brea dire wolf diets varied seasonally and if these variations can be explained by wolf or prey migration. We collected serial samples from 10 dire wolf teeth which were analyzed for their δ¹³C and δ¹⁸O isotope values. δ¹³C acts as a measure of both diet and, to a lesser extent, climate. For predators such as dire wolves, C isotope compositions reflect the herbivores they consume, and the herbivores’ isotopic composition is reflected by the plants they consume. The δ¹³C value of a plant varies based on several factors, most notably, its photosynthetic pathway. C₃ plants are adapted for cooler, wetter areas and have lower δ¹³C values around -14.5‰. Conversely, C₄ photosynthetic plants are adapted for warmer, drier climates and have higher values around 2.5‰. Oxygen isotopes, on the other hand, are derived from drinking water and record seasonality and climate. Low δ¹⁸O values reflect cool periods while warmer temperatures produce higher δ¹⁸O values. The average δ¹³C value in the teeth we have sampled is -8.3±1.2‰ (±1σ) while the average δ¹⁸O value is -3.9±1.4‰. These values are similar to previously published isotope compositions for RLB herbivores (horses, bison), which suggests a very small percentage of C₄ plants in the diet of dire wolf prey, consistent with Rancho La Brea as a primarily C3 ecosystem. We also found an inverse relationship between carbon and oxygen isotope compositions within individual teeth. This suggests a dietary shift towards more open-habitat prey during warmer periods. A shift to greater fat consumption in dire wolf diets during warm periods could also cause this inverse relationship, as fat is depleted in ¹³C. Both of these interpretations imply a seasonal component to diet choices, which may be due to movement of prey, wolves, or both. However, given the small amount of variation in both δ¹³C and δ¹⁸O compositions in the teeth sampled, it is most likely that these seasonal changes are not due to substantial movement of dire wolves. As such, we predict that dire wolves did not follow migrating animals, and instead remained within the area around Rancho La Brea year-round. Future work will expand on these data with increased sample sizes and with other isotopic systems such as nitrogen and strontium.