Wildlife information at the click of a mouse--
|Fallow deer (Dama dama)||Red deer (Cervus elaphus)|
|Yearling = Fawn
Second Year = Pricket
Third Year = Sorel
Fourth Year = Sore
Fifth Year = Bare Buck
Sixth Year = Buck
Seventh + Year = Great Buck
|Yearling = Calf
Second Year = Brocket
Third Year = Spayad
Fourth Year = Staggard
Fifth Year = Stag
Sixth Year = Hart
Seventh + Year = Great Hart
The age of the deer: The table above gives the names assigned to deer of different ages, often based on the development of their antlers.
A mature Red stag may well have 12 to 15 branches (called tines or points) to his antlers and stags are often named according to the number of these points. Deer with their first set of short, simple, unbranched antlers (i.e. at two years old) are referred to as prickets (Fallow) or brockets (Red - see below, left). Over subsequent years, the antlers should become progressively larger and branched (up until the stag is about 10 years old, after which the number of tines starts to decline). A Red deer with 12 points (six per antler) to his antler is called a Royal stag, while 14 points make an Imperial stag and an animal with 16 points or more is referred to as a Monarch. In his article for South Coast Today (a Massachusetts news and current affairs website), outdoor writer Marc Folco describes how hunters speak in terms of “pointers”. Mr Folco explains that a deer with five tines each side is a five-pointer, while one with six either side is a six-pointer. In cases where the antlers are asymmetrical (i.e. different number of tines each side), the two values are given separated by an “X” – thus, a deer with six tines on one antler and five on the other is a “6 X 5”, rather than an 11-pointer. In Fallow bucks, the palmation extends with subsequent antler sets as do the number of points, called spellers in this species.
That time of the month
How do we know it’s testosterone that regulates the antler cycle? Moreover, what controls the changes in testosterone? It was Greek philosopher Aristotle who first noticed that the genitals had an important connection to the development of antlers and, around 350 BC, he wrote in his Historia Animālium that: “If stags be mutilated, when, by reason of their age, they have as yet no horns, they never grow horns at all; if they be mutilated when they have horns, the horns remain unchanged in size and the animal does not lose them.” It would be several centuries until Aristotle’s musing would be confirmed experimentally when, in 1913, two German anatomists established that castration of a deer with cleaned or polished antlers led to casting and growth of new antlers from which the velvet is never shed. Subsequent experimentation by Richard Goss found that both androgens (testosterone) and oestrogens (oestradiol) were able to prevent old antlers from being shed, inhibit the growth of new antlers and cause the velvet to shed prematurely from growing antlers. It has also been shown that castration of a stag growing antlers prevents the antlers from becoming completely calcified and the velvet from being shed; the result is that the antlers may continue to grow and lead to what Donald Chapman describes as “antler monstrosities”, including the benign tumours that Dr Goss called ‘antleromas’. Studies by various authors -- especially the Red Deer Research Group on Rum and Goss -- during the 1970s and 1980s provided further confirmation that testosterone was the regulator; it was found that fitting castrated stags with testosterone implants allowed them to resume a normal antler cycle and testosterone injections initiated pedicle development in females.
The question of what regulates testosterone is less straight forward to answer, although for temperate deer at least (equatorial deer appear to be a special case, as will become apparent) light is critically important. More specifically, it is the number of hours of light and subsequent darkness that the deer receives – this is referred to as the photoperiod and expressed as light:dark ratio, such that 12L:12D would mean 12 hours of light followed by 12 hours of darkness. Temperate deer are highly seasonal animals and their biological (or circadian) rhythms are heavily influenced by the changing daylength. Melatonin, the so-called ‘hormone of darkness’, is produced by the deer’s pineal gland in response to darkness and the more hours of darkness the animal is exposed to, the more melatonin is produced. There are some ambiguous data about the role of melatonin on sex hormones in mammals, particularly humans at the moment, but in short-day breeders (i.e. animals that breed during the winter) it seems that gonadal functions are activated by an increase in melatonin. Thus, an increase in melatonin indirectly stimulates the testes to increase their production and secretion of testosterone. The increase in circulating testosterone then terminates bone formation in the antler and triggers the death of the velvet. Indeed, biologists at Aberdeen’s Rowett Research Institute reported, in a paper to the Journal of Reproduction and Fertility during 1986, that melatonin is a key hormone in regulating the antler cycle; they demonstrated that velvet shedding and rutting could be induced up to five weeks early in a stag given feed pellets containing the hormone.
