Hollow hairs trap the deer's body heat. The deer family is the only group of animals in the world that grow antlers. The deer family includes elk, moose, caribou, and mule deer. It sounds silly, but deer are actually aged by checking their teeth, not by counting points on antlers. A trained biologist looks at both the type of teeth the deer has as well as how worn they are.
Deer have a set of baby teeth just like humans.
They are born with four teeth and in the next few months develop baby incisors and premolars. By the time they are about 18 months old, permanent teeth will have replaced the baby teeth. An adult will have six incisors and two canines in the lower front, six premolars and six molars in the lower back and the same in the upper back. A quick glance at the premolars tells a biologist whether the animal is a fawn, youth or an adult. After all of the white-tail's permanent teeth have come in, biologists look for the amount of wear on the molars, which lose about 1 millimeter of height each year.