Science Spotlight

A collared bear near Blue Canyon in Placer County.

For years, California Department of Fish and Wildlife scientists were trying to locate dens for California’s Sierra Nevada red fox — a rare and threatened species whose population has decreased substantially. Scientists had a general idea that some of the foxes denned at high elevations in the Lassen Peak area, but aside from vague descriptions written in the 1920s, the den locations had never been documented.

In 2018, a team of scientists led by CDFW wildlife biologist Jennifer Carlson put GPS satellite collars on several Sierra Nevada Red Foxes. They were able to locate several dens in 2018 and 2019 and are in the process of verifying at least two more. Through collar technology and field work they have also verified that the population they were tracking has successfully raised litters.

“Knowing basic information about where the foxes live and breed will help us develop conservation actions to benefit the species,” said Pete Figura, a CDFW wildlife management supervisor who has experience collaring many types of wildlife including deer, elk, Pacific fishers and band-tailed pigeons.

The conservation benefits of collar technology have been well documented. Perhaps slightly less well known is that collars are designed and deployed with animal welfare in mind, allowing study animals to reproduce, get the food they need, maintain a healthy weight and live full lives.

Scientists strive to use collars that weigh five percent or less of an animal’s body weight, and in some cases can use collars that weigh as little as one-and-a-half percent of an animal’s body weight. When possible, scientists use collars that feature a drop-off mechanism which releases the collar before its battery life runs out. The drop-off mechanism also ensures that the animal does not have to live the remainder of its life with an inactive collar. Drop-off collar technology isn’t yet available on some of the smallest collars but may be in the future. Biologists also use expandable collars for young animals that haven’t reached full size, allowing them to grow without being impeded by the collar.

CDFW uses collars that provide two types of telemetry data. The data is derived from three basic approaches:

1. Collars that send out a VHF radio signal which researchers can detect or listen to with an antenna to determine an animal’s general location and whether it is alive or dead. Researchers must be relatively close to detect the VHF signal.
2. Collars that passively receive a radio signal from satellites and collect GPS location and other data such as movement and temperature. The data are usually stored onboard the unit which needs to be retrieved and downloaded onto a computer. Some of these collars also allow researchers to download data remotely using a hand-held device, but researchers must be relatively close to the animal. These collars also incorporate the VHF radio signal technology described above.
3. Collars that also communicate actively to satellites allowing researchers to access location data on their computers and communicate with the collars to change settings remotely (e.g., to change the data collection schedule during migration).

While CDFW collars many types of animals, ungulates (hooved mammals) are the largest group. At any given time, CDFW is collecting data from 500 or more collared ungulates across the state including deer, elk, pronghorn and bighorn sheep. The department collects data on collared ungulates for conservation purposes and to inform hunting limits. Scientists track movement, habitat use and survival and can collect data on everything from ambient temperature to the direction and speed the animal is traveling. Collars can also tell scientists when and where an animal has died and whether it is moving, feeding, or resting.

“We take very seriously our ethical obligation not to harm animals or unnecessarily cause them discomfort. We take great strides to ensure collars have the right fit and weight for the animal wearing them,” said David Casady, a CDFW wildlife biologist with extensive collaring experience.

Collars are typically made from foam and leather with a circuit board housed in strong metal or plastic casing. For data purposes, collars are designed so that the animal doesn’t behave differently than the rest of the population.

“The data we collect from a collared animal needs to be representative of the population at large or it’s not very applicable to our management and conservation efforts,” said Casady.

Data from collars allow CDFW to make well-informed, science-based management decisions. Although scientists in the field often have a solid understanding of the wildlife they research, thoroughly vetted data is what counts in the eyes of decision makers.

Wildlife Biologist Justin Dellinger researches mountain lions (and wolves) for CDFW. He’s seen valuable data come from the 70 to 80 collared lions currently being monitored throughout the state.

“With collared males that move around and look for new territory, data can show us where there’s a lack of habitat connectivity. We can use those data in developing movement corridors and road crossings. It can ultimately help our state’s lion population live full lives,” he said.