The importance of photoperiod on antler cycle has been recognised for more that half a century when, in 1954, Polish biologist Zbigniew Jaczewski demonstrated that Red deer could produce two sets of antlers in a single year if exposed to a ‘sped up’ photoperiod. Despite these initial results the ways in which photoperiod regulated antler development was, and to an extent still is, poorly understood. A series of classical experiments on Sika deer conducted by Richard Goss at Brown University in Rhode Island, however, improved our understanding. During the seven year study, captive deer were held under the artificial conditions of Brown University’s Animal Care Facility, where the photoperiod and temperature were strictly controlled – the idea was to expose the deer to various combinations of light and dark and see what effect it had on their antler cycle. Goss found that, if the photoperiod was reversed (i.e. the deer thought it was winter when it was actually summer) they cast and re-grew their antlers six months out-of-sync with the outside world. It was also possible to corroborate and expand upon Jaczewski’s studies. Goss demonstrated that, if the photoperiod was reduced to replicate a shorter year, the deer could be made to regenerate up to four sets of (albeit stunted) antlers in a single calendar year. Interestingly, Goss observed that when the photoperiod was set to mimic six ‘years in one’ the deer didn’t produce six sets of antlers, instead they returned to their normal circannual cycle (i.e. one set per year); likewise when the photoperiod was increased (making one year last for two) the deer still cast their antlers each calendar year as normal. Overall, Goss concluded that all the time the deer could entrain to (determine) the photoperiod they could cast and re-grow their antlers accordingly, even if this led to increased or irregular production. If, however, the photoperiod was one that they couldn’t determine they simply reverted to their usual circannual rhythm of casting and renewal. In other words, there is a limit to the photoperiod that deer can adapt to.
Flow diagram showing the basic antler cycle, which is heavily influenced by photoperiod acting upon hormone levels.
Before we leave the subject of photoperiodic control of antler cycles, we should give some consideration to deer living in equatorial regions, where there are no seasons. Equatorial deer can be found with antlers at any time of the year, but curiously many (although not all) seem to cast annually, so they must have a method of keeping track of time; precisely how this is achieved is still enigmatic. Dr Goss’ work has shown us that there is something peculiar about an equinoxial photoperiod (i.e. 12L:12D), his deer cast normally when given 12L but fewer dark hours and vice versa, but when given 12L:12D they failed to cast and casting was abated for up to four years in one animal. At first, Goss thought it might have been the L:D ratio of one that caused it, but exposing the deer to shorter durations of the same ratio (i.e. 5L:5D, 6L:6D, 8L:8D and so on) caused the deer to cast as normal, so there was something special about 12 hours of light followed by 12 hours of darkness that caused disruption to the cycle and a failure to cast. We still don’t know what happens or how equatorial deer manage to keep track of the year, although there is the suggestion that they are sensitive to how many alternating periods of light and dark they experience and associate a certain number with one year. Goss’ work has provided interesting data to suggest that deer may at least be able to keep track of photoperiods. By splitting deer into two groups and exposing one to a regular short-day to long-day photoperiod (i.e. 4L:20D to 20L:4D) and the other to the opposite, Goss found that antlers were shed and re-grown in synchrony with every alternative change in day length, regardless of the direction (i.e. lengthening or shortening) of the change. In other words, it didn’t matter whether the days got longer or shorter, the important thing was that the deer registered that there had been a switch in the photoperiod and because the deer would normally experience two shifts per year, they adjusted their cycle to shed every second change. Given what we know about the function of hormones in this cycle, it’s difficult to see how it could only be the number of shifts in photoperiod that a deer registers as opposed to the amount of light the shifts bring, which would have a direct stimulatory or suppressive impact on melatonin production.
So, in the end, we have a cycle of bone growth and loss that is under the control of light; light stimulates the production of melatonin, which probably acts on the anterior pituitary stimulating the testis to produce (or stop producing) the testosterone that regulates the deposition of bone within the antler, and the blood supply to the velvet surrounding it. The next step is to address what controls how the antlers look and how large they grow.
A 'Royal' Red stag, with 12 antler points, in the New Forest, Hampshire.
All shapes and sizes
For centuries the size of a deer’s antlers has been of interest to humans and there are many collections of deer antlers and heads around the world – the largest collection of White-tailed deer (Odocoileus virginianus) heads, for example, currently resides in the Buckhorn Hall of Horns in Texas. In years gone by Red deer were transported from English parks to the hillsides of Scotland in a bid to improve the quality (body size and ultimately antler size) of the native stock; this implies an element of supposed genetic control over the development of antlers. Indeed, many have commented that it is possible to identify a stag over successive years by the shape of his antlers; the Red Deer Research Group on Rum have apparently become rather adept at this. The evidence, however, is far from unanimous and, in his book The Roe Deer, Richard Prior wrote: “A buck’s age, let alone his identity or his value as breeding stock, cannot be judged by his antlers alone.” It is actually translocation studies that have shown us that how large a deer’s antlers grow is not solely a reflection of genes; diet is a crucial factor. In his Whitehead Encyclopedia of Deer, G Kenneth Whitehead described how Scottish hill stags (which are typically rather small-antlered animals) transferred to superior habitat -- a deer park, for example -- are capable of producing antlers comparable to stags in other high quality habitats, such as deciduous woodlands. Indeed, Prior stated that it was habitat, rather than bloodline, that had overriding influence on the antler size of Roe deer. In his A Life for Deer, vet John Fletcher made much the same point when he said: “undoubtedly the limiting factor in the productivity of Highland red deer is very rarely the genetics of the deer but rather the environment: food and shelter”. There are other population studies, largely on Rum, suggesting that the average antler size declines with increasing population density – more deer means more competition for food such that each animal typically gets less, is in poorer condition and thus produces smaller antlers. The Rum biologists have also found that the weather and early life-history of the deer can also affect the length of their first antler set – as may be expected, light calves born late in the year and growing up in bad weather (mainly low temperatures) developed smaller antlers than heavier calves born during good conditions.
