In the dry heart of California’s Central Valley lives a fox so small it could sit in a shoebox.
The San Joaquin kit fox once ranged across the whole valley floor, and today it has almost nowhere left to go.
So when a few of them turned up inside the fenced rows of two solar farms, biologists were not sure whether to feel relieved or worried.
What the trackers recorded over the next three years surprised nearly everyone who had feared the fences.
A five pound fox running out of room
The San Joaquin kit fox is the smallest fox in North America, barely five pounds of ears, legs and tail.
It lives nowhere else on Earth, only in central California, and hunts at night to beat the desert heat.
House cat sized and shy, it shelters underground by day and comes out after dark to catch rodents, insects and rabbits.
It is so well suited to dry country that it can go its whole life without drinking water, pulling moisture straight from its prey.
Farming, roads, housing and energy projects have swallowed almost 95 percent of its original habitat.
Fewer than 3,000 are thought to remain, and the species is federally listed as endangered.
As early as 1979, biologists reckoned only about 7 percent of the valley floor south of Stanislaus County was still wild.
To hang on, the fox needs open ground, steady prey, and many dens to slip between as it dodges coyotes.
Why biologists put collars on foxes inside a solar farm
The question was simple and uncomfortable.
With its wild home nearly gone, could this fox live inside a human built landscape instead?
Between 2014 and 2017, a team led by biologist Brian Cypher set out to find an answer.
They ran two separate three year studies, one at the Topaz Solar Farm and one at the California Valley Solar Ranch.
Foxes inside the fences were collared and tracked against foxes living outside, in open country nearby.
At each site, the collared animals were compared against foxes at a nearby reference site out in the open valley.
The work was later presented at the UC Santa Barbara Bren School and written up in a 2019 report.
The foxes did not just survive, they thrived
The results were hard to argue with.
Reproductive success was identical on both sites, at almost 87 percent.
The average territory a fox needed inside the solar farm was no larger than in open country, roughly two and a half square kilometers against about three.
Den use and diet looked much the same in both places, and the foxes moved between dozens of burrows just as their wild neighbors did.
In one detail that caught the team off guard, the males inside the solar farm were actually a little heavier.
By almost every measure, life inside the fence was as good as life beyond it.
The trick was hiding in the fence
The reason turns out to sit in the one thing meant to keep wildlife out.
The perimeter fence was built with gaps sized so a small kit fox can slip right through.
A coyote, its main predator, is too broad in the shoulders to squeeze through the same gap.
Coyotes kill these foxes at brutal rates in the open, so a barrier they cannot cross is a gift.
Inside, the ground under the panels was thick with kangaroo rats and other rodents, so food was everywhere.
The panels threw shade that steadied the temperature, and human disturbance was low once the building stopped.
Crews even grazed sheep between the rows to keep the grass short, which kept the rodents the foxes eat healthy and easy to catch.
A fence drawn to protect machinery had quietly become a fox refuge.
Why this is not a blank check for solar
It would be easy to read this as proof that solar farms are good for animals.
The researchers were careful to say it is not.
Both sites came with heavy conservation work, including artificial escape dens, open movement corridors, managed vegetation, and control of dogs, traffic and poisons.
Strip those measures away, the team warned, and the same fenced site could have become a trap instead of a refuge.
The honest lesson is narrower and more useful.
Built and run with a species in mind, a solar site can feed the grid and still leave room for life.
Wild animals have surprised us this way before, from seals hunting inside an offshore wind farm to horned lizards clawing back in Texas.
For a five pound fox with almost nowhere left to go, a field of panels turned out to be a place it could live.