The extent of the Sierra Nevada red fox populations is an area of active research. In Oregon, ongoing studies at Mount Hood and Central Oregon were prompted by observations in 2012 and 2013. Recent genetic evidence also suggests range expansion into western Oregon since the 1940s. In California, detections occurred in northern Yosemite National Park the winter of 2014-15, the Stanislaus National Forest in late 2015, and in Lassen Volcanic National Park in 2018. The first two areas are near Sonora Pass, but it has not been confirmed the individuals are part of the Sonora Pass population.