Despite a considerable body of evidence implicating habitat quality in regulating antler size, this doesn’t mean that genetics are unimportant. Ultimately, all structures in the body are built according to the ‘blueprint’ laid out by the genome and it is well established that there is a species-specific pattern to antlers (i.e. you can tell the species of a deer by looking at its antlers), which is presumably encoded in their genes. Indeed, we now have several studies showing that there is a degree of heritability in both size and, perhaps more importantly, shape of the antlers; the researchers on Rum estimate that antler size is roughly 20% heritable. Overall, the data suggest that, where food is not limiting and hormonal aberrations (accidental castration, etc.) are absent, genetics play a fundamental role in determining how large and into what shape a given set of antlers will grow. When dietary and hormonal aberrations are encountered, these can override the impact of the genes because such secondary sexual characteristics (i.e. attractive features that aren’t necessary for the physical act of reproduction) generally have low growth priorities. It has also been established that a deer’s antlers become progressively larger (in height and thickness) and more branched as the animal ages, although there does not, however, appear to be a correlation between the age of a deer and the number of tines on its antlers.
The understanding that antler size and complexity increases throughout the stag or buck’s lifetime explains why it should be necessary to cast the antlers they have invested so much energy growing. The sensitive velvet skin is necessary for the antler to develop, but must be removed before the antler can be used in combat. Removing the velvet, however, effectively ‘kills’ the antler (despite some bone cell activity the antler cannot grow or heal). Thus, if the antlers are to grow with the deer’s body, it becomes necessary to replace them regularly. As we shall see shortly, antler size and complexity appear to help females judge how ‘fit’ a stag or buck is, as well as potentially letting other males decide whether he’s worth fighting with. Thus, if the antlers were never replaced, the male would either spend his entire life with simple spikes that said nothing about his reproductive prowess, or would need a set that were impracticably large for a yearling to carry. All this casting and re-growing, however, doesn’t come without a price. (Photo: Reeve's muntjac antlers. Note the pedicles are much larger than those of other species, being roughly the same length as the antlers. If you're unsure where the pedicle stops and antler starts, hover your mouse over the image to highlight the coronet.)
The cost of antlers
Antlers come in all shapes and sizes, from the small (10cm / 4 in.) simple spikes of the muntjac (Muntiacus reevesi) to the 1.5m (5 ft) organs sported by American wapiti (Cervus canadensis). Moreover, we have seen that antlers are grown quickly. Indeed, they are the fastest growing mammalian tissue; they exhibit a typical sigmoidal growth curve (i.e. start slowly, speed up and then slow down just prior to cleaning), growing several centimetres per day during their peak growth period, and are complete within 12 to 16 weeks. Such rapid growth requires a considerable amount of minerals, namely calcium and phosphorous. In 1985, Paul Muir and his colleagues at the University of Canterbury in New Zealand calculated that a Red stag producing three kilos (just over 6.5 lbs) of hard antler deposits just over half-a-kilo (19 oz.) of calcium and up to one kilo (just over 2 lbs) of other minerals; during the final ten weeks of development (when growth is at its most rapid), calcium is deposited at a rate of around 5g (1/5 oz.) per day. This corresponds well with values given by Donald Chapman who, in his Mammal Review article, notes that antlers are composed of about 50% minerals (the rest is largely ash and protein) and, of this, 45% is calcium and 19% phosphorous. So, as Chapman points out, for an ‘average’ Red stag producing a pair of antlers weighing around 13kg (just under 3 lbs) in 130 days, this means that the deer must produce an average of 100g (3.5 oz.) of bone per day.
So, how do the deer find such large quantities of bone growing minerals in such a short time? There is some evidence that minerals can be sequestered internally, from the deer’s skeleton. Work by biologists in the USA -- on Mule deer (Odocoileus hemionus) in the Rocky Mountains of Colorado and Reindeer kept at Washington State University, to name a couple -- has documented a decrease in bone mass, caused by osteoporosis or osteoresorption. The researchers have recorded minerals being removed from the skeleton, with the ribs and long bones (e.g. the metacarpus and tibia of the legs) yielding a combined decrease in density of almost 60%; the bulk of this resorption seems to occur during the middle of the antler’s growth period. It seems that, where resorption occurs, it is more common in trabecular bone than in cortical bone, probably because the former is more metabolically active.
Cast antler from a mature Fallow buck. The extent of the palmation and the number and size of the spellers ('finger fringes') increase with the age and condition of the buck.
