Elk Die-Off Mystery Solved In February 2004 approximately 600 elk died in southern Wyoming, and no one knew why, despite batteries of tests on blood samples. Now, Wyoming biologists have the answer, and the key clue in the investigation came through an article from 1964 that one of the biologists read in which he learned of cattle and sheep dying from usinc acid in lichen that they ate. Returning to where the elk had been found, biologists collected lichen and, returning to a lab in Laramie, attempted to feed it to three captive elk. The two that ate it died within 10 days of being introduced to the food. The third elk, which wouldn’t eat the lichen, has survived.