The Irish Elk stood about tall at the shoulders, and it had the largest antlers of any known cervid (a maximum of from tip to tip and weighing up to ). A significant collection of M. giganteus skeletons can be found at the Natural History Museum in Dublin.
The size of Irish Elk antlers is distinctive, and several theories have arisen as to their evolution. One theory was that their antlers, under constant and strong sexual selection, increased in size because males were using them in combat for access to females; it was also suggested that they eventually became so unwieldy that the Irish Elk could not carry on the normal business of life and so became extinct. However, it was not until Stephen Jay Gould's important 1974 essay on Megaloceros that this theory was tested rigorously.
Gould demonstrated that for deer in general, species with larger body size have antlers that are more than proportionately larger, a consequence of allometry, or differential growth rate of body size and antler size during development. In fact, Irish Elk had antlers of just the size one would predict from their body size. Note this does not mean that sexual selection played no part in maintaining large antler size, only that the antlers of the species' ancestors were already large to begin with. Indeed, Gould concluded that the large antler size and their position on the skull was very much maintained by sexual selection: They were morphologically ill-suited for combat between males, but their position was ideal to present them to intimidate rivals or impress females. Unlike other deer, M. giganteus did not even have to turn its head to present the antlers to best effect, but could accomplish this by simply looking straight ahead.
More recent research pointed out that high amounts of calcium and phosphate compounds are required to form antlers, and therefore large quantities of these minerals are required for the massive structures of the Irish Elk. The males (and male deer in general) met this requirement partly from their bones, replenishing them from foodplants after the antlers were grown or reclaiming the nutrients from discarded antlers (as has been observed in extant deer). Thus, in the antler growth phase, male deer from Ireland were suffering from a condition similar to osteoporosis. When the climate changed at the end of the last Ice Age, the vegetation in the animal's habitat also changed towards species that presumably could not deliver sufficient amounts of the required minerals, at least in the western part of its range. The most recent specimen of M. giganteus in northern Siberia, dated to 7,700 years ago - well after the end of the last Ice Age -, shows no sign of nutrient stress. This is actually quite unsurprising, as they come from a region with continental climate where the proposed vegetation changes had not (yet) occurred.
In conclusion, it is easy to advance a number of hypotheses regarding the disappearance of the more localized populations of this species. The situation is less clear regarding the final demise of the Irish Elk in continental Eurasia east of the Urals however. Stuart et al. (2004) tentatively suggest that a combination of human presence along rivers and slow decrease in habitat quality in upland presented the last Irish Elk with the choice of good habitat but considerable hunting pressure, or general absence of humans in suboptimal habitat.