Each year, male white-tailed deer grow and shed bone formations from the top of their head known as antlers. Most male animals in the deer family (Cervidae) have antlers. Caribou are unique to the deer family in that both male and female caribou typically grow antlers. In rare instances, female white-tailed deer may grow antlers. When this occurs it is generally the result of abnormally high levels of testosterone in the doe's body.
Antlers vs. horns
Deer grow antlers; they do not grow horns. Horns differ from antlers in that horns are composed of a continuously growing bony core which is surrounded by an outer keratinous sheath. By comparison, antlers are solid bone and they are shed and re-grown each year. Horns are similar to antlers only in that they grow from the forehead.
Antler terminology
Figure 1 illustrates the basic terminology for antlers of white-tailed deer.
Pedicle development
The two circular areas that grow antlers from a buck's skull are known as pedicles. Antler pedicles start developing on the frontal bones of male white-tailed deer fawns during late fetal development. After birth, early stimulation from testosterone is then required for final pedicle development. When bucks reach 4-6 months of age, the antlers begin to grow from the pedicle. Overall, pedicle formation occurs through a process known as intramembranous ossification, whereas antlers grow by endochondral ossification.
Annual antler development
Spring/Summer - Annually, deer antler growth begins nationwide between mid-March and early-April. In response to changes in photoperiod, growth hormones from the pituitary gland trigger the release of Insulin-like Growth Factor (IGF), which stimulates antler growth. During this time, the soft growing antler is covered with hairy skin, called "velvet". When antlers are in the velvet stage they are full of thousands of blood vessels, cartilage and nervous tissue. Growth of antlers is very rapid, and some deer species are capable of growing nearly 1 inch of antler per day.
Antlers continue to grow through August or September. As day length decreases in autumn, the pituitary gland indirectly increases the secretion of testosterone and testicular volume increases Figure 2.
Fall/Winter - In response to rising testosterone levels and testicular volume, antlers harden through calcification. In this process, soft tissue is converted directly to bone by the deposition of minerals (mineralization) within the matrix of cartilage and blood vessels in the developing antlers. The velvet subsequently dries up and sloughs off. Occasionally, bucks rub their antlers against brush to aid in velvet removal.
During the rut, antlers remain as hard, polished bone with sharp tines. Hard antlers enable bucks to fight, posture for dominance among rival males, and display for does. Between late December and early January, testosterone levels decrease and cause the union between the antlers and pedicles to weaken and the antlers then are shed or broken off. Once the antlers are shed, skin grows over the wound left by the cast antler. The entire process starts over again in the spring.
How deer age affects antler characteristics
Buck fawns grow two small bump-like antlers, or "buttons", their first year and will grow their first true set of antlers during the following spring and summer. The second rack will be bigger than the first, and with sufficient nutrition, each set of antlers will grow even larger until the buck passes his prime (usually 5-7 years old). Pedicle diameter also increases with age, with new bone grown annually in concentric layers. Older bucks typically cast their antlers before young bucks, and immature or unhealthy bucks may not shed their antlers until early spring (March-April).
Mineral requirements for antler growth
Mineral requirements for antler growth exceed those of normal skeletal growth and maintenance (studies have shown that deer are constantly undergoing skeleton rebuilding). In some species, antler requirements for minerals may be 3 times as high as that required to maintain the skeleton. Mineral demand for antler growth is satisfied from both the diet and from bone resorption.
Where do deer get the calcium and phosphorus needed for growing antlers?
Diet provides the greatest proportion of calcium and phosphorus for antler growth and mineralization. However, antler growth will never exceed genetic potential, even if a deer consumes these elements above optimum levels. During times of peak antler growth, antler demand for minerals forces the thyroid gland to release calcitonin; this hormone allows the deer to "steal" minerals from its internal bone structure. This process is known as "physiological" or "temporary" osteoporosis. Bones such as the ribs and shoulder blades contribute the most to this temporary mineral deficiency, and they may lose as much as 40% of their calcium content while antlers are hardening. However, by September, deer can fully replace the minerals borrowed from their skeleton.
Antler composition and physiology
Composition of growing (soft) antlers in velvet (Spring/Summer):
Velvet antlers are High in water and Low in dry matter:
20% dry matter & minerals
80% crude protein
20% ash (22% calcium and 11% phosphorus)
80% organic material (water, etc.)
Composition of hard & polished antlers (Fall/Winter):
Hard antlers are Low in water and High in dry matter:
60% dry matter & minerals
40-45% crude protein
54-60% ash (25-40% calcium and 19% phosphorus)
40% organic material (water, etc.)
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