Obviously there is a limit to the amount of minerals that can be sequestered from the skeleton, before the bone becomes too brittle to support the animal; it seems unlikely that the complete antler set could be constructed in this way. This is even more apparent when we consider that antlers are allometric organs, which means that larger deer sport larger antlers even after correcting for body weight. Consequently, deer must get some of their minerals from their diet and one very good source is antlers. Bone (and specifically antler) eating is apparently a fairly common behaviour in deer and cast antlers represent a substantial ‘mineral bank’ – I have even come across reports of deer chewing on another’s antlers while still attached! In their 1982 Red Deer: Behaviour and Ecology of Two Sexes, Tim Clutton-Brock, Fiona Guinness and Steve Albon note that both stags and hinds on Rum chewed bones and cast antlers, especially during the spring and early summer (when stags are in velvet). In a fascinating paper to the journal Mammalia during 1985, Cyrille Barrette described the antler eating behaviour of wild Axis deer (Axis axis) in the Wilpattu National Park, Sri Lanka. Prof. Barrette witnessed 102 instances of osteophagia (literally ‘bone eating’) during his two years of fieldwork, with all ages and both sexes indulging – it was, however, the males in velvet that were seen to chew bones most often. Apparently chewing bouts were fairly lengthy, lasting on average nearly 40 minutes, if the ‘chewer’ wasn’t disturbed; in some cases the antler was chewed down to the coronet! Describing the chewing behaviour, Barrette wrote:
“In most cases, the deer did not pick up the bone off the ground but only lifted the sharper end, chewing it with molars and premolars in the ‘cigar-like’ manner described by Sutcliffe (1973). The heavier end of the bone rested on the ground and saliva could be seen dripping while the animal worked the bone in its mouth”
Deer-chewed antlers collected by Prof. Barrette during the aforementioned study. The top set are an intact and chewed antler from an Axis deer (Axis axis), while the bottom examples are from a Sambar deer (Rusa unicolor). In both cases, deer have chewed the antlers almost down to the coronet. Photo used with permission.
Incidentally, as something of a side-line, the reason that we don’t find ourselves swimming in cast antlers when we go walking in the woods (in fact, they’re pretty scarce) is probably because many are eaten by deer and rodents. Where antlers are found, it is not uncommon to find scrapes and indentations on their surface characteristic of having been gnawed by one or more rodents and, in a brief communication to the journal Science in 1940, former University of Toronto zoologist Alan F. Coventry told of a Red squirrel (Tamiasciurus hudsonicus) regularly visiting a moose skull outside his cabin on an island in Ontario’s Lake Temagami, to gnaw at the bony projection. (Photo: A typical gnaw mark, from a rodent, on a Fallow antler.)
Given the need for replacing antlers each year and the significant metabolic drain involved in doing so, why should the deer go through all this? What do they use these antlers for?
Swiss Army antlers
The function of antlers has long been a subject of debate, with suggestions ranging from the lucidly apparent to the rather bizarre. In 1937, for example, the eminent German zoologist Han Krieg suggested that antlers were a method of removing excessive minerals consumed in the diet. Today, there is little contention over the purpose antlers serve or the reasons for their evolution. Before we look at the currently accepted theory of purpose, let’s take a moment to look at some of the competing ideas.
One thing we can be fairly certain of is that, given the high energetic cost associated with growing antlers, if deer didn’t have a good ‘reason’ for doing so, they almost certainly wouldn’t. Nature is in finite balance and animals that waste their energy on frivolous organs are lost from the population. Consequently, having antlers must convey an advantage that is genetically heritable – in other words, having antlers must make it more likely that you’ll survive and reproduce, thereby sending your genes (which also contain the instructions for building antlers) into future generations.
In a short paper to the journal Nature during 1968, Bernard Stonehouse at the University of Canterbury suggested -- based on various anatomical observations, including the large number of blood vessels, lack of fat under the skin and branching that provides a large surface area -- that: “Thermoregulation may thus be the function which primarily determines the form and proportions of antlers, and necessitates their annual renewal”. In other words, antlers might have evolved to help deer regulate their temperature by dissipating heat like the ears of an elephant. The immediate problem with this idea, you might be thinking, is that generally only males grow antlers; surely if it was a heat loss adaptation females would grow them too? Well, Stonehouse argued that males have a bigger problem losing heat because they put on weight more rapidly (and maintain larger fat reserves) than females during the summer months. In his review of the function of antlers published in Behaviour during 1982, however, Tim Clutton-Brock suggested that the anatomical peculiarities that Stonehouse listed were perhaps better explained as a means to allow rapid growth of the antlers. Clutton-Brock also pointed out that, not only do some deer grow antlers during the winter (the Roe deer, for example), when they presumably have little need to lose excessive heat, but also that: “in tropical cervids there is no close association between antler growth and temperature” and “the antlers of temperate deer species tend to be larger, relative to their body size, than those of tropical species”.
An alternative suggestion was that antlers evolved as a method of defence against predators. This was first suggested by Charles Darwin in his 1871 book The Descent of Man and this may well be part of the story although, as Dr Clutton-Brock pointed out, it seems unlikely that antlers evolved principally as a means of defence given that only one sex grows them. Indeed, females and their young are arguably more vulnerable to attack by predators during the summer months than stags/bucks are. That said, it is interesting to note that Reindeer females grow antlers during the calving season and, unlike many deer species, the calves accompany their mother as soon as they can walk rather than being left lying in cover while the mother feeds. It seems likely that antlers could allow the female to offer an extra element of protection to her calf.
