Antlers are usually found only on males. Only reindeer (known as caribou in North America) have antlers on the females, and these are normally smaller than those of the males. Nevertheless, fertile from other species of deer have the capacity to produce antlers on occasion, usually due to increased testosterone levels. The "horns" of a pronghorn (which is not a cervid but a giraffoid) meet some of the criteria of antlers, but are not considered true antlers because they contain keratin.