Unlike horns, which are permanent structures, antlers are shed and re-grown each year. Some deer begin growing their new antlers almost immediately after the old ones have been shed, while others may exhibit a delay between shedding and re-growth.
Horns consist of a bony projection of the frontal (forehead) bone enclosed by a sleeve of keratin – they maintain a venous (blood) and nervous supply that allows them to grow throughout the animal’s lifetime. Antlers, by contrast, are made of bone and develop from a point on the top of the male’s skull called the pedicle, rather than from the skull itself. The antler grows out of the pedicles and, during its formation, it’s covered with hairy skin called velvet that’s pinky-grey in colour and packed with blood vessels and nerves, making them highly sensitive to the touch – a stag in velvet is still sometimes referred to by the 16th Century term “pollard”. Should the velvet become damaged, the antlers can become deformed. The potential rejuvenating power of deer velvet has led to the marketing of tablets made from the velvet of farmed deer; the tablets are said to provide relief from various ailments spanning impotence to arthritis, whilst also having immune enhancing properties.
When the antler’s growth is complete, the velvet dries up and is shed – at this point, the deer is said to be “in tatters”. The shedding process appears to be under hormonal control and usually takes less than 24 hours. Deer antlers have androgen (male sex hormones) receptors and it appears that an increase in testosterone levels (probably related to increasing day length) causes the blood supply to the antler velvet to be severed, causing the velvet to die and dry out. When the antlers have been cleaned by the stag or buck (i.e. the velvet has been completely removed), the animal is said to be in the misnomer of “hard horn”. At this point, the antler is dead bone – it no longer has a nervous or blood supply and it cannot repair itself should damage occur. In her 1991 book Deer, biologist Norma Chapman explains that the basic antler pattern is genetically-fixed for a species, although the exact form and size of the antlers are affected both by parental characters and quality of food.
A study conducted by Uwe and Horst Kierdorf at the University of Giessen in Germany found that Roe deer antlers form by a different process to those of either Red or Fallow. In Red and Fallow deer a cartilage ‘model’ is built that is then turned to bone (a process called modified endochondral ossification), while in Roe deer it’s the connective tissue membrane that’s turned to bone in a process known as intramembraneous ossification. The study also found that although formation of Roe antlers was different, the antler growth proceeds by endochordal ossification.
Casting of antlers is initiated by a drop in blood testosterone. Many studies on captive deer have demonstrated that if a stag is castrated while in full antler (i.e. hard horn), he will still cast and re-grow his antlers, but the velvet will never dry out and the antlers will not be shed. Under normal circumstances, antlers are shed and re-grown annually to coincide with the deer’s breeding season. Red, Fallow, Sika and Muntjac shed their antlers during April and May and the new growth is complete and cleaned by August/September. Roe shed their antlers in November/December and re-grow them over the winter and early spring such that they’re cleaned during April/May.
Shed antlers are sometimes eaten or licked by deer and other animals, providing a valuable source of calcium and phosphorous; hence, it is best not to collect antlers if you find them in the forest. Indeed, the antlers and velvet represent a veritable goldmine of nutrients for many animals. The antler itself is composed of various types of structural cells and there is an apparent negative correlation between calcium content and fat concentration along the antler – calcium levels increase towards the base of the antler, whilst lipid concentrations are highest at the tip. The antlers and associated velvet contain many of the essential dietary elements including calcium, phosphorous, sulphur, magnesium, potassium, sodium and iron. The velvet itself contains various amino acids including all eight essential ones (i.e. those that are required in the diet and can’t be synthesized by the animal).
For many decades scientists have hotly debated the function of deer antlers. The most widely accepted theory is that antlers evolved as weapons where deer compete for resources, predominantly (although not limited to) mates. In the first instance the antlers are a sign of fitness – they require a considerable amount of energetic and nutritional expenditure to produce and a large antler set typically represents an animal in good condition, although there’s an element of genetic control and habitat quality involved too. They can also be used by would-be interlopers to assess their chances in a fight and are used as physical weapons to both repel an attack from, and initiate a challenge to, a contender. Recent studies on moose in Europe have suggested that the antlers may also act as parabolic reflectors of sound, so moose with antlers have more sensitive hearing than those without. Logically, other species with palmate antlers (e.g. Fallow) may also gain a similar advantage.
For a more comprehensive overview of how antlers form and what function they serve, the reader is directed to the antler Q/A.