The Sierra Nevada Bighorn Sheep (IMAGE) PLOS Caption The Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae) was granted protection under the U.S. Endangered Species Act in 2000, when population estimates were as low as 100 individuals. Credit Chris Horsch/USFWS, CC0 (https://creativecommons.org/publicdomain/zero/1.0/) Usage Restrictions No restrictions. License CC0 Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.