Dellinger can also attest to the fact that collared lions are able to reproduce. “A 10-year-old female collared lion that we’re monitoring recently had her fifth litter of kittens,” he said.

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CDFW Photos. Top Photo: A collared deer at Bonita Meadows in Tulare County in 2017. The deer was collared as part of a long-term monitoring project.

Media Contact:
Ken Paglia, CDFW Communications, (916) 825-7120

Camouflaged trap used to capture foxes for the study. CDFW photo by Jennifer Carlson.


Sierra Nevada red fox bounds back to its native habitat after capture and study. CDFW photo by Scientific Aide Corrie McFarland.

The Sierra Nevada red fox has been the subject of intensified study by CDFW over the past decade. As they are notoriously tough to track and even tougher to trap, there are many unanswered questions regarding this elusive animal.

In an effort to better understand this state-listed threatened species, an ongoing research project seeks to capture and affix GPS tracking collars to them. The data collected will help biologists better understand the size and characteristics of the fox’s home range, its denning and resting areas, and its foraging habits.

The species has been outfoxing researchers for some time -- to the point where in the 1980s, it was presumed to have vanished forever from its historically occupied habitat in the Cascade and Sierra Nevada ranges. In March 1993, thanks to the then-emerging technology of infrared trail cameras, US Forest Service employees detected a single red fox in the Lassen National Forest.

That discovery prompted a wider study of foxes and other meso-carnivores in and around Lassen Volcanic National Park. In the late 1990s and early 2000s, Dr. John Perrine of the University of California, Berkeley, captured five individuals, primarily in the park, and placed radio collars on them to study their home range (both summer and winter), food habits and resting sites. Unfortunately, two of the collared individuals died within a year and none of the females reproduced during the course of the study.

Years later, CDFW launched a new study to determine the foxes’ current distribution in northern California and to address potential impacts on the species from activities including recreation and timber harvest. Initial efforts in 2008 used scat-detector dogs to survey portions of Lassen Volcanic National Park and the adjacent Caribou Wilderness. Then, from 2009 to 2011, trail cameras and hair-snaring devices were employed to survey high-elevation habitats in the Cascade Range from Mount Shasta to Lassen Peak. Yet foxes were only detected in the Lassen Peak area.

CDFW biologists have continued to survey for foxes with trail cameras, hair-snaring devices and scat surveys. Scats and photos are often obtained along Lassen Volcanic National Park and Forest Service hiking trails, because, like many other animals, red foxes frequent trails as they move through their territories. Analysis of the DNA contained in the collected scats and hair identified 22 individuals from 2007-2016. Some of these foxes are long-lived – samples collected over time from the same individual indicate that five of those individuals lived at least five and a half years.

CDFW efforts to capture and collar Sierra Nevada red foxes since 2013 were unsuccessful – until early February 2018. The nearly two decade-long dry spell came to an end at last when CDFW captured a Sierra Nevada red fox, a male that weighed about 10 pounds. It was captured in a “log cabin” style trap on National Forest land just outside of Lassen Volcanic National Park, near the town of Mineral. The fox was collared and released at the capture location, and CDFW biologists have been impressed by the distances he has regularly been covering since (five to six miles per day) despite the rough terrain and high elevation.

“Persistence played a large role in our success, because there are many days when we do not have any fox detections,” said CDFW Wildlife Biologist Jennifer Carlson. “We also ramped up our efforts this year by hiring two scientific aids rather than just one, which allowed us to literally double our efforts by putting more traps out across the study area.”

CDFW hopes to capture as many as four more red foxes this year. Scientists are using box traps, cage traps and a “log cabin” style trap that researchers have used in other states to capture both red foxes and wolverines. Capturing foxes is not an easy task given the cold temperatures and snowstorms, but as the Lassen population may only consist of around 20 individuals, it is imperative for the department to learn as much as it can about this stealthy animal.

For more information, please visit the Sierra Nevada red fox page.

Top photo: Senior Environmental Scientist Pete Figura and Senior Wildlife Veterinarian Deana Clifford release a red fox study subject. CDFW photo by Corrie McFarland