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Sunday, 29 November 2020

Megaloceros giganteus: behind the antlers

Megaloceros giganteus galloping about: a rare scene in palaeoart, but probably a common sight in life. Famed for its size and headgear, we mostly ignore the fact that Megaloceros was among the most cursorial of all deer. What else about this amazing animal is hidden behind those antlers?
Megaloceros giganteus, otherwise known as the Irish elk or giant deer, can genuinely be regarded as a game-changing species for the study of extinct life. First discovered in 1695, it was a founding taxon of palaeontological science - a species that sent early scientists a clear message about the existence of life before the age of humanity and the reality of extinction. The significance of the Irish Elk - which, of course, is not strictly Irish nor anything to do with any living 'elk' species - was perhaps first fully grasped by Baron Georges Cuvier, who published a series of studies on giant deer in the late 1700s and early 1800s. Since then, Megaloceros has been an unwavering presence in paleontological media of all kinds: the focus of numerous scientific studies, a# case study for animal evolution, and a frequent star of palaeoartworks and popular texts. From a historic perspective, Megaloceros has demonstrated strong cultural staying power.

Among the earliest life reconstructions of Megaloceros is this small illustration from an 1854 issue of the periodical Die Gartenlaube. Its presence may have been influenced by the Crystal Palace Megaloceroscovered in the same magazine earlier that year. Although a relatively unremarkable restoration in most respects, it shows the same regal pose that has stereotyped portrayals of Megaloceros for nearly two centuries. If you've drawn or sculpted Megaloceros at some point, chances are it was standing somewhat like this.

An upshot of Megaloceros always being 'around' is that it's easy to overlook some of the cooler aspects of its palaeobiology. Its size and impressive antlers have become the principle takehome about its existence, such that other facts about its anatomy and ecology are ignored or forgotten. Recently, I've had cause to dig into the palaeobiology and research history of Megalocerosand, wow, what an animal. Cervids are genuinely fascinating anyway: charismatic, complex and awesome animals which, behind their soft, doleful eyes, have a physiology and life history evolved in partnership with Michael Bay. And it's within this context of deer being awesome, unusual animals that we can view Megaloceros as a kind of "superdeer": a species that represents the known extreme of many trends in cervid evolution, and that should be known for much more than its size and antler spread.

Giant deer 101

M. giganteus is a well-known taxon represented by thousands of fossils and with a wide but latitudinally-restricted distribution through Pleistocene and Holocene deposits of Europe and Asia (Lister 1994; Geist 1999; Vislobokova 2012; Lister et al. 2019). Like many Ice Age species, its distribution was continually shrinking and expanding in response to glacial growth and it never occupied the full extent of its known range at any one time. As with the mastodons discussed here recently, M. giganteus was an Ice Age species that avoided the cold. Although an adaptable animal that seems to have lived in a range of environments, Megaloceros appears to have thrived mostly during interglacials when the well-vegetated, herbaceous habitats it preferred were at their broadest (Vislobokova 2012). It lived in largely open, flat and well-watered settings termed ‘boreal parkland’ or ‘boreal steppe-woodland’, which were situated between cold steppe-tundra in the north and warmer, arid regions to the south. This parkland habitat provided Megaloceros with a diversity of grazing and browsing opportunities thanks to the presence of pine and spruce trees, as well as dense shrubs and herbs. Theprecise habitat preferences of Megaloceros changed over time and is reflected in antler shape: early subspecies had smaller, more erect antlers and seems to have preferred slightly more forested settings, for example (Lister 1994). Access to good quality food and water were likely critical to Megaloceros life histories due to the nutritional demands of growing those huge antlers each year (Geist 1999), and their need for highly productive plant communities likely prevented them from attaining the wider geographic ranges of other famous Pleistocene megafauna (Vislobokova 2012).