Some authors have suggested that antlers may have evolved as a tool for gaining access to food. Indeed, Reindeer have been seen to dig craters in the snow with their antlers to get at lichens and there are various reports of deer using their antlers to knock fruit from trees and even operate farm equipment to cut themselves some carrots. On reflection, however, it seems unlikely that such tasks would require antlers of increasing size and complexity and a more likely explanation is that the antler can be used to gain access to certain foods (just as they can be used to scratch hard to reach places on the back), but they did not evolve as a response to these tasks.
We now arrive at a series of theories that actually tie together under the umbrella of ‘social apparatus’. The most widely regarded explanation for the evolution of antlers is that they may be both an advertisement of fitness (both to females and other males) and weapons for use in intraspecific combat (i.e. between members of the same species). In his 1998 book, Deer of the World, Valerius Geist argued that antlers evolved as a response to feeding out in the open; being out in the open makes you more vulnerable to predators, which permits the formation of herds because more eyes and ears makes it less likely any one individual is going to get attacked. The downside to living in a group is that there are many mouths going for the same food, in other words there’s competition, and this leads to aggression and in-fighting. Now, as Dr Geist elucidated:
“Weapons that maximize wounding ultimately attract predators to the group, disrupt normal functions, and increase the cost of daily living to the group. Statistically, an individual that leaves the group due to wounds or fright reduces the security of each individual remaining in the group.”
So, animals living in a herd need a way of establishing a ‘pecking order’ without causing any serious injury – complex antlers do just this, by allowing largely ‘bloodless’ wrestling and sparring matches. The antlers serve to catch the charge of an opponent and hold onto his head so that wrestling can commence. Geist gives a detailed coverage of the evolution of antler forms and the reader is directed there for a more complete picture, but suffice to say that the precursors to modern antlers (so-called protoantlers) were bony, skin-covered, hairy extensions of the skull possessed by the mid-Miocene deer Dicroceros elegans and used in defence (curved upper canines were the offensive weapons). These protoantlers probably evolved into bony lumps on the head as a response to a need for protecting the head against bites. According to Geist, it was the bone protecting the underlying skull structure that was the origin of the early true antlers; such antlers are first recorded on small deer from Old World Europe that looked similar to modern day muntjac. (Photo: Fallow buck with antler buds.)
Of course, competition for food is not the only form of contest that herding animals experience; competition for mates is equally important and antlers appear to have evolved to allow males to compete with each other for access to females. Indeed, as Dr Clutton-Block noted in his review: “The occurrence of antlers in males and their absence in females (who do not have to fight for access to mating partners) is in accordance with the theory that they evolved as weapons.” So, can we prove that antlers are primarily used as weapons for intraspecific competition? Yes, by experimentally removing the antlers of captive deer. Many such experiments were carried out by Gerald Lincoln during the late 1960s and early 1970s and the results showed that when antlers were removed, the stags fell to the bottom of the hierarchy – the antlerless stags were challenged immediately by other members of the group. Similar observations were made by Michael Abbleby working with Red deer in Scotland and by Ludek Bartos and Vaclav Perner on the white Red deer herd kept on the Zehusice Game Reserve in Czechoslovakia. Red stags form bachelor groups outside of the breeding season and these scientists reported that as the stags cast their antlers they rapidly dropped in the hierarchy; once all the deer had cast, the pecking order was reinstated. A loss of status seems to correspond to poor breeding success in wild populations, because without antlers the males cannot compete with other antlered males and thus fail to maintain a harem. In a paper to the Journal of Experimental Zoology during 1972, Dr Lincoln wrote:
“... the loss of antlers can radically impair the rutting performance, and prevent animals from taking a harem and participating in mating ...”
The relationship between antler size and fighting ability or dominance is less clear. Some authors, such as Clutton-Brock in his Behaviour review, point out that the relationship between antler size and dominance is not a particularly close one and an individual’s fighting ability changes throughout the rutting season, while his antler size remains constant. There are, however, multiple studies confirming a positive correlation between antler size and body size so if, as the studies suggest, body size is the major factor influencing rutting success some have argued that it should be possible to gauge a stag’s fighting prowess based on his antler size. There is some evidence, from a study using stuffed heads with different antler sizes, that stags can do this, but Clutton-Brock considers that a stag basing a decision whether or not to fight another based solely on its antlers is likely to make the wrong choice. In the 1982 book, Clutton-Brock and his colleagues point out:
“... among stags over five years old in our study area, neither fighting success nor reproductive success was related to antler length ...”
