Replacing worn-out batteries in animal tracking devices can be a time-consuming, expensive duty for wildlife scientists. It’s stressful for the animals, too. Now, inspired by the technology behind a self-charging smartwatch, researchers have invented a tracker powered by the animals’ own movement. The approach could help researchers monitor animals across their entire life spans, making things easier for both the scientists and the creatures they follow.

“Batteries are often the greatest limitation to development of animal tracking technology,” says Todd Katzner, a research biologist at the U.S. Geological Survey who has tracked wildlife for more than 20 years. “Any system that overcomes current constraints has the potential to be immensely valuable.”

Many wildlife trackers are solar-powered. These don’t work for nocturnal creatures or for animals that live in the water, Sun-blocking forests, or caves, such as pandas, tigers, and sea lions. Nonrechargeable lithium-ion batteries—heralded by manufacturers as lasting for decades—are another option, but field experience shows very hot or cold temperatures considerably shorten their life spans.

That’s always frustrated Rasmus Worsøe Havmøller, a biologist at the Natural History Museum of Denmark. He often accompanies biologist Linnea Worsøe Havmøller, his spouse who is also at the museum, on field studies of endangered Asian wild dogs, or dholes (Cuon alpinus). Fewer than 2000 of the small-headed, long-bodied canines remain in the wild.

Watching the dogs run across the dry savannas of southern Asia, the pair wondered whether it was possible to convert that running energy into electrical power. Rasmus Worsøe Havmøller learned researchers had already developed prototypes for tracking devices powered by the vibrations of an animal’s movement, but these were too bulky to be used for most species. However, he also discovered a Swiss smartwatch powered by a tiny generator that converts the wearer’s movement into an electromagnetic current that powers the device.

Inspired, the Worsøe Havmøllers and colleagues teamed up with Troels Gregersen, an engineer at the Max Planck Institute of Animal Behavior, to see whether they could adapt the technology to work in animal trackers. The watch’s tiny generator, roughly the size and weight of a couple of stacked poker chips, harvests energy from side-to-side movement. As a pendulum within swings, it generates current by sliding a ferromagnetic ring across a copper coil. That energy gets stored in a lithium supercapacitor, which can withstand 40 times as many charge cycles as a standard lithium battery.

“Suddenly we actually had the device to deliver power, and suddenly we had the device to store the power,” Rasmus Worsøe Havmøller says.

The researchers packaged their new battery with a low-power GPS tracker, accelerometer, and a tiny computer inside a waterproof, fang-resistant case. The tracking device and collar weigh a mere 5.3 ounces—light enough that a striped skunk could safely wear it, Gregersen says.

The scientists first tested the equipment on their own pet dogs, affixing the trackers to the collars or harnesses of Balto, Pekka, and Comet. Just an hour and a half of dog-walking was enough to generate power for daily GPS location pings, they report in PLOS ONE.

The scientists bolted the devices to sturdy wildlife collars, which they placed on a European bison roaming a raised bog east of Aalborg, Denmark, and an Exmoor pony living on grassy wetlands at the southern tip of the Danish island of Langeland. Both animals are being used to munch overgrowing vegetation and fertilize the soil to “rewild” the landscapes. They, too, produced enough energy to power regular GPS pings, as well as at least one accelerometer reading per day, Rasmus Worsøe Havmøller says. “For management and conservation, that should be plenty.”

These trackers could theoretically last an animal’s whole lifetime, he says. The Exmoor pony’s tracker was still going strong as of the time of the paper’s publication, 147 days after researchers affixed it.

The schematics for the device are included in the paper and freely available to other scientists. The researchers say they hope others will build on their work to create even more efficient and lighter weight versions.

The technology could be “a huge game changer” for tracking larger animals, says Joshua Guilbert, a bat biologist at the Biodiversity Research Institute. Not only do the long-lasting batteries mean less human interaction with wildlife, but they also work day or night in virtually any environment. And unlike devices powered by solar panels, they don’t need to stay clean in order to work properly. Unfortunately, he notes, the new trackers are still too heavy to use on bats.

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