Distribution of M. giganteus sites recently compiled by Lister and Stuart (2019). Note the relatively limited latitudinal range.
Staying put in food-rich environments is probably one reason Megaloceros became so large and well-ornamented. Although not actually the largest extinct deer, the biggest stags are estimated to have massed an impressive 550-600 kg (or more) and stood about 2 m at the shoulder. Their antlers were the biggest of any cervid, weighing 35-45 kg and spanning up to 3.5 m. Reinforced skull bones and mandibular mineral storage were just some of their cranial adaptations to housing these immense organs (Lister 1994). Our knowledge of Megalceros does is comparably poor compared to that of stags, as our giant deer record strongly skews towards male specimens. So far as can be ascertained from a handful of skulls - the only elements we can routinely refer to either sex - females were of generally similar size to males, albeit about 80% of their mass and lacking antlers (Geist 1999). It's thought that our male-dominated fossil record reflects stags foraging separately to females and seeking aquatic vegetation rich in nutrients for antler growth. This, in turn, brought them more routinely into settings conducive to fossilisation. Although some fossil localities contain dozens or hundreds of Megalceros fossils, they are generally rare outside of these exceptional sites. This may reflect genuinely low populations of Megaloceros in most ancient environments (Lister 1994; Geist 1999), perhaps as a consequence of their resource needs.

Discussions and depictions of Megaloceros are heavily biased towards male specimens but - guess what? There were females too. Their fossils are significantly rarer than those of males but seem to have been of similar linear dimensions, though they were perhaps more slender overall. This skeleton is on display in the Natural History Museum, London.

Since the mid-1800s scholars have discussed where Megaloceros fits within deer phylogeny. Although firmly placed within Cervinae - the Old World deer clade - opinions differ over which extant deer are its closest relatives: Cervus (e.g. red deer, wapiti, sika deer - Kuehn et al. 2005) or Dama (fallow deer - Lister 1994; Geist 1999; Lister et al. 2005; Hughes 2005). The weight of morphological and DNA evidence for the Dama link is probably heavier at present, although this only resolves part of our struggles: how Megaloceros is related to other extinct deer is also uncertain. The composition, and even existence of a Megaloceros-line clade, Megacerini, is currently debated, as is whether Dama is the last surviving megacerine (see Vislobokova 2012 for a review).

Extreme deer, extreme speed

Prehistoric animals tend to become popular because they represent some kind of biological extreme - the biggest, the heaviest and so on. It's curious, therefore, that the running capacity of Megaloceros is neither widely remarked upon nor commonly depicted in palaeoartworks. A wealth of anatomical data shows that Megaloceros was among the most cursorial of all deer - a species adapted for running far and long over wide, open ground (Geist 1999; Janis et al. 2012). Deer exhibit a range of running styles but Megaloceros seems to have been most suited to long-distance galloping with low limb ground clearance, in contrast to species which bound at height over rough terrain or high-step at speed through dense vegetation. Reindeer and wapiti are similarly adapted to long-distance running, though neither is comparable in cursorial adaptation to Megaloceros. Along with cursorial limb proportions, Megaloceros also has an evident muscle bulk consistent with running habits, highly 'hinged' shoulders permitting an enhanced forelimb stride length, and an expanded trunk volume for an enlarged heart and lungs (Geist 1999). Relatively small hooves indicate a preference running over firm ground. I'm not aware of any specific estimates of Megaloceros running speed, but Geist (1999) considered Megaloceros comparable to the fastest living deer - perhaps 80 kph (50 mph) - and assumed similarly commendable levels of stamina. The increased bulk of Megaloceros would have made for slower acceleration but, once at cruising speed, it might have been capable of running for hours and hours without stopping.

Many aspects of Megaloceros osteology - from limb proportions to chest volume, antler configuration and knee morphology - are consistent with a strongly cursorial lifestyle. It's ironic that this energetic, fast-running deer is often depicted in static poses in palaeoart when dynamic and active compositions typify the genre as a whole. Image by James St. John, from Wikimedia Commons, CC-BY 2.0.