Indeed, Clutton-Brock questions whether correlations between antler size and dominance, fighting ability and reproductive success are picking up on a relationship that is actually a by-product of the well known correlation between body size and dominance. This would make sense given that antler breakage doesn’t seem to have any significant impact on rutting performance. In a study of the Tule elk (Cervus cannadensis nannodes), for example, California Fish and Game biologist Heather Johnson and her colleagues found that antler breakage, regardless of severity, had no effect on either the fighting success or harem-holding success of bull elks in Owens Valley, California. Even if breakage were a problem, it appears to be rare and, in a 1971 note to the journal Nature, John Henshaw wrote of how he had only come across one instance of breakage of the main beam in his observations of “approximately a quarter of a million cervids of nine species”. We still cannot be sure what visual cues females use when assessing a potential mate, or that stags use when assessing a potential challenger, but it is thought that antler size may be part of the story, providing a ‘by proxy’ clue.
Deer use their antlers to settle disputes. During the rut, as males compete for access for females, these Fallow bucks lock antlers and try to push each other back. The winner is the deer that manages to push the other one back, forcing a hasty retreat.
In 2005 a team of Spanish biologists lead by Aurelio Malo at the Museo Nacional de Ciencias Naturales in Madrid found that Red stags sporting large, complex antlers had relatively larger testes and faster sperm than those with smaller, simpler appendages. Subsequently, in a 2007 paper to The American Naturalist, a team of French and Swedish scientists argued that antler size may “provide an honest signal of male phenotypic quality in roe deer” – in other words, Roe does may be able to tell the quality of a male by the size of his antlers. Thus, females may be able to use the size of a stag or buck’s antlers as a cue to their quality as a mate, because large antlers generally indicate a buck in good condition, that is, a fit buck. If females then actively chose males with large, complex antlers over those with smaller, simpler ones then there would be a selective pressure towards males with large and complex ornaments. Data from the Rum population provide some evidence for this, showing that the mating success of stags between the ages of seven and ten years is associated with the number of points on their antlers -- those with more points generally have more matings that those with fewer points -- although the relationship is not always clear.
Studies on the mechanical properties of antler bone have lent additional support to the theory that they evolved primarily as weapons. A study by University of York biologist John Currey and his colleagues assessed how effective antler of varying ‘moistness’ was at standing up to force. The researchers compared various measures of physical strength (elasticity, work-to-fracture ratio, etc.) of antler samples and sections of wet femur; they discovered that antler could withstand almost two-and-a-half times more sustained force (i.e. force attempting to bend the antler to breaking point) and six-and-a-half times more blunt impact force than wet femur before it broke. The biologists suggested that the antlers begin to dry out once the velvet is shed, but only sufficient moisture is lost to improve the mechanical properties of the bone. Thus, if the antler was too wet it would simply distort under the pressure applied during a clash, but if it were too dry it would be stiffer, more brittle and thus more likely to break. Indeed, Richard Prior points to a similar scenario in his 1995 book, suggesting that Roe antlers gain density, perhaps by absorbing resins and so on through fraying – Prior points out that most late-shot bucks seem to have very hard, dense antler compared with those shot just out of velvet, implying a gradual drying out of the antlers following casting. (Photo: A Roe deer buck in his winter coat and with antlers in velvet. Roe are unusual among deer in growing their antlers during the winter.)
Before we leave the subject of what use antlers may serve, it is worth briefly mentioning work by University of Guelph biologist George Bubenik and Cleveland State University mathematician Peter Bubenik. In a 2008 paper to the European Journal of Wildlife Research these scientists presented data showing that the palmated (broad, flat) antlers of Alaskan moose (Alces alces) might serve as a parabolic reflector of sound. The researchers constructed a ‘fake ear’ and attached it to an antler, pointing it at various different angles. The data showed that the loudest response was registered when the ear was facing the centre of the antler; the sound pressure was 119% that of when the ear was facing forwards (i.e. away from the antler). The authors wrote:
“These findings strongly indicate that the palm of moose antlers may serve as an effective, parabolic reflector which increases the acoustic pressure of the incoming sound”
Some researchers have suggested that large, flat, palmate antlers may serve to funnel sound into the ears of their owner (a bull moose, above), improving the deer's ability to locate sound sources.
In other words, the sound hits the antler and is bounced off into the ear, thereby improving the moose’s hearing. It seems unlikely that antler evolved in moose in response to a need for better hearing, but it reinforces the idea that such structures may convey more advantages than we might initially realise.
Not all antlers grow and develop in the pattern typical of the species. Sometimes aberrations occur, including atypical antler growth in female deer.
Abnormalities and curiosities
Perhaps one of the most curious aspects of antler biology is when they are grown by animals that don’t generally grow them, or when animals that should normally grow them fail to do so. We have already seen that female Reindeer grow antlers, but they have been documented in other deer too. Most frequently such reports come from Roe deer and, as Richard Prior alludes to in his The Roe Deer book, it is not unusual for older females to possess antlers. Prior explains:
“Old does quite commonly grow short antlers in velvet, usually not more than five centimetres long. They are normal breeders, but probably towards the end of their reproductive life.”
