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