For Geist (1999), a running lifestyle is integral to explaining the development of Megaloceros size and antler spread. It stands to reason that the largest deer will also be the fastest and fittest, and antler size is a direct, honest reflection of stag health. These factors alone could have promoted the evolution of enormous antlers in a large cervid living in uncluttered habitats, but cursoriality encourages antler size in other ways too. For mammals, a consequence of evolving fast running is that juveniles have to be strong and coordinated enough to keep up with their parents, especially if running is critical to escaping danger. Megaloceros fawns must thus have also been capable runners from a very early age, requiring their parents to not only give birth to highly developed, precocial offspring but also supply them with milk rich enough to sustain long-distance, high-velocity running. Lo and behold, studies show that antler size correlates not only with offspring health and size, but also the amount of fat and nutrients in milk (Geist 1999). Put together, these factors mean Megaloceros wasn't a fast-running deer that happened to have huge antlers: its antlers were a direct consequence of its hyper-cursorial lifestyle.

But seriously though, those antlers

As is already evident, it's difficult to write about Megaloceros without frequent mention of its headgear, so let's tackle that topic head-on. There's a lot more to talk about here than just size. Antler structure, function and history of interpretation are also fascinating. Megaloceros antlers are extremely long - up to 1.7 m each - and broaden into great palms in their distal regions, with huge tines erupting from the burr, beam and palm margins. The first tine, which emerges just above the antler base, is broadened to differing extents in different subspecies, and is especially large in early representatives of the lineage (Lister 1994). This likely served as some sort of eye protector during combat (see below).

The size and elaboration of M. giganteus antlers placed it at the heart of a historic discussion about the fitness, evolution and extinction of fossil animals. M. giganteus stags were considered examples par excellence of orthogenic evolution in the late 19th and early 20th century when, as reviewed by Gould (1974), it was a species that had evolved itself into a corner: the result of a runaway, one-way evolutionary process that encouraged the creation of vastly oversized and biologically untenable antlers. Eventually, it was suggested, the antlers became so large and heavy that their owners were forever being caught in vegetation, mired in mud and bogs, or even suffered catastrophic brain haemorrhages caused by redirected blood flow from antler velvet. Extinction was inevitable for such sorry creatures.

Of course, even an elementary grasp of modern principles of natural selection shows these ideas as naive, quaint and totally wrong - there is no conceivable mechanism through which species can evolve structures that are so hazardous to their health. But it wasn't until the 1930s that a sensible alternative explanation for Megaloceros antler size was proposed (positive allometry), and we waited until the 1970s for someone to actually test whether Megaloceros antlers were actually oversized, cumbersome organs or consistent in proportion to what we'd expect from living deer. This study came in the form of Stephen J. Gould's famous 1974 paper "The origin and function of 'bizarre' structures: antler size and skull size in the 'Irish Elk,' Megaloceros giganteus", which demonstrated that Megaloceros antler dimensions were not only consistent with those predicted from living deer, but that their evolution was explainable through regular old sexual selection - no crazy, uncontrolled evolutionary mechanism required. Further work has shown that, relative to body size, Megaloceros antlers were no heavier than those of fallow deer and, indeed, were actually lighter than those of reindeer (Geist 1999). Gould's results were an important grounding of Megaloceros in the wider context of deer biology. Rather than seeing it as a weird, dead-end outlier, Gould showed that Megaloceros represented the known limit of antlers operating as 'honest' signals of stag health and virility. Their size is a predictable outcome of cervine evolution should species develop into large-bodied, open-country specialists.

Were Megaloceros antlers exceptionally large? In an absolute sense, yes, but in a relative sense, no: they're appropriate for a deer of its body size. This graph from Gould's landmark 1974 paper is a little crude - neither shoulder height nor antler length are the best measure of body proportions (masses would be preferable) but this study marked the beginning of more informed and quantified investigations into the evolution, function and significance of giant deer antlers.

