NEXT time you reach for the mineral supplements, spare a thought for the
Irish elk. If it hadn’t been so dependent on minerals, this mighty beast might
still be around today. As it was, its expensive tastes meant that the Irish elk,
famed for its enormous antlers, died out over 10 000 years ago.
The memory of Megaloceros giganteus lives on today in college
textbooks, where it appears as a cautionary tale of runaway sexual selection.
Females, so the theory goes, drove the males to evolve unfeasibly large antlers
by only mating with the most well-endowed stags. Exactly how this led to their
demise is hotly contested. Explanations range from widespread drownings in peat
bogs due to excessive antler weight, to mass epilepsy caused by huge rushes of
blood to the head when males shed the velvet on their racks. But the latest
research indicates that malnutrition was the key to the great beasts’ downfall.
They simply couldn’t keep up with the mineral needs of their giant antlers.
To add insult to injury, Irish elk are completely misnamed. For a start,
they’re not elks. Their closest living relatives are the dainty white-spotted
fallow deer. True elk—or moose, as they are known in the US—are only
distant cousins. Nor are they uniquely Irish—Megaloceros once
roamed across much of Eurasia from Lake Baikal to southern Spain. Its bones have
even been found in the clays under London’s Trafalgar Square. Fossils from
mainland Europe date from 400 000 to around 11 300 years ago when, experts
believe, Palaeolithic hunters wiped it out. But the last remaining population,
in Ireland, could not have suffered the same fate. Humans did not reach the
Emerald Isle until some 1500 years after the giant deer became extinct. What
strange twist of fortune tipped these great beasts over the edge?
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The key to the mystery lies in the size of the Irish elk’s antlers. Then, as
today, antlers would have served to impress potential mates as well as to
intimidate rivals. “A male with large antlers is showing that it has a
hereditary endowment for superior foraging and is able to divert nutrients from
maintenance to antler production,” says Valerius Geist from the University of
Calgary, Alberta. By mating with such stags, females would be likely to produce
large, fast-growing offspring with the best chance of outrunning predators on
open plains. “In females such genes would serve milk production and the
development of the fetal skeleton,” adds Geist. So the benefits would also pass
down to the next generation.
Although this explains how the deer lumbered themselves with huge antlers, it
doesn’t show how the appendages led to their downfall. “I’ve been impressed by
the Irish elk ever since I was an undergraduate,” says Ron Moen from the
University of Minnesota, Duluth. “I was sure mating-based explanations did not
tell the whole story of its extinction.” So, with university colleagues John
Pastor and Yosef Cohen from the University of Minnesota, Twin Cities, Moen began
to build up a picture of the ancient deer’s metabolic needs and the mineral
content of the plants it lived on. “From my experience with moose, I felt
minerals had to be involved in the extinction story,” says Moen.
Standing 2 metres tall at the shoulder and weighing 600 kilograms, an adult
Megaloceros stag was the size of the largest male moose living today.
Yet the Irish elk’s antlers were almost a third heavier than those of the modern
moose. An adult male’s rack could weigh 40 kilograms and span over 3.5
metres—as big as those of the largest deer ever, the 1200-kilogram
broad-fronted elk which lived in Eurasia around the time that
Megaloceros evolved. Using a computer model, the researchers simulated the
Irish elk’s metabolism and growth patterns. One of their first discoveries was
that its antlers were actually about as large as one would expect for a deer of
that size. “It is the moose that have smaller ones,” says Moen.
But as the team delved deeper, they realised that the physiological demands
of growing such huge antlers were almost as stupendous as the antlers
themselves. Stags grow and shed their antlers every year—a costly habit,
because antlers are made of bone and are among the fastest growing vertebrate
tissues. Although antler size normally scales with a deer’s body size, the
physiological demands of building antlers increases with its square. An adult
male moose puts half as much energy into growing antlers as it deposits in
energy reserves in the form of stored fat and protein. But Moen’s calculations
indicate that in Irish elk this proportion must have been as much as
three-quarters. “Megaloceros was putting some 20 per cent of its total
summer energy budget into its antlers,” says Moen. “This figure is without
parallel among today’s deer.”
