Grizzly bears used to roam across the entire U.S., but they have been eliminated from 98% of their range. There used to be 100,000 bears in the lower 48 states, but now there are only around 1,000 left. Yellowstone National Park holds one of the densest populations.

Because bears are such a hit with tourists, concessionaires in Yellowstone drew the bears in by baiting them with garbage to locations near lodges. Hand-feeding bears along the roadside also became popular. Between 1931 and 1941, an average of 59 tourists were injured every year by bears.

Officials knew they needed to get things under control. In 1959, National Geographic hired two naturalists to study the grizzlies of Yellowstone and offer recommendations. John and Frank Craighead were twins with a long history in biological sciences.

The Craigheads pioneered the use of radio telemetry, improving radio collars so much that they vastly increased knowledge of grizzly behavior. They found that 90% of grizzlies in Yellowstone wandered outside of the park. Of those, 45% were killed by landowners. Although grizzlies normally live up to 30 years, the average Yellowstone grizzly died by age six, which was problematic since females don’t mate until they turn five. Only one of the grizzlies died of old age. Females mate once every three years, giving birth to between one and three cubs, and 40% of bear cubs die in their first year of life. Only the musk ox has a slower reproduction rate. They discovered that a male grizzly will roam a territory equal in size to the state of Rhode Island; that females will babysit and sometimes adopt cubs that don’t belong to them; that males will kill a cub that isn’t theirs. They discovered that bears build a new den every year and never re-use any bear’s old den.

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What was supposed to be a two-year study lasted 12 years. The Craighead brothers and their assistants performed 9,000 person-days of research, hiked over 162,000 miles, and captured, marked, and studied 256 grizzlies.

Then, in 1967, two young women were killed on the same night by grizzlies in nearby Glacier National Park. As a result, officials in Yellowstone wanted to close all the dumps where bears fed immediately. The Craigheads objected to this plan. The bears depended on the dumps as one of their main sources of food; closing them all at once would have catastrophic results, forcing the bears into campgrounds in search of food, sending them after livestock, increasing interactions with humans, and leading to starvation among bears unable to find another food source on short notice. They advocated for closing the dumps gradually, allowing the bears enough time to readjust their feeding habits.

Yellowstone officials disagreed. For their efforts, the Craigheads were banned from the park. Their laboratory was bulldozed. And all of their predictions came true: the grizzly population of Yellowstone dropped catastrophically, until grizzlies were ultimately placed on the Endangered Species list in 1975. Frank and John Craighead wrote a book about their discoveries, “The Grizzlies of Yellowstone” and went on to enjoy long careers as naturalists and researchers.

By the time Frank died in 2001 and John died at the age of 100 in 2016, agencies throughout the Yellowstone region had adopted all of the recommendations they had advocated: Have all agencies work together. Eliminate sport hunting and impose stiff penalties for poaching. Institute public education. Place strict rules for food storage on public lands. Cancel livestock allotments near park boundaries. The population of grizzlies doubled, all thanks to the Craighead twins.