Why Is a Growing Antler Furry? It Is Building the Fastest Bone There Is

The growing antler is living tissue rather than dead bone, and it is covered by a specialised skin carrying a dense network of blood vessels and nerves. That skin supplies everything the growth requires, delivering the enormous quantities of calcium, phosphorus and protein needed to build bone at extraordinary rates, and it is warm to the touch and bleeds readily if damaged. The nerves make the growing structure sensitive, and deer visibly avoid knocking them. Once the antler reaches full size, the blood supply is cut off by a ring of tissue at the base, the covering dies, and the animal rubs it off against vegetation, revealing the dead bone beneath.

The rates involved are without parallel among mammals:

• •A large male can add over two centimetres of bone per day at peak growth
• •A full set is built in three to four months
• •That set may weigh many kilograms in the largest species
• •The mineral required exceeds what the diet can supply at that rate
• •The deficit is drawn from the animal's own skeleton, which thins measurably
• •The skeleton is rebuilt afterwards from the diet over the rest of the year

Building and discarding a substantial structure annually is expensive and the reason is competition between males. Antlers are used in contests over access to females, both as weapons in pushing matches and as a signal of condition that allows rivals to assess each other without fighting, since a large symmetrical set can only be grown by an animal that fed well. Growing them fresh each year means the signal is current rather than historical, so an animal in poor condition this year cannot display last year's achievement. Shedding after the breeding season removes the weight and the cost of carrying them through winter. The cycle is driven by day length acting through hormones.

The covering is harvested commercially and the practice is contentious. Farmed deer in several countries, principally New Zealand, China and Korea, are restrained and the growing antler is cut off partway through its development, dried and sold, mainly into traditional medicine markets where it is used for a wide range of claimed effects. The evidence for those effects is weak, with reviews finding studies that are small, poorly controlled and frequently conducted by interested parties. The procedure is painful without anaesthesia, since the structure carries nerves, and several countries require veterinary supervision and pain relief while others prohibit it entirely. The industry is substantial and is a recurring subject of welfare regulation.

The process attracts serious medical attention for reasons unrelated to deer. It is the only case in mammals of an appendage regenerating completely and repeatedly from a defined site, which makes it the obvious model for understanding why mammals generally cannot regrow limbs while several other groups can. The growth involves cells dividing at rates comparable to a tumour while remaining precisely controlled and stopping on schedule, which is of obvious interest to cancer research. The nerves regenerate along with the tissue, which is studied for nerve repair. And the cycle of drawing mineral from the skeleton and replacing it is studied as a natural model of bone loss and recovery.

Growing antlers are living tissue covered by skin carrying dense blood vessels and nerves, supplying bone built at over two centimetres a day, and the covering dies and is rubbed off when the blood supply is cut. The mineral demand exceeds the diet, so it is drawn from the animal's own skeleton and replaced later. The annual cycle keeps the signal current, and the regeneration is studied as the only mammalian case of its kind.