The need for minerals is also huge. Moen calculated that a 40-kilogram set of
antlers would have contained nearly 20 kilograms of mineral elements, including
7.6 kilograms of calcium and 3.8 kilograms of phosphorus. By comparison, the
skeleton of a 70-kilogram human contains less than half that amount of
calcium.
During the 60-day period of antler mineralisation, a male
Megaloceros would have had to lay down at least 60 grams of calcium and
more than 30 grams of phosphorus every day. This means that 1 in every 3 calcium
atoms consumed would go to its antlers. Studies of the digestive physiology of
deer led Moen and his colleagues to conclude that even vegetation that was
jam-packed with minerals wouldn’t have been up to the job. So, around 10 per
cent of these minerals had to be absorbed from the deer’s own skeleton, to be
replenished after the breeding season. “It is very likely that this huge animal
had a survival strategy partly based on seasonal osteoporosis,” says Moen.
Achilles’ heel
Dependency on minerals was, it seems, the Irish elk’s Achilles’ heel. Its
seasonal buy-now-pay-later system could operate effectively only in an
environment where plants high in calcium and phosphorus were abundant. The first
elk immigrants to Ireland would have munched on a mineral-rich mosaic of willow,
juniper, birch, crowberry and nutritious grasses. But, about 5000 years later,
fate dealt Megaloceros a bitter blow from which it never recovered.
Between 13 000 and 11 000 years ago, the Irish climate was similar to
today’s. Then, as now, this clement situation was largely due to an upwelling of
warm tropical water between Ireland and Iceland. “But around 10 900 years ago
the North American ice sheet dumped a huge volume of cold, low-saline water into
the North Atlantic,” says Fraser Mitchell from Trinity College, Dublin. As a
result, the land temperature in Ireland dropped precipitously, by between 7 and
12 °C. “Ireland went from temperate to glacial temperatures in little over
100 years,” says Mitchell. This climatic nose-dive was the onset of the
Nahanagan stadial, a little ice age that affected the whole of Ireland for about
500 years. Unable to migrate to warmer climes, the world’s last population of
Megaloceroshad no choice but to tough it out.
Pollen records from this time show a rapid transition from a lush,
shrub-dotted grassland to tough, cold-dwarfed tundra. The new vegetation was
dominated by low-growing herbs like peppercress, knotweed and dwarf brassica,
which would have grown less quickly and decomposed more slowly than their
predecessors. In addition, the shorter spring would have given the deer less
prime-time foraging on juicy new leaves and shoots. These were lean times for a
giant vegetarian. An adult Megaloceros stag needed about 40 kilograms of
forage daily to keep hoof and hide together.
To make matters worse, the new vegetation would have contained much less
calcium and phosphorus than the old. Moen’s calculations of mineral flow for the
tundra communities are ominous: the cold climate plants probably contained
barely one-third of the critical minerals of the plants they replaced. This was
simply not enough to support an antler-growing mega-stag. Death rates would have
risen sharply, especially during the winter when males, exhausted from the rut,
were at their lowest ebb.
Skeletal evidence shows that, faced with a colder climate and poorer forage,
the deer became about 10 per cent smaller in just a couple of generations. But
even this downsizing was not enough to save them. “To survive, they probably
should have gotten smaller still, but I think sexual selection pressures for
larger antlers opposed selection pressures from the environmental changes,” says
Moen. “This would have placed tremendous stress on the mineral metabolism of the
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But, in a final twist, it transpires that even if climate change hadn’t
sealed their fate, the deer’s own appetites would probably have finished them
off. Over the 5000 years when Megaloceros thrived in Ireland, the
original vegetation, dominated by willow, juniper, birch and crowberry,
gradually gave way to a grassland where these mineral-rich shrubs were
increasingly rare. Mitchell and Richard Bradshaw from the Danish Geological
Survey lay the blame squarely on the deer’s jaws.
