Deciduous antlers, composed of bone, are peculiar to deer and are the most conspicuous identifying features of the Cervidae. There is a substantial literature on this topic including Brown (1983). Deer evolved to live within northern temperate regions and, consequently, their breeding cycle is controlled by the seasons and mediated by day length (Lincoln 1971a). In general, antlers grow during the period of the year when animals are sexually quiescent; they mineralise and die under the influence of testosterone in preparation for the mating season. The hard antler is, therefore, a male secondary sexual character. Some species, such as water deer (Hydropotes inermis), do not grow antlers; while in some species such as reindeer, females also routinely grow antlers. See Figure 10.1 (see also figures from Chapter 1) for an overview of the terms used to describe antlers.

Antlers grow by endochondral ossification from stem cells derived from the periosteum of an anatomically distinct protuberance of the frontal bone, the pedicle. Fallow, red and roe deer visibly grow their antler pedicles in utero in response to testosterone, although by birth they may no longer be visible (Li and Suttie 2001; Kierdorf et al. 2023). It is often quoted that antlers may grow up to 2 cm/day (making it the fastest growing mammalian tissue); in some species, it provides a valuable model for the investigation of bone formation (Qin et al. 2023).

Studies of the interface between the skin and the antler pedicle bone have contributed in part to the development of transcutaneous implants for limb amputation in humans and animals without the complications of infection (Pendegrass et al. 2006; Golachowski et al. 2019).

The growing antler is cartilaginous and covered by heavily vascularised skin (velvet) with large numbers of sebaceous glands and hair; the deer are described as being ‘in velvet’. As the antlers harden, in response to rising levels of testosterone, the cartilage ossifies and the antler becomes effectively a dead bone attached to the pedicle. In some countries, antler velvet is harvested for processing into medicinal products (Pearse 2022; Li et al. 2023). The study of antler velvet has shed light on the processes involved in tissue wound healing (Sinha et al. 2022).

By the end of their first winter in red deer, the pedicles have made rapid growth stimulated by testosterone. From these pedicles (Figure 10.2a,b), during their first year of life, red deer males develop their first antlers at a rate influenced by levels of nutrition. Having cast their first antlers, often just simple spikes, usually at about two years of age, stags grow normal adult antlers whenever testosterone levels are very low. In red deer and other species, differentiation of the pedicle in early life is apparently stimulated by testosterone. By contrast, antlers grow only when testosterone levels are at very low, baseline, levels. Therefore, growth is controlled by testosterone levels, which are in turn controlled by changing day length: in spring, in red deer, as a response to increasing day length, testosterone levels decline causing the antlers to fall off or ‘cast’. New antlers immediately begin to grow and continue to do so very rapidly through the early summer when testosterone levels are low.

With the summer solstice, luteinising hormone levels and subsequently testosterone levels begin to rise and, influenced by the declining day length, red deer stags harden or mineralise their antlers. By late summer red and fallow deer shed, or ‘clean’, their velvet. This covering of the live antler becomes frayed and is eventually lost, leaving the characteristic hard antler that the stag uses to exert his dominance over competing males. The rut, which in the Cervinae in the northern hemisphere may be considered to begin in September, effectively ends in late October although younger males may rut later and males may mate hinds throughout the winter. After the rut, as testosterone concentrations decline the antlers are shed in the spring. Regrowth commences and the cycle is repeated (Lincoln 1971a,b).

If male deer are castrated as calves before the pedicle has differentiated, then no antlers can be formed. If deer are castrated when they are older, that is, after the pedicle has become active, then antlers are formed, but in the absence of testosterone, the antlers remain in velvet. If the male deer is castrated in the hard antler phase these antlers are cast, usually in about three weeks, new antlers grow and the antlers remain soft and live, covered in velvet for the rest of the castrate’s life. If the deer is castrated when he is growing his antlers, then the antlers will remain in velvet and never harden. Thus, in all cases, castration of the adult males results in him remaining permanently in velvet.

Castrated red deer, with soft antlers, will almost always be subordinate to stags that are in hard antlers during the winter and may be excluded from feed sources. They do not, however, suffer the weight losses that intact stags undergo during the rut and their welfare is not compromised except in the rare situation where the velvet growth becomes disorganised, creating a ‘perruque’ or ‘wig’. This is a disorganised mixture of bone, cartilage and skin which loses any semblance of biaxial antler formation and may cover the frontal, occipital and even maxillary regions of the head. Whilst the formation of large perruque antlers is rare in castrated red deer (Figure 10.3a), it is common in roe deer castrates and in bucks that have suffered testicular pathology. Perruque heads in roe bucks may eventually overwhelm the buck and kill it (Figure 10.3b).

The response of fallow deer to castration is similar to that of red deer and the resultant antlers have been described as antleromas (Kierdorf et al. 2017). Wild white-tailed deer (Odocoileus virginianus) have occasionally been reported to have antleroma-type (bony) lesions and these are usually associated with alterations in circulating levels of testosterone (Munk et al. 2015).

The term antleroma has been recommended to describe any tumour or neoplasm composed of antler-derived tissues including one or more of the velvet skin, bone, cartilage and fibrous connective tissue (Munk et al. 2015). Antleromas are associated with reduced testosterone which may be due to castration, cryptorchidism or other form of hypogonadic state (Bubenik 1990a). Viral infections such as epizootic haemorrhagic disease virus may affect the testes and reduce testosterone levels leading to antler abnormalities (Fox et al. 2015).

The Capreolinae have an alternative antler cycle. Roe deer of both species (Capreoluscapreolus and Capreolus pygargus) in the northern hemisphere are seasonally different in growing their antlers during the winter and remaining in hard antler in the summer when they rut. Some tropical species such as muntjac are sexually active all year round but have a seasonal antler cycle, which corresponds with red, fallow and sika when they are translocated to the northern hemisphere.

Veterinary surgeons may be asked to treat adult male deer that have been castrated and are consequently in a state of persistent velvet which may have developed haematoma or have hypertrophied into a perruque. As early as 1935, Georg Blauel demonstrated that oestradiol is effective in causing castrate roe bucks to clean the velvet from their perruques (Blauel 1935). Subsequently, it was shown that oestradiol was more than 10 times as effective in mineralising velvet antlers than testosterone (Goss 1968; Bubenik 1990b). The velvet antlers of castrate red deer have also been shown to be readily cleaned using oestradiol (Fletcher and Short 1974). The more common approach based on the physiological control of antler growth has been to administer exogenous testosterone. However, this will make the deer rut and become aggressive – something that castration was meant to avoid. Once the administration of exogenous hormones is stopped the antlers will be shed (although in some deer this may not be the case) and subsequently revert to new velvet growth but it will take some months for the perruque to regrow and that may be considered sufficient therapy.

Deer may grow, usually contralaterally, asymmetric antlers as a result of lameness or limb amputation; the exact pathomechanism of this phenemenon is not well understood (Marburger et al. 1972; Davis 1983; Wild et al. 2022). Parasitism (gut and lung nematodes, liver fluke), systemic disease, poor nutrition, trauma from fighting and damage to the pedicle may also affect antler growth and form (Brown 1983; Esattore et al. 2024). Selective breeding for multipoint antlers has shown that there is a genetic component of antler size and shape, with genetic markers identified (Elblinger et al. 2022). It is not uncommon for individual deer to occasionally clean or cast their antlers at abnormal times for no apparent reason.

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