Picture a white-tailed deer and chances are you will envision an animal bearing antlers. After all, a deer sighting may be charming, but if that deer is a buck with a large rack of antlers, the encounter becomes a “wow” moment. But visual impact isn’t the only exciting thing about antlers. Deer antlers are the fastest growing mammalian bones and may be the only mammalian tissues capable of full regeneration. Their annual replacement and growth provide a fascinating, and potentially valuable, model of tissue development.
Antlers are bony structures that arise from the pedicle of the skull’s frontal bone. In almost all members of the deer family (Cervidae), including our local white-tailed deer (Odocoelus virginianus), only males bear antlers. (Caribou, in which both males and females have antlers, are the exception.) The functions of those antlers are primarily for establishing reproductive success. First, the antlers are used directly in male-male contests for dominance. In autumn, males lock horns in battle for the right to secure primary access to females. Secondarily, the antlers serve as a signal to females. A larger rack signifies a male that has lived a long time and has successfully found high-nutrient foods (rack size increases with age and food quality). So, females seek out mates with large antlers, indicative of the likelihood that they will produce high-quality offspring.
While antlers do provide those important autumnal functions, they also have their down side. Antlers are heavy, costing energy to carry around. They potentially create challenges for unimpeded travel through woods and undergrowth, too. So, despite the costs of regrowing them, antlers are shed on an annual basis. Males typically lose their antlers sometime during winter. Growth of the new set begins immediately but may not be visible for a couple of months. Through spring and into summer, the antlers grow to full size.
Like other bones, antlers are built from minerals (especially calcium-phosphate) deposited around a matrix of protein (collagen). So, creating those structures requires both the building blocks—protein and minerals—and the blood supply for delivering those materials where they’re needed. Growing antlers arise from the pedicles and, under hormonal influence, blood vessels grow out from there. The energy required for a buck to grow a full set of antlers rivals that for a doe to produce a fawn. And indeed, the material demands of growing antlers may well outstrip the diet’s supply capacity. If so, then minerals are “borrowed” from other bones, especially non-weight-bearing bones like the ribs, which consequently suffer a seasonal weakening, or osteoporosis.
While growing, the antlers are somewhat soft and are covered in a specialized hairy coating called velvet. As fall arrives and antlers reach full size, testosterone levels surge; in response, the last stages of antler mineralization occur, blood flow ceases, the antlers harden, and the velvet dries and is scraped away. The antlers then are ready for combat. However, even while deer go head to head with their competitors, cells in the antler pedicles remain active. Osteoclasts reabsorb calcium at the junction with the antler, and by winter that connections weakens to the point that the antlers fall off to the forest floor. There, they are valued as food by rodents and provide nutrients back to the ecosystem.
As noted above, the size of a buck’s antlers is determined by three main factors. The first is age. Antlers in white-tailed deer tend to increase in size with age up to about six years. (In Ohio, with its active population of hunters, a minority of bucks may make it to their sixth birthday.) The second factor is genetics. Antler size does have an inheritable contribution, transmitted by both the does and the bucks. And the third factor is nutrition. While genetics help to determine the potential size of a rack, that potential can be achieved only if the buck’s diet includes adequate minerals and, especially, protein.
The rapid growth of deer antlers represents a tremendous mobilization of tissues. Antlers may increase in size by as much a 2 cm per day, and the nerves and blood vessels supplying them must match that pace. The factors that both enable and control rapid antler growth may provide insights into biomedical circumstances ranging from hair loss and replacement to repair of damaged organs and control of cancer proliferation. Unraveling those factors turns out to be a complex puzzle, with different players involved in different stages of the antler cycle. Regulation comes from both outside of the antlers, such as hormonal responses to environmental cues, and from internal elements, like patterns of gene activation and stem cell differentiation that are specific to particular antler tissues and stages of growth. One set of genes may activate rapid growth of cartilage and conversion to bone, while a separate set simultaneously prevents that growth from progressing in an unconstrained, cancer-like direction. One research article on this topic (Journal of Anatomy 207: 603 – 618) is entitled, “Deer antlers: a zoological curiosity or the key to understanding organ regeneration in mammals?” It turns out that the answer to that question almost certainly is: Both!
As spring advances toward summer, female deer are giving birth to their fawns. Bucks, on the other hand, are generally lying low this time of year. But those tissues on their heads—from the base of the pedicles to the tips of the tines—are busy at work, building another season’s rack of antlers—even bigger and better this year than last!
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.