Furthermore, ideas that Megaloceros antlers were somehow evolving out of control ignores nuances of their structure and evolution. There's no doubt that - as with all stags - giant deer males were physiologically stressed by antler production. Even assuming development occurred over several months, centimetres of growth were required each day and it's doubted that enough calcium and phosphate could be processed directly from their food fast enough to meet demand (Moen et al. 1999). Megaloceros responded with the standard cervid adaptation of borrowing bony material from its skeleton, and physiological models suggest up to 80% of their available resorbable calcium was required to achieve a full antler spread - about twice that of moose (Moen et al. 1999). Assuming access to suitable plants, these resorbed minerals could be replaced in time to rebuild a strong, durable skeleton for the rut, but osteoporosis remained a risk for stags unable to recapture that material. It's thus unsurprising that Megaloceros antlers are, on average, not quite as solid as those of other deer. All antlers are a blend of compact bone (very dense and heavy, equating to a high mineral cost) and a spongier component (lighter and less dense, so lower mineral cost), and Megaloceros evolved a higher ratio of lower-density bone to better manage the physiological demands of antler growth. Even this was not enough to prevent their antlers from becoming a burden at times, however: average antler size is seen to reduce on occasion throughout their evolutionary history. These responses are the exact opposite of what we'd expect in an orthogenic 'runaway evolution' model, and are evidence of how ordinary selection pressures were keeping Megaloceros anatomy in step with environmental conditions.

Megaloceros antlers were exceptional communication devices, appearing enormous from multiple angles, and especially so from the front. Unlike other deer, Megaloceros would not have to pose to show off its antler palms: they were unmissable however you saw it. Screengrab of a 3D scan by National Museums Scotland that is loads of fun to play with (and forms a useful reference for artists!).

The relatively low mineral value and size of Megaloceros antlers has led to discussions about their function: were they used for visual communication only? Palmate antler regions, which occur today in species like fallow deer and moose, are thought to serve primarily in communication, and it stands to reason that this applied to Megaloceros too. But while living palmate stags use head and neck motion to draw attention to their antlers (Gould 1974; Geist 1999), Megaloceros headgear was rotated in such a way that stags could exhibit their huge palms by simply standing still. This precluded the need to wave 40 kg of bone around for days on end, which was probably terrific news for the neck and head tissues of Megaloceros stags (Gould 1974). This said, Megaloceros skulls, necks and shoulders were heavily augmented to support their antlers (the skull roof was 30 mm thick! - Lister 1994) and it's probable that they were capable of moving them around with speed and precision, perhaps even violently. Although some authors (Gould 1974) have regarded Megaloceros stags as incapable of wrestling with one another, studies of their antler histology and stress distribution suggest such matches were possible (Lister 1994; Klinkhamer et al. 2019). The effects of clashing antlers have yet to be modelled - this being fraught with variables that are difficult to estimate at present - but Megaloceros antlers perform well under pushing and twisting regimes provided the stresses are primarily experienced in their proximal regions. They appear to have been especially resistant to rotational forces, and we might imagine stags locking their antlers together at close range, faces low to the ground, shoving and twisting each other to the floor (Lister 1994). Their antlers are so long that forces incurred at their tips might have been amplified to potentially bone-snapping levels however, so it's possible Megaloceros may have avoided especially violent, unpredictable fighting styles. Again, this is consistent with trends seen across deer: as stags become larger, and species become more sociable, their fights generally become more ritualised and lower risk. Megaloceros may be the ultimate example of this correlation (Geist 1999).

