The recent question from R. Goodge in ST about ?self-healing? antlers (Letters, 24 May) is one that highlights some remarkable aspects of the biology of deer antlers while also challenging some of the established preconceptions.
Testosterone stimulates antler development and maintenance, with the lowest blood levels recorded just prior to casting, but the hormone prolactin also has a role. Hormonal influence promotes the death of the protective ?velvet?, which has carried blood vessels to feed growth and mineralisation. Once removed, the antler dries and hardens to aid combat, and externally is ?dead?, no longer having a blood supply. Inside, however, it is a different story.
In red and fallow deer antlers, vascular cores have been shown to carry blood after completion of antler hardening. Researchers in Poland have noted that broken antler beams on red stags bled after damage during fighting, with the blood carrying a ?rut taint?, indicating vascular tissue still being fed by the arterial blood supply.
The cross-section of the fallow antler beam (image 1) shows clear differentiation between ossified and blood-carrying vascular tissues, familiar to anyone who has made a walking stick from a fallow antler coronet and brow tine, with the vascular tissue more easily worked to receive a hazel stick. My saw drew blood eight years after casting ? smears can be seen on the hard antler.
In red and fallow deer, scientific investigations have found that continuous remodelling of bone takes place within the antler and that the pedicle carries a rich supply of blood vessels that feed directly into the main core of the antler. Blood is considered to keep the antler in a ?moist? state that affords higher impact resistance. In fallow, the antler has been shown to remain in this state until some three weeks before casting, with residual blood left in the core.
Casting is the result of the progressive die-back and then dense ossification of the blood vessels within the pedicle (again stimulated by the reduction in testosterone), creating a brittle point that facilitates a clean break ? a layer of pedicle bone cells can be seen on the base of cast antlers (image 2), which acts as an effective seal. By contrast, the horns of sheep and cattle are permanent living structures made of keratin (the same types of cells make up hair and nails). The horn is a continuous growth and if damaged never fully recovers. The cross-section of a ram?s horn reveals an interesting similarity to antlers, however (image 3), showing the presence of a core of vascular tissue within this living structure ? just like the fallow deer antler cross-section.
X-ray studies of roebuck antlers, polished and in velvet, have shown a similar structure of blood vessels, without any change with increasing age. The core in clean antlers may show less clear differentiation than in fallow, but close examination of the cross-section (image 4) reveals residual vascular pockets in the main beam.
Antlers certainly have a self-healing capacity when in velvet, with fractures to tines being repaired through a process of deposition that strengthens the fused bone as shown in this example of a Lake District red deer top tine (image 5). After the removal of the velvet, however, the external repair process is halted, as the tissue is dead. So why do we find these other smoothed breaks?
It is likely the external section of the antler becomes smoothed or polished through physical wear and tear, and the degree of polishing may be relative to when general antler cleaning and fraying is carried out. Antler bone is made up of cells that can be worked and polished by friction; roebucks have been recorded engaged in fraying over periods of 40 to 50 minutes at a time. Internal irritation of nerve endings near the break may also contribute to rubbing activity.
If the break occurs in an area that still has a good blood supply, it is logical to expect some wound healing (re-modelling) of the subsurface layers. The photograph illustrating R. Goodge?s letter shows damage close to both the beam and coronet, where the blood supply would be strongest. The exposed rougher internal bone also readily picks up soil or other contaminants that mask the break, which may also give an impression of healing on ?dead? areas of antler.
To summarise, research suggests that the outer layer of ossified antler tissue does not self-heal after removal of the velvet, but may get modified through polishing from fraying activity or rubbing. The inner core has a longer active life and the capacity to rebuild bone tissue, with some variation between antler regions and deer species. Science aside, antlers remain fascinating structures, but I suspect few will be taking a saw to their trophies to explore the theory further!