MICROEVOLUTIONARY CHANGES IN GRAY FOXES (UROCYON CINEREOARGENTEUS) FROM RANCHO LA BREA, LOS ANGELES, CALIFORNIA

Towards the end of the Pleistocene epoch, two major disturbances—megafaunal extinction and

climate change—changed the landscape and interactions among fauna. The surviving small

carnivores tell a story of species adapting to these rapid changes. The purpose of this study was to document how the gray fox (Urocyon cinereoargenteus) responded to these ecological and environmental shifts. Using the fossil collection of gray foxes at the La Brea Tar Pits and Museum and modern representatives at the Natural History Museum of Los Angeles County, we collected linear measurements and calculated biomechanical indices on skulls, teeth, and limb bones to quantify variation within the modern population and compare with the fossil population. We analyzed 43 modern individuals and 60 fossil specimens of the crania and 12 modern individuals and 157 fossil specimens of the postcrania. We found that modern gray foxes were slightly sexually dimorphic, with males having longer upper canines and humeri than females. Across California, foxes from Santa Cruz County differed from others in having larger molars, while foxes from Orange and Ventura Counties had longer tibiae. These geographic differences across extant foxes likely stem from differences in prey availability and habitat, with more mesic regions possibly providing greater prey biomass and foxes in more xeric regions possibly foraging and digging more. Compared to extant foxes, fossil foxes had longer lower carnassials and carnassial blades as well as smaller relative grinding areas on the upper molars, and smaller areas for muscle attachment on the ulna and femur. These results suggest that fossil gray foxes were larger and more carnivorous than their modern omnivorous counterparts. As well, the postcranial differences suggest that modern gray foxes may climb or dig more than fossil representatives. Megafaunal extinctions may have driven these ecomorphological changes by releasing mesocarnivores from intraguild predation while enabling exploration of new options in diet and habitat, producing their current widespread distribution throughout California. These ecomorphological changes document a history of survival during drastic, large-scale disturbances that may inform the response of modern grey foxes to new challenges from anthropogenic pressures.