Prior goes on to recount the story of a doe he hand-reared, which developed antlers in velvet from the age of eight; the antlers grew larger in successive years (lengthening to 15cm / 6 in.) and by the time she died, the doe had developed an abnormality called ‘perruque head’ (see below). A post mortem of the doe revealed some testicular cells in the vicinity of her ovaries. Indeed, the development of antlers in female deer typically seems to be related to a hormonal imbalance, which appears to occur in old age. Alternatively, it may be that the testicular cells may always be present, but their influence is overpowered by the oestrogens secreted by the ovaries – as the doe ages, perhaps a drop in oestrogen secretion allows testosterone to have an influence. Either way, in their 2008 review of Roe Deer natural history Mark Hewison and Brian Staines note that antlers in does are often associated with hermaphroditism, both ‘true’ (where both male and female genital tissue is present) and ‘pseudo’ (where one sex displays the sexual organs of the other). The biologists also point out that pregnant antlered Roe deer are known and that, although doe antlers generally fail to shed velvet, does in ‘hard horn’ have been reported. Roe are not the only species in which females can uncharacteristically grow antlers – in White-tailed deer, for example, antlers are estimated to occur in roughly 0.1% (i.e. 1 in every 1000) of females. (Photo: Mule deer, Odocoileus hemionus, that has failed to shed the velvet on his right antler.)
In some cases, stags and bucks fail to develop antlers – such deer are called hummels or, in parts of south-west England, notts. Early theories proposed that the lack of antlers was either the result of accidental damage to the testicles, or that it was a heritable genetic condition (i.e. hummels bred to produce hummels). It should be noted, at this point, that true hummels are distinct from haviers -- which are antlerless as a result of castration -- and that, although lacking antlers, they are fertile. Some stalkers believed that hummels were actually at an advantage over antlered stags because, spared the drains of antler development, they could grow larger and would therefore be more successful in obtaining a harem during the rut. Consequently, hummels tended to be shot on sight in a bid to improve the local trophy standards. In a paper to the Deer journal during 1970, however, Brian Mitchell and Tim Parish presented data showing that hummels do not necessarily grow larger than antlered stags, while the studies by Gerald Lincoln discussed earlier have demonstrated that antlerless stags are at a disadvantage during the rutting season. Our understanding of hummels took a sizeable step forward when a ‘congenitally polled’ (polled means ‘without horns’) Red stag was caught at Braemar Lodge in the Scottish Highlands during 1969. Breeding studies conducted by Gerald Lincoln and John Fletcher clearly demonstrated that the hummel condition was not genetic – the stag sired multiple antlered progeny, which also sired antlered stags even when crossed with their sisters (ruling out a recessive gene). This stag also provided scientists with a better understanding of what causes the lack of antlers in hummels.
In 1974, Polish anatomists Zbigniew Jaczewski and Krystyna Krzywinska reported that amputation of the tip of the pedicle of a castrated stag would sometimes cause it to grow an antler, as if the act of wounding the pedicle was the stimulation for antlerogenesis. In a 1976 paper to the Journal of Experimental Zoology, Gerald Lincoln and John Fletcher presented the results of their surgical study on the Braemar Lodge hummel, which had “rudimentary pedicles” but failed to grow antlers from them during five years of observation. The biologists found that if they amputated the tip of the stag’s right pedicle, the deer grew a complete (albeit stunted) antler on this side (no growth was documented on the left pedicle), which was subsequently cleaned and then cast in the normal way. The stag died shortly after the experiment and dissection revealed a substantial increase in the thickness of the right pedicle compared to the left. Lincoln and Fletcher concluded that hummels weren’t physiologically incapable of producing antlers; instead a failure to develop fully formed pedicles meant that the antlers had no base from which to differentiate. The researchers wrote:
“... it was possible to induce antler growth in the hummel by apparently simulating the process of ‘wounding’ that naturally occurs at the time of antler casting.”
Skull of the Braemar Lodge hummel, showing a hypertrophied right pedicle (circled) following surgical wounding. Photo used with permission.
In other words, it may be the wounding of the pedicle caused when the antler is cast that, in conjunction with a drop in testosterone secretion, triggers the growth of the antler. So, why might stags fail to develop normal pedicles? Lincoln and Fletcher suggest that, given most hummels are found among the Red deer of the Scottish hillsides, they may come about under conditions of low food availability; low nutrient levels experienced as a calf may starve the pedicles of the minerals needed during a crucial growth period. Alternatively, researchers at the Croatian Forestry Society in Zagreb proposed, in a 2008 paper to the Croatian journal Sumarski List, that:
“As pedicle growth depends on androgenic [testosterone] stimulation, low levels of circulating androgens or a low density of androgen receptors in the antlerogenic periosteum could lead to poor pedicle growth and in consequence to a complete or almost complete lack of antler growth.”
Hummels are relatively rare (in 1990, G. Kenneth Whitehead put the figure at less than 1% of Scottish Red deer), so the data we have are from only a few individuals and the results may not be representative of all cases. However, off-hand it seems that, if the hummels were a response to a lack of androgen receptors in the pedicle, this is unlikely to be corrected by simple surgical wounding of the tissue. Ultimately we are still unsure as to the exact cause of this condition, but it seems likely that both early malnutrition and abnormally low testosterone levels are possible contenders. It should be mentioned, incidentally, that although hummels appear more common in Red deer than other species, it is not a condition unique to Cervus elaphus; there are occasional records of Roe bucks without antlers.
