06/10/2026
To save the rarest mammal in North America, the United States government built a drone that flies over the prairie and shoots peanut-butter-flavored pellets at rodents.
That sentence is completely true.
The black-footed ferret eats prairie dogs. Prairie dogs make up roughly ninety percent of its diet. A ferret needs to kill about three prairie dogs per week to survive, and it lives in the burrows of the prairie dogs it has already eaten. When plague sweeps through a prairie dog colony, it can kill ninety-eight percent of the population within weeks. The ferrets that depend on that colony either starve or contract plague themselves and die.
Every reintroduction of captive-bred ferrets into the wild since the 1990s has faced the same problem. You release the ferrets. The plague arrives. The prairie dogs collapse. The ferrets collapse with them. We covered the Meeteetse discovery on this page, where a ranch dog named Shep brought home a dead ferret and accidentally revealed the last wild population. By 1987, eighteen ferrets remained on earth. The species has been clawing back from that number ever since, and plague keeps undoing the progress.
Between 2001 and 2008, researchers at the USGS National Wildlife Health Center and the University of Wisconsin developed an oral vaccine against sylvatic plague that could be baked into a pellet and eaten by a prairie dog. The pellet needed to be small enough for a prairie dog to pick up, durable enough to survive being dropped from height, and appealing enough that a prairie dog would eat it instead of ignoring it. They tested blueberry flavor.
They tested sweet potato. They tested peanut butter. Tonia Rocke, the USGS wildlife disease specialist who led the development, said peanut butter was the overwhelming favorite.
The pellets are roughly the size and weight of an M&M. They are dyed blue so prairie dogs can see them against the brown dirt of their colonies. Each pellet contains a single dose of the sylvatic plague vaccine, which provides protection for approximately nine months. The target density is fifty pellets per acre, spaced roughly thirty feet apart, so that no single prairie dog eats them all. If seventy-five percent of the colony eats a pellet, the entire colony benefits from herd immunity.
The problem was scale. A single prairie dog colony can cover thousands of acres. Distributing pellets by hand, which is how the first field trials were conducted in 2012, required technicians walking grid patterns across open prairie dropping one pellet at a time. It worked on fifty-acre test plots. It did not work on the fifty-six thousand acres of UL Bend National Wildlife Refuge in northeastern Montana, one of the most important ferret reintroduction sites in the country, where plague had already wiped out two thousand of three thousand acres of prairie dog habitat.
In 2016, USFWS wildlife biologist Randy Matchett and a team of engineers from World Wildlife Fund, Model Avionics, and Support XXL built three mechanized delivery systems. The first was a custom eight-bladed octocopter drone with a GPS-guided pellet dispenser that shoots vaccine baits out thirty feet on either side of the aircraft as it flies predetermined grid patterns over prairie dog colonies.
The second was an ATV-mounted single-pellet dispenser for terrain the drone cannot cover. The third was an ATV-mounted triple shooter that drops three pellets simultaneously for faster ground coverage.
The drone can vaccinate a prairie dog colony at roughly six times the speed of a person walking the same ground on foot. It flies a programmed route, drops one pellet every thirty feet based on GPS coordinates, and returns to base for reloading. The pellets are delivered frozen to keep the vaccine viable in summer heat. When they thaw during application, jamming occasionally occurs in the dispenser mechanism, which engineers are still refining.
At Rocky Mountain Arsenal National Wildlife Refuge near Denver, drones cannot be used because the refuge sits under Denver International Airport's flight path. There, technicians use the ATV-mounted dispensers instead. The prairie dogs do not distinguish between pellets dropped from a drone and pellets dropped from an ATV. They eat both.
The vaccine does not protect the ferrets directly. It protects the prairie dogs, which protects the food supply, which protects the ferrets. The ferrets themselves are vaccinated against plague individually by injection before they are released from captivity. But an individually vaccinated ferret released into a colony where ninety-eight percent of the prairie dogs have just died of plague is still a dead ferret. It will not starve from plague. It will starve from having nothing to eat.
The entire chain of survival for the rarest mammal in North America runs through a one-gram peanut-butter pellet dyed blue and dropped from a drone onto a prairie dog town in Montana. The ferret depends on the prairie dog. The prairie dog depends on the vaccine. The vaccine depends on the drone, the ATV, the GPS grid, the frozen storage, the peanut butter flavoring that a USGS researcher tested against blueberry and sweet potato because she needed to know which flavor a rodent would eat voluntarily in the field.
Three hundred ferrets in the wild. Eighteen in 1987. Every one of them alive today because a ranch dog found a dead one in 1981 and a woman at a taxidermy shop recognized what it was. And now, forty years later, the species' survival depends in part on whether a frozen peanut-butter pellet the size of an M&M thaws at the right speed inside a drone-mounted dispenser flying a GPS grid over a prairie dog colony that has not yet been hit by a disease the prairie dog has no natural defense against.
Source: U.S. Fish and Wildlife Service / USGS National Wildlife Health Center / National Geographic / Sierra Club / World Wildlife Fund.