They hold that the giant deer’s grazing patterns caused a fundamental change
in the flora of prehistoric Ireland, in much the same way that elephants
influence plant communities in Africa today. Their evidence comes from microwear
patterns on the ancient deer’s teeth. Diets dominated by woody shrubs leave
different scratch patterns on the enamel to those dominated by grasses. Bradshaw
and Mitchell’s analysis shows scratches characteristic of grass becoming more
frequent over time. This suggests Irish elks may have chomped their way to
vulnerability, so that they were already teetering on the brink of mineral
deficiency when climate change tipped them towards oblivion.
By this time, seasonal osteoporosis had probably become a permanent state as
stags sought to subsidise the environment’s meagre minerals with stores from
their own bodies. The end for the Irish elk can hardly have been splendid:
enfeebled females would have borne progressively weaker offspring and the last
male would have been starved, stressed and spavined when, 10 600 years ago, his
great antlers finally hit the ground.
EXACTLY what the Irish elk’s great antlers were used for is still hotly
debated. Valerius Geist from the University of Calgary, Alberta, believes they
were more for show than fighting. “Irish elk stags were most likely lek
breeders,” he says. “One can imagine a group of stags on a knoll flashing their
huge antlers. It would have attracted females from many miles to their communal
breeding ground.” Anthony Barnosky from the University of California at Berkeley
agrees. “Combat with 40-kilogram antlers would have involved tremendous
mechanical stresses in the neck muscles,” he says. “I believe the antlers were
used for display. They were too fragile for fighting.”
Others don’t buy this interpretation. “From all we know about deer, Irish elk
personify the type with polygamous, harem-holding males and an intensely
competitive rut,” says Dale Guthrie, an emeritus professor from the University
of Alaska, Fairbanks. He points out that the neck vertebrae between the
shoulders of Irish elk stags were specially strengthened and had huge tendons
attached that would have acted both as shock absorbers and to help raise and
lower the heavy head. What’s more, analysis of Megaloceros antlers
reveals hydroxyapatite crystals oriented along stress lines. “This indicates
that stags fought to establish dominance,” says Ron Moen from the University of
Minnesota, Duluth.
Either way, late summer would have been a testing time for the stags as they
vied for does. For most of the year, however, male and female Irish elk would
probably have gone their separate ways, as red deer do today. This idea
certainly fits with findings at Ballybetagh Bog, 15 kilometres south of Dublin.
From the mid-17th century onwards, it has been the prime source of Irish elk
remains. Around 100 or so Megaloceros skeletons have been excavated
from the site. Not one of them is female.
What is now a lens of peat 900 metres long by 100 metres wide, was once a
shallow, sheltered, shrub-fringed lake, between 12 300 and 10 600 years ago. In
the mid-1980s, before the site was converted into a cattle shed and lost to
science forever, an intrigued Barnosky excavated a portion of the bog for more
bones. His analysis of how the bones lay relative to each other revealed how and
where the stags had died. He concluded that most died among lakeside thickets
and their bodies or bones then either slowly slipped into the waters or were
accidentally kicked there by other Irish elk.
But what were these giant deer doing there, and why were they all male?
Barnosky’s analysis of the animals’ antlers and tooth size revealed that most
were either very old or very young adults. None were in their reproductive
prime. Because the antlers had not been shed, Barnosky deduced the animals had
died in winter. Again, minerals seem to be the key. Previous studies had found
fragments of willow, which is high in calcium and phosphorus, on teeth from the
Ballybetagh skulls. This led Barnosky to believe that the lake was a winter
congregation point for bachelor deer, attracted by the vegetation that might
help them balance their mineral deficits.
“Those males that died were exhausted from the late summer rut and simply did
not have the stored resources to make it,” says Barnosky. Such winterkill is
still the commonest cause of male mortality in deer with intensely competitive
breeding systems.
On the horns of a dilemma
- Further reading: The palaeoecological approach to reconstructing former
grazing-vegetation interactions by Richard Bradshaw and Frank Mitchell,
Forest Ecology and Management, vol 120, p 3 (1999) - Antler growth and extinction of Irish elk by Ron Moen and others,
Evolutionary Ecology Research, vol 1, p 235 (1999) - The evolution of the giant deer, Megaloceros giganteus (Blumenbach)
by Adrian Lister, Zoological Journal of the Linnean Society, vol 112, p
65 (1994)