Giving Megaloceros the hump

Among the most powerfully haunting images of modern Irish art is a study of the giant Irish deer, Megaloceros giganteus, painted by Barrie Cooke in 1983. A great spiked shield of antlers flares out of the shadows, and the deer's intense presence condenses below them in the splash of sun across a russet hide. The beast is not so much the monarch of the glen as its animistic god.

Cooke's painting puts incandescent flesh on the lanky and intimidating pair of skeletons that peer over the heads of visitors to the Natural History Museum in Dublin. It also made a brilliant bookjacket for a recent cultural history of nature in Ireland. But now, it seems, there's something zoologically amiss with this Megaloceros. It's pagan, it's powerful, but it doesn't have a hump.

The notion that the giant deer possessed a prominent, boldly-coloured hump is comparatively new, and owes everything to paintings left on cave walls in France by the Palaeolithic artists of more than 30,000 years ago. It was first found depicted in the cave of Cougnac, in south-central France, in the 1950s, and finally clinched in two portraits discovered at Chauvet in 1994.

The "Irish elk" (to use the common description) was, as Stephen Jay Gould likes to point out in his periodic essays on the animal, "neither exclusively Irish, nor an elk." In the later Ice Age, it lived widely if sparsely throughout Europe and northern Asia, wandering after grasslands as they shifted with changes in climate. It was the largest deer that ever lived: more than three metres from the hooves to the top of the antlers.

But in body-size, at least, the deer was a quite a close match for the American moose, and some early scientists believed it might still survive in the forest wilderness of Canada. In Europe the moose is called elk; hence the initial confusion. But the identification with Ireland came simply because this island produced so many fossil specimens so early on in natural history. Bones and antlers were superbly preserved in lake sediments beneath the bogs.

The most famous site for them is Ballybetagh Bog, near Kiltiernan in Co Dublin, where 19th century collectors made quite an industry from assembling skeletons and selling them to museums throughout the world. Even in the last half-century about 40 finds have been investigated there, and one bone has been radiocarbon dated to 10,600 years ago. This is the latest-known record of a species which began roaming Europe at least 400,000 years before.

In Ireland it reached its peak in the grassland phase which followed the first big thawing from the Ice Age, only to be struck down - "like Lucifer in full flight", as the late Frank Mitchell wrote - by a sudden, if temporary, reversal of the warming. The image of the giant stags mired in frozen lakes, huge antlers weighed down with ice, belongs to the iconography of extinction, not all of it accurate. The bare-skulled females died too, and from the same hunger.

The deer were long gone by the time the first post-glacial hunters arrived in Ireland, and Frank Mitchell was one of many scientists who wondered how far Megaloceros could ever have interacted with people, even in continental Europe. As he pointed out 50 years ago: "There are no representations of the Giant Deer in Palaeolithic cave art."

Stephen Jay Gould quotes this in the latest collection of the marvellous essays he writes for America's Natural History magazine (Leonardo's Mountain of Clams and the Diet of Worms, published by Jonathan Cape at £18 in the UK). The Harvard professor is ecstatic about the newly-discovered paintings, chiefly because they provide "a wondrous rescue . . . from history's great shredding machine." But the new hump on Megaloceros also offers grist to his enthusiasm for evolutionary argument.

Gould first came to Ireland in 1971, "yardstick in hand" to measure the skulls and antlers of Irish elks. In the Natural History Museum, then in its pre-restoration gloom, he sat in thick grime on top of the cabinets to reach the deer-heads mounted on the walls. He went on to measure more of them in the trophy rooms of country manors and even one, with cigar in its jaws, in the banqueting hall of Bunratty.

He was assembling evidence to test arguments about the significance of such monstrous and unwieldy antlers. For anti-Darwinians their inefficiency made a telling case against the supposed workings of natural selection. Darwinians countered that proportionately larger antlers went with bigger body size and were not adaptive in themselves.

Gould's measurements backed this idea, but he shied away from the thought that the 30-kilo antlers, so clumsy as weapons and so costly to grow from scratch each year, had lost any primary function. He argued that the broadpalmed antlers finally made sense as devices for ritualised display, avoiding actual battles between stags and suitably impressing the females.

In his latest essay, he admits to believing he was wrong in making this suggestion. Work by other scientists has convinced him that giant deer "almost surely used their antlers in actual combat". But he still sees them as a major element in posturing and display, especially in the "parallel walk" between challenging stags (which can be seen among the red deer of Killarney).

This is also where the hump may come in. From the cave paintings, it was far more than the ridge which developed above the spines on the deer's dorsal vertebrae, helping to hold up its hugely-burdened head. It was a big, exaggerated, boldly-coloured bump, joined to radiating lines on the body: just the thing, perhaps, to help intimidate a rival male.

For Gould, it provides a splendid illustration of a well-established evolutionary principle - the way in which physical features, arising for one function, can be "co-opted" and developed by natural selection for another, specialised role. But mostly he loves the hump because, as soft tissue, it could never have survived as a fossil and its existence could never have been deduced. But for the human creative urge, we would never have known about it.