"If you see a porcupine, it's like finding a unicorn," biologist Jessy Coltrane told me. She was referring to the part of Montana west of the Continental Divide, where these once-common rodents have nearly disappeared.

This week's story is all about Montana's porcupines. Four winters ago, I had a series of remarkable experiences with one particular porcupine near Helena. Read about this one gnawing on ice, eating snow and Russian-olive fruits, and denning in an abandoned culvert. Then I get into the biology of these long-lived rodents and the bigger questions about porcupine disappearances.