Antler deformities come in various shapes and sizes and, although none are particularly common, they are typically associated with hormonal imbalances. In her 1991 book, Deer, Norma Chapman notes that the so-called double-head condition is well known in Fallow bucks from Denmark and Germany, with rare reports in German Roe deer and Scottish Red deer. The condition manifests as either a failure to cast the antlers before the new set begin growing, leading to four antlers being present simultaneously from two pedicles, or the formation of an additional one or two pedicles on the skull from which antlers will grow. The phenomenon of failing to cast before new growth begins can take a more serious turn in a compound-growth condition known as perruque. Perruque, from the French meaning ‘wig’, is a condition where the antlers continue to grow during subsequent years without casting; the result is the growth of what can, in advanced cases, be a rather grotesque and heavy lump of bone that cascades down across the face and obscures the eyes. In particularly warm periods any damage to the velvet tissue, which is often retained indefinitely, may become infected. Perruque seems to be caused by damage to the testicles. To the best of my knowledge, the only documentation of the onset and progress of perruque in Roe deer comes from this single Roe Prior cared for. Although most commonly reported in Roe bucks, there are occasional records of less dramatic perruqueing in Fallow and Red deer.
Roe bucks have been recorded with coalesced antlers (see right), where the main beams merge into a single thick mass; they are shed as a single unit. Coalescence seems to be the result of larger, thicker pedicles being situated closer together than normal; in his 1995 book, Prior notes that this condition may be more common in elderly bucks, as the pedicles thicken and shorten with successive antler castings. Injuries to limbs have also been implicated in the abnormal development of antlers. There are surgical experiments in Sambar (Cervus unicolor) and Indian muntjac (Muntiacus muntjak) deer showing that amputation of part of one hind limb can lead to stunted growth of the opposite antler – amputation of the left hind leg, for example, would produce stunted growth of the right antler. In his Encyclopedia of Deer, Whitehead suggests that this may be a form of ‘bilateral compensation’, recounting a case where a Sambar deer grew a left antler three-to-four times heavier than the right, apparently to compensate (balance out) a weakened, handicapped left hind leg. Similar stories are discussed by John Fletcher in his A Life for Deer, in which he tells how deer stalkers long spoke of how an injured stag would “grow a twisted horn”. Fletcher mentions that, although deer have an extraordinary capacity to repair bone fractures spontaneously, where a fracture coincides with antler growth an asymmetrical antler with distorted growth is almost inevitable; he speculates that this might be related to liberation of endorphins (natural pain killing chemicals) by the body.
Hormones tend to be involved in antler abnormalities in one form or another, but in some cases parasites have been implicated. There are some rare examples of deer (generally Red stags) growing twisted, or corkscrew, antlers; the origin of such abnormalities is currently unknown, but it has been suggested that it may be related to internal parasite -- specifically lungworm (Dictyocaulus spp.) -- load.
So, in summary we have established that antlers are deciduous bony structures that develop from extensions of the stag’s (and in some species, hind’s) skull. The first antlers are usually simple, unbranched spikes that grow as extensions to the pedicle and thus lack a coronet; subsequent antlers become progressively larger and more branched. Casting and re-growing of the antlers is under seasonal influence, which acts upon androgen levels in the deer’s blood – in spring, low testosterone levels cause the antlers to be cast and new growth to start, while in the autumn a rise in testosterone (in preparation for the rut) causes the velvet to be shed. There are times when antler development goes awry and these are generally associated with hormonal imbalances, as are the atypical growth of antlers in females. Antlers are used as weapons during combat between stags for mating rights and, in reindeer, may help find food buried under the snow and compete with bulls for access to precious winter resources. (Back to Menu)
Antler Growth References
Bauer, E.A. (1995). Antlers: The antlered animals of Europe and North America. SwanHill Press, Shrewsbury.
Chapman, D.I. (1975). Antlers - bones of contention. Mamm. Rev. 5(4): 121-172.
Goss, R.J. (1983). Deer antlers: Regeneration, function, and evolution. Academic Press, USA.
Goss, R.J. et al. (1992). The mechanism of antler casting in the fallow deer. J. Exp. Zool. 264(4): 429-436.
Kierdorf, U. et al. (2009). Improbable appendages: Deer antler renewal as a unique case of mammalian regeneration. Sem. Cell Devel. Biol. 20(5): 535-542.
Li, C. et al. (2007). Antler Regeneration: A dependent process of stem tissue primed via interaction with its enveloping skin. J. Exp. Zool. 307A(2): 95-105.
Price, J.S. et al. (2005). Deer antlers: a zoological curiosity or the key to understanding organ regeneration in mammals? J. Anat. 207: 603-618.
Putman, R. (1988). The Natural History of Deer. Comstock Publishing Associates, New York. (Chapter 7).