Heavy Hangs Over Thy Head

Heavy Hangs Over Thy Head!

By Karen Vail Photos by Karen Vail

Wanderings this spring has turned up a plethora of fascinating headgear. I have found moose, deer and elk antlers, and an old bighorn sheep horn. Wait, aren’t they all horns or antlers? Nope. The more I dug up on animal headgear, the fuller the horn of plenty became. (har, har!)

It started with a very large moose antler. From base to tip it was longer than my outstretched arm. Not that I could hold it up easily to measure it along my arm. That sucker was HEAVY! I put it up to my head trying to imagine what kind of neck muscles this animal must have had to support two of these! A smaller moose antler found later was very recently shed with a bright red base with bits of hair and skin.

Let’s start with the basics; what is an antler and a horn? Antlers are made entirely of bone and are grown and shed yearly. Over time antlers become increasingly branched. The Cervids are the wearers of antlers: deer, elk, moose. True horns are not shed (we’ll look at the outlier in a moment), are unbranched, and are bone surrounded by a layer of keratin (like what our nails are made of) then covered by a keratinized epidermis. In our area, Rocky Mountain bighorn sheep, pronghorn, bison and the nonnative mountain goat have horns. How antlers and horns appear out of the top of an animal’s head is truly magical.

Just imagine you are a 900 pound male moose. Each year you add up to 60 pounds of bone that grows out of the top of your head that spans 40 to 50 inches across. You are also required to walk (without tipping over!) and avoid tangling yourself in shrubs or doing the limbo between tree limbs. Let’s begin the process of growing antlers after the animal has shed them in late winter. Antlers begin growing from two spots on a male’s skull called pedicles. After the antlers have been shed the pedicle is open like a wound, eventually developing a scab-like covering called a wound epithelium within 2-3 weeks. In most deer family members antler growth begins shortly after the wound is healed, although moose antlers do not begin growth until 2-3 months after antler shed. Beneath this wound epithelium are cells that will create the antler growth zones. As I was reading a description of antler formation, I was surprised at how similar this process is to root and shoot growth in plants, all growing from tissues at their tips that are nourished from vascular tissues and protected by hardened tissues. Nifty, huh?! Antler bone begins to grow inside a nourishing skin covering the pedicels called velvet. Beneath the velvet is a thicker protective membrane, and beneath this is the actively growing area, the mesenchyme. Young cells form columnar structures as the building blocks for bone to build the antler. Underneath the furry membrane is a rich supply of blood and nutrients for the fast growing antlers; up to an inch a day for elk and a half inch per day for moose. During this stage the antlers feel alive and warm because of all the living tissue, and the animals are very protective of their antlers. There are two types of bone within an antler. Spongy bone is found in the center, is soft and porous and is where nutrients and hormones are transported during growth. Compact bone forms the outer shell and is dense and stiff. The days of summer shorten stimulating a rise in testosterone levels and the growth cycle begins to slow, initiating the process of hardening or mineralizing the antlers. Blood flow is restricted causing the velvet to slough off with aggressive thrashing on shrubs and small trees to remove it. (I would think that it’s got to itch too, huh?!) The velvet is quickly scavenged by mice and other animals because of its high nutrient content. After breeding antlers become a burden going into winter and are shed. Where the antler meets the pedicel cells called osteoclasts form which demineralize the bone along an abscission layer and the antler eventually falls. Again, like a leaf in the fall forming an abscission layer and floating to the ground.

Horns have been classified into 4 groups, 2 of which are pertinent to our area: true horns of the Bovidae (mountain goat, bighorn sheep) and pronghorn of the (ahem) pronghorn. Horns of both classes are formed from a keratinous sheath that grows from a core of live bone tissue, and horns are found on both male and female animals. Whereas bighorn sheep add horn material each year, male pronghorn shed their sheath each year. Also bighorn sheep have unbranched horns and male pronghorn have two branches, the forward pointing tine earning them the name pronghorn. Bighorn sheep (which is our Colorado state mammal!) are born with nubs of horncores which eventually elongate with a keratin sheath surrounding this. The sheath develops slowly and because the proteins grow faster at the outer edge of the horn, the growth is directed in a spiral inward on the rams. Within each horn is a living core that provides a continuous flow of blood below the hard sheath. Different growth rates of horn are laid down from summer to winter, creating a growth ring like a tree’s annual growth ring. These annuli rings can be used to fairly accurately determine the age of a ram. The inner bone is highly porous and acts as a shock absorber for the high impact duels the males have. New research has shown that even with adaptations for these high impacts, trauma to the brain still can occur. Pronghorn, a family unique to North America, evolved their deciduous keratinous horns (that’s a mouthful!) independently from the bovids. Male pronghorn develop branched horns, females sometimes develop a bony knob where a horn develops. These “deciduous” horns consist of a pair of permanent bony points on the skull covered with a layer of epidermal cells (the sheath) that form horn (keratin) but not velvet. This sheath is shed each year and replaced with new horn material around the bony core. Mosse antler below.

Bighorn Sheep Horn