Unlike most Alaska national wildlife refuges, Yukon Delta National Wildlife Refuge supports a low density of big-game animals. This can make hunting here difficult and impractical. However, the refuge does offer one of the few opportunities in the United States to hunt a most unusual big-game animal: musk ox. A species from the Pleistocene Era, 10,000 years ago, musk oxen roam the tundra with their long, wooly coats and large, curved horns. They are best known for standing defensively in a circle, with smaller musk oxen in the center, to protect against wolves. Musk oxen were hunted to dangerously low levels in the early 20th century. A hunting prohibition allowed the herds to recover. The refuge offers good guided hunts for this species, principally on Nunivak Island. The excellent herd management of musk oxen means that large-sized bulls are quite common, and a selective hunter has an excellent chance of taking a record-size animal.