Nearly all deer have a facial gland, in front of each eye, that contains a strongly scented substance, or pheromone, used to mark its territory. The males of many species open these glands wide when angry or excited. All deer except the musk deer have a liver without a gallbladder. The musk deer, along with the Chinese water deer, also differs from other species in that it doesn't have antlers and bears upper canine teeth that have developed into tusks. The antlers of deer are solid and bony unlike the hollow, permanent horns of other ruminants.
Antlers form only on males and their growth is controlled by the male sex hormone.
One exception is the caribou where both sexes grow antlers. Arising from the frontal bones and nourished by a highly vascularized, fine-haired skin, called velvet, antlers complete their growth, which requires great amounts of calcium, within a few months. Circulation is then cut off, and the resulting dead skin is rubbed off as the animal rubs its antlers against trees and other brush. Antlers are used to slash territorial markings on trees or bushes, to make threatening displays, and to combat other males. Usually the fighting is stylized and harmless, but occasionally males of large species lock antlers and die of exhaustion or starvation.