Mr. Big - Plains Bison by Georgie Alexon Via Flickr: Both male and female bison have horns beginning around two years old. They then have a stage called a "spike-horn," where the horns develop at a 45-degree angle. This lasts until they are around four years old. Horns start black but turn grey as the buffalo ages. Adult horns curve upward, and the tips start to become blunted and shorter after about age eight. This photo was taken in Elk Island Provincial Park, in Alberta, Canada. - Bison bison bison
Source: flickr.com