John Craighead and his twin brother, Frank, were intrepid explorers and pioneering conservation biologists who helped reshape our scientific understanding of grizzly bears, the migration patterns of predators, and the ecology of the greater Yellowstone system. They chronicled their explorations in the pages of National Geographic magazine, starting in 1936, when they were 20 years old.

John passed away this week at the age of 100. Frank passed away in 2001.

The brothers tracked grizzly bears with radio collars in the 1950s and '60s, and were even early users of satellite data. They discovered just how far bears roamed beyond the borders of Yellowstone, which has had a big impact both on how the park is managed for conservation and on our broader understanding of predators.

The Craigheads' work was referenced in the May 2016 special issue of National Geographic magazine, which focused on the lasting importance of Yellowstone as a natural sanctuary and laboratory and as a symbol of the power of parks around the world.

As the magazine wrote, the Craigheads "championed the idea that the parks should be managed as part of a Greater Yellowstone Ecosystem—an idea that took decades to catch on." (Read John's 1976 article about using satellite data to understand bears.)

In his final years, John spent time in a tepee outside his house in Missoula, Montana, where he lived for decades with his family.

In 2004, he said, "Frank and I had seen plenty of beautiful mountains in Pennsylvania, but the day our '28 Chevy topped this hill in Wyoming and we spotted the Tetons, it was like our souls got sucked right into the Rocky Mountains," as recorded in the book Grizzlies and Grizzled Old Men by Mike Lapinski.

"We knew right then and there that our calling was out West, and that any professional endeavor would have to somehow be centered in those mountains."

In 1998, John Craighead received the Aldo Leopold Award, and both twins were named among America's top scientists of the 20th century by the Audubon Society.

They were born on August 14, 1916, in Washington, D.C. They attended Penn State and worked for the Navy during World War II, developing a survival school and training program for pilots. In 1943 they wrote a survival guide.