Tracking One of California's Rarest Mammals

A man crouches behind a woman kneeling who is releasing a red fox into a snow-covered forest.
Pete Figura (left) and Deana Clifford of California Department of Fish and Wildlife release
a male red fox back into the wild on February 13, 2018. CDFW photo by Corrie McFarland.

CDFW Photo

In winter of 2018, California Department of Fish and Wildlife (CDFW) biologists began an intensive camera survey effort in and around Lassen Volcanic National Park. By March, they had captured and collared one male and two female Sierra Nevada red foxes. These three foxes are the first of their kind captured in over a decade and offer hope of better understanding this state-listed threatened species.

Data collected from the GPS tracking collars will provide significant insights into the ecology of the Sierra Nevada red fox (SNRF). This includes the size and characteristics of its home range, how it uses habitat, where it dens, and its reproductive biology.

CDFW Environmental Scientist Jennifer Carlson discussed data gathered from the first collared fox, “we have already been surprised by the large area the fox has been using and the distance it has traveled—it has averaged over seven straight-line miles per day in very rugged terrain.”

SNRF is a genetically and geographically distinct subspecies of red fox that prefers to live at high elevations, usually above 5,000 feet. A handful of subtle adaptations aid its survival in harsh, winter conditions. Dense fur on its toe pads provide insulation and snowshoe-like float. An unusually dense winter coat provides warmth and insulation against the elements.

Three of North America's ten red fox subspecies reside in high elevation areas: Sierra Nevada (V. v. necator), Cascade (V. v. cascadensis), and Rocky Mountains (V. v. macroura). In California, the Sacramento Valley red fox occupies portions of the Sacramento Valley and other non-native red foxes are widespread in low-elevation habitats.

Although once found throughout the Sierra Nevada and Southern Cascade mountain ranges, the SNRF is now one of the rarest mammals in California. The subspecies' abundance and distribution had declined dramatically in the last century. Today, the only known populations reside in Lassen Volcanic National Park and the surrounding Lassen National Forest (known as the Lassen area) and Sonora Pass area, near Yosemite National Park.

The reason for the decline is unknown. Research, which began as early as 1937 under biologist Joseph Grinnell, has proven difficult due to the animals' low density and rugged habitat. Likely impacts include trapping (prohibited in 1974), reduced prey populations resulting from historic meadow over-grazing, competition from coyotes and American martens, and climate change effects such as reduced snowfall.

Success in current survey efforts may provide our best opportunity for understanding and fostering Lassen's native red fox. CDFW researchers Corrie McFarland, Maria Immel, Pete Figura, Jennifer Carlson, and Deana Clifford made a breakthrough last winter in locating what is likely the first identified SNRF den.

In the earlier 1990s studies, collared females did not reproduce during the study period. One of the two females captured and collared last winter was pregnant and appears to have successfully reproduced. GPS tracking data led the team to the southeast flank of Lassen Peak where a single pup was observed near a likely den site. To confirm the den site is in fact the first ever recorded for this species in the park, researchers must witness the female entering or exiting the den and analyze scats collected nearby.

Successful reproduction is key to SNRF recovery. Researchers estimate the Lassen area population consists of only about 20 individuals, based on survey findings from 2009 to 2011. Such a small number is likely too few to sustain a population under ideal conditions and could lead to in-breeding. Under better circumstances, they would disperse to other areas where they could mate with genetically distinct species.

Analysis of DNA contained in blood, scat, and hair samples is contributing to a gradually growing database that can aid in understanding SNRF genetics and how individuals are related. The continuing challenge is locating and obtaining samples from this elusive species.

The first study of the Lassen population began in the late 1990s, when Lassen National Forest biologists detected SNRF in surveys for medium-sized carnivores including the American marten, fisher, and wolverine. Researchers were able to affix radio telemetry collars to fewer than a dozen SNRF in the following decade. The resulting data highlighted an extremely large seasonal home range and pronounced migration to lower elevations in winter seasons. Analysis of scat revealed that the SNRF were predominately scavenging on mule deer carcasses and eating pocket gophers. Secondary food items included voles, ground squirrels, birds, insects, berries, and human garbage.

In 2008, CDFW used scat-detector dogs to survey portions of Lassen Volcanic National Park and the adjacent Caribou Wilderness. After initial surveys identified SNRF populations, researchers set out remote cameras along trails in these areas expecting that the foxes would use the easiest method of travel— human trails. The premise proved true when SNRF were detected along trails within the park.

Two years later, the range of SNRF expanded beyond the Lassen area when a motion-activated camera revealed a red fox in the Sonora Pass area near Yosemite National Park. Up until then, the Lassen population was thought to be the sole remnant of the species in California. Additional SNRF have since been detected within the Sonora Pass area as well as within Yosemite National Park.

The three SNRF captured and collared in 2018 represent our newest hope of gaining a deeper understanding of this mysterious animal. New sophisticated, light-weight collars provide location information every three hours and have already generated rich data on the individuals' movements.

CDFW researchers will continue to track the collared foxes and attempt to capture and collar additional SNRF in the Lassen area. You can assist with their efforts by reporting any red fox sightings in the park or in the Sierra Nevada, Southern Cascade, or Klamath mountain ranges above 2,500 feet elevation.

As we continue to learn more about SNRF, the species is afforded some protection in its listing as
Threatened by CDFW. The Yosemite population has also been designated a Candidate Species for listing under the Federal Endangered Species Act.

To help manage the SNRF species into the future, Lassen Volcanic National Park has joined other federal, state, academic, and non-government entities in the Sierra Nevada Red Fox Working Group. Through scientific study, the group strives to give the native Sierra Nevada red fox its best chance for recovery.

Last updated: October 27, 2018