After 11 years, longest-migrating mule deer dies in Wyoming’s Red Desert
A Wyoming mule deer that migrated farther than any other deer known to science has died. Deer 255’s life of migrations ended in the Red Desert of Wyoming, about 200 miles from her most recent summer range in Jackson Hole. She was 10 years and 10 months old.
Born in June 2013, Deer 255’s life spanned a significant period in the field of migration science and conservation. Her lifetime saw Wyoming map and designate its first migration corridor in 2016 and ended at a time when 182 migrations had been mapped across 10 states in the American West.
Biologists designated her Deer 255 because she was the 55th animal collared in the second year of a collaborative study led by Matthew Kauffman of the U.S. Geological Survey’s (USGS) Wyoming Cooperative Fish and Wildlife Research Unit and the Monteith Shop at the University of Wyoming (UW), with strong collaboration from the Bureau of Land Management (BLM) and the Wyoming Game and Fish Department.
Deer 255’s story
Biologists first captured Deer 255 on March 11, 2016 in the Leucite Hills near Superior where she spent the winter. UW PhD Student Anna Ortega and a research team were astounded in June 2016 when Deer 255 – having already migrated 150 miles to the Hoback Basin – broke off from the rest of her herd and migrated an additional 90 miles to Idaho.
Deer 255’s walkabout ran from Bondurant and skirted Cache Creek above the town of Jackson. From there, she traversed the shoulder of Jackson Peak and crossed the Gros Ventre River east of Slide Lake.
She passed near Moran Junction and less than a mile from Jackson Lake Lodge, then bounded over a highway to traverse the John D. Rockefeller Jr. Memorial Parkway.
After swimming the Snake River near the inlet of Jackson Lake, she climbed over the low north end of the Teton Range, then forded the Fall River in the southwest corner of Yellowstone National Park.
Her final 2016 summer range was four miles from Island Park, Idaho – 242 miles from where she started.
At the time, no deer had migrated such a great distance from winter range to summer range. Across North America, only Arctic caribou are known to migrate farther.
Ortega and the team could not yet tell if this incredible movement was a true seasonal migration or a dispersal event. Before they could observe her fall 2016 migration to confirm if she would return to the Red Desert, Deer 255’s Global Positioning System (GPS) collar malfunctioned on Aug. 7, 2016.
Deer 255’s whereabouts were unknown for the next 19 months until March 12, 2018, when Ortega and crew were recapturing study animals on their Red Desert winter range near Superior.
The helicopter pilot spotted a deer on Steamboat Mountain with a brown GPS collar that they could see but not hear on the radio. They captured the doe, and Ortega – with a quick glance at the collar serial number etched into her memory – immediately recognized her as the long-lost deer.
From this point on, biologists had an unbroken record of migration data for Deer 255, during which they tracked her nearly 3,300 miles across seven spring and seven fall migrations.
Her migration tied together a vast swath of Wyoming and Idaho, spanning private working ranches and public lands administered by the BLM, the U.S. Forest Service and the National Park Service.
Death as a prey animal
Then, Deer 255’s vast yet incredibly regular movements ended in a patch of sagebrush. She died three days into her 2024 spring migration, having traveled about 16 miles north from her Leucite Hills winter range straddling the Continental Divide near Superior.
She died around noon on April 11, according to GPS collar data monitored by current UW PhD Student Luke Wilde.
Wilde and Wyoming Migration Initiative Writer and Filmmaker Gregory Nickerson found Deer 255 five miles north of Steamboat Mountain, a volcanic uplift near the Killpecker Sand Dunes in Sweetwater County. This area is an important spring migration stopover for Red Desert mule deer.
In the modern West, where deer often die by disease, habitat loss, starvation, fence entanglement or vehicle collisions, Deer 255 died playing her role as a prey animal in an intact ecosystem.
Signs on Deer 255’s body suggested predation as the probable cause of death, most likely a mountain lion.
Deer 255 was healthy as of March 12, when last captured for a field checkup. She was pregnant with twins and had a body fat percentage of six percent, which put her in the top 12 percent of all animals spending the past winter near Superior.
As a long-distance migrant, Deer 255 typically had more body fat than medium- or short-distance migrants that winter in the Red Desert but migrate to the southern Wind River Range or Steamboat Mountain, respectively.
She also had a larger body size than average – 170 pounds, compared to most female mule deer at around 140 pounds – and a notched left ear from an unknown injury.
Throughout her life, she had crossed highways dozens of times and jumped more than 1,000 fences. No doubt she had avoided numerous predators, just as she had eluded field biologists trying to spot her in the field, which earned her a reputation as the wiliest deer in the study.
Long migrations
The long-term research project focuses on the migration dynamics of deer that winter near Superior and make the Red Desert-to-Hoback migration. Biologist Hall Sawyer with Western EcoSystems Technology, Inc. initially documented this 150-mile mule deer migration in 2014.
The state of Wyoming officially designated the broader Sublette mule deer corridor as “vital habitat” in 2016.
Although mule deer display extreme fidelity to their migration routes year after year, Deer 255 was notable for her ultra-long-distance migration.
Although Deer 255 summered in different spots over the years, she always used the same exact migration route to get there and back. Ortega tracked Deer 255 across the full 242-mile migration from the Red Desert to Island Park, Idaho in both 2016 and 2018.
In 2020-21, Deer 255 spent the summer near Mount Berry and Mount Reid near Jackson Lake in Wyoming, both known locations along her past routes to Idaho.
For the summers of 2019, 2022 and 2023, Deer 255 spent the summer in the Bridger-Teton National Forest in Jackson Hole. Those years, she foraged and nursed her young in habitats 14.5 miles east of the summit of the Grand Teton, near Dry Lake and the Triangle X Ranch.
Using field ultrasounds in 2016 and 2018-2024, biologists documented Deer 255 had eight pregnancies, including seven sets of twins.
Both Ortega and Wilde attempted to collar an offspring of Deer 255 to see if one of her fawns would learn her long migration and carry the knowledge forward for another generation. Despite her multiple successes in bringing twin fawns to winter range in the Red Desert, biologists were unable to track any of her fawns migrating back to summer range.
Researchers suspect other deer make Deer 255’s record-breaking migration, but her specific migration tactic is rare and has yet to be recorded in any other collared deer.
Of the hundreds of animals tracked in the Red Desert, only one other deer has been known to migrate to Idaho – Deer 665, which spends the summer on Teton Pass and made an exploratory movement to Ririe, Idaho in 2022.
As of this writing, Deer 255’s distance record for mule deer still stands, though biologists expect it will be broken as migration tracking efforts continue across the West.
This story was originally published on June 11 by UW Ag News and can be found at uwagnews.com.