Life appearance

Charles Knight's 1906 take on Megaloceros in a traditional, red deer/wapiti form. Cervus-like reconstructions have typified this genus since at least the 1850s, when Waterhouse Hawkins reconstructed Megaloceros for the Crystal Palace Park. Image from Wikimedia Commons, in public domain.
Although no soft-tissue Megaloceros fossils are known, a reasonable amount has been inferred about its life appearance through predictions of ecology and interpretation of Palaeolithic art (e.g. Lister 1994; Geist 1999; Guthrie 2006; also see Darren Naish's summary at Tetrapod Zoology). Although these differ in some details, a consensus has formed around some aspects, including the presence of a large shoulder hump (a diagnostic feature of Megaloceros cave art) and a fallow deer-like prominent laryngeal region. These features seem common to both males and females, implying a use beyond male competition and sexual signalling. It's tempting to link large larynxes with deep, guttural calling, akin to that produced by Dama, and with both genders sporting them, perhaps Megaloceros was an especially vociferous deer? Palaeolithic artists depicted the shoulder hump as large and bulging in both genders, so it probably does not merely represent the augmented, antler-carrying shoulder and neck muscles of stags. Geist (1999) speculated that it represents a fatty hump: an energy reserve for periods of seasonal stress that would not, by being concentrated in one place, cause the same overheating risk as distributing fatty tissues across the body, as occurs in some reindeer. Zebu cattle, white rhinos, bison and buffalo are similarly adapted running species (Geist 1999). There is no artistic evidence of a mane or bell, as seen in some other large cervids, although some Palaeolithic art hints at longer hairs on the hump.

A take on Megaloceros body patterning based on a very literalistic reading of certain Palaeolithic artworks - note the stripe running from the shoulder to hindlimb on the stag. The shoulder hump on the doe should probably be larger.
Predictions about colouration have also been presented, although no-one should be under the illusion that we have a robust idea of colours and patterns in this species. Our most informative insights into this are cave art showing dark colours at the shoulder hump which taper into dark stripes extending across the body and neck (diagonally across the flank from the shoulder to the knee region, as well as vertically across the neck-torso junction). A further dark stripe or band is recorded at the top of the neck, possibly in relation to the enlarged laryngeal region. Another vertical stripe may have framed a rump patch - a feature consistent with Dama. How we interpret these drawings isn't 100% clear, with the full-body restorations of Geist (1999) and Guthrie (2006) differing in some details. Geist regarded Megaloceros as pale all over with a conspicuously dark transverse body stripe and dorsal midline, while Guthrie portrayed more regionalised colouration with darker hindquarters grading into paler hues around the shoulder, framed by prominent stripes. Lister (1994) was more conservative, merely noting the likelihood of the dark shoulder hump.

One of the most informative Palaeolithic artworks of Megaloceros is featured in the Megaloceros Gallery in France's famous Chauvet Cave. Unlike other examples, it seems to show shaded areas that could reflect darker body regions, similar to the banded Coelodonta depicted on the opposite wall. A second Megaloceros is featured a little further in, also showing a dark neck band. Alas, other Megaloceros panel drawings lack similar details, precluding determination of their reliability. Cropped from Google Arts and Culture - make sure to check out the Megaloceros Gallery in 3D!

The challenges of interpreting Megaloceros colouration from cave art alone are many. Opinions differ on how many panel drawings we have - perhaps up to 40, but of which maybe only 15 or 16 are confidently identified as M. giganteus. Most are body outlines without details of colours or patterns and, to my knowledge, only 4-5 pieces feature those dark humps and body stripes. This is enough to assume that they are recording something about Megaloceros life appearance, but what, exactly, might that be? Are the lines radiating from the shoulder humps stripes, or something else, like the boundaries between colour regions? Are those simpler 'body outline' artworks actually depicting Megaloceros with uniform colouration? Do these differences reflect seasonal or regional colour variation, or else varying artistic approaches? The question of how literally we should take Palaeolithic depictions is not new, and it doesn't have a straightforward answer (Guthrie 2006).

