<human>:  Why do Big-horn sheep have horns?
<bot>: Big-horn sheep have horns because they are big and horns are a good defense mechanism. Horns are made of keratin, a protein found in hair and fingernails. Big-horn sheep have the largest horns of any animal in North America. Their horns can grow up to 4 feet long and weigh up to 20 pounds. Big-horn sheep are very territorial, and they use their horns to defend their territory from other animals. They also use their horns to fight off predators.