Select Palaeolithic art of Megaloceros as interpreted by Guthrie (2006) - arrows indicate features of interest such as shoulder humps, expanded laryngeal regions etc. Note the absence of obvious colour and pattern data in most depictions - what significance, if any, might that have? This compilation of Guthrie's work is swiped from Tetrapod Zoology.
But while Palaeolithic art does not give us a clear answer on Megaloceros life appearance, ideas of it being a highly-specialised 'extreme' deer species help us explore likely colour schemes a little further. We can look to deer and other ungulates that match Megaloceros in behaviour and ecology to make some broad-brush predictions of what an especially large, well-ornamented, communicative, open-habitat and cursorial deer species may have looked like. Geist (1999) calculated that an energetic 600 kg deer living in an open, shadeless environment could be sensitive to solar insolation, especially during sustained ruts and runs, and predicted light-colours across the body to reflect sunlight and avoid overheating. Thoughts that Megaloceros may have been subject to heat stress are not baseless: some readers may recall that moose are highly prone to overheating, even in winter, despite living in shadier settings and having a higher surface area:volume ratio than Megaloceros (Dussault et al. 2004; van Beest and Milner 2013). Geist also predicted a rump patch - a feature seen in Dama as well as possibly one piece of ancient art - as this feature has convergently developed in a number of social, cursorial ungulates. The possibility of dark body regions or stripes has some functional explanation, too, with Geist (1999), Guthrie (2006) and Lister (1994) all noting the utility of dark patterning in ornamented deer, and Geist suggesting this would be especially effective in an open environment against a light sky. It's difficult to know how correct these predictions are of course, but I admire the fact that they consider the unique lifestyle of Megaloceros rather than, as has been palaeoart tradition until recently, merely dressing Megaloceros as a red deer or wapiti.

Demise and extinction

Charles Knight's depiction of a Megaloceros stag being hunted by early humans. Did we contribute to the extinction of giant deer? Possibly, although evidence for our interaction with this species is uncommon. From Knight (1949).
It seems remiss to have written all this about Megaloceros without mentioning modern ideas of why it is no longer with us. This remains a topic of ongoing research, and a synthesis provided by Lister and Stewart (2019) gives a good overview of the current state of the debate. Our vast sample size and detailed understanding of Quaternary sediments means that the history of Megaloceros can be read in detail, providing a substantial (though still developing) insight into the major factors that contributed to its demise. Once thought to have been an exclusively Pleistocene species that held out longest in northwest Europe, recent discoveries and dating of Megaloceros sites in Siberia have not only confirmed its survival well into the Holocene, but shown that Asia held Megaloceros populations longer than anywhere else - as recently as 7,600 years ago.

With concepts that Megaloceros antlers were some sort of doomed anatomy pulling giant deer to extinction firmly disregarded, more sensible discussions about Megaloceros demise consider environmental change, their high productivity demands, and human hunting likely extinction causes. Generally, it seems that local Megaloceros extinctions coincide with habitats changing from parkland habitat to wooded or tundra-like settings, both of which heralded reduction in optimal foraging conditions as well as losses of plants rich in calcium. Megaloceros populations in those areas may have thus dwindled or been forced to find more suitable places to live. There is evidence that Megaloceros anatomy responded somewhat to nutrient-stressed conditions, but ideas that dwarfed or otherwise stunted populations developed in islands or other refugia have not been bourne out: late-surviving Megaloceros were only something like 2% smaller than earlier specimens. The lack of more dramatic body size reduction may be related to predation pressures to stay large and fast, especially as new data shows that humans co-existed with Megaloceros for longer than was once thought. Evidence for our hunting giant deer is relatively rare so we may not have been a major factor in their extinction, but as suitable habitats reduced, even low hunting frequencies may have been enough to push Megaloceros over the edge.

As is always the case with a well-studied species, there's much more we could say about Megaloceros. We are still learning about its palaeobiogeography, behaviour, evolution and extinction from new discoveries and detailed examination of archived specimens. Digging into the literature on these animals has proven genuinely fascinating. As someone used to studying Mesozoic reptiles, the amount of information and insight we have on this species is almost overwhelming, and it seems that understanding its palaeobiology is challenged more by having to consider so much data rather than, as I'm more accustomed to, having too little information to draw meaningful conclusions. And yet, despite an excellent fossil record and centuries of study, some major aspects of its biology remain poorly understood or contested. Food for thought, indeed, about our capacity to interpret fossil tetrapods from more typical palaeontological datasets.

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