Measuring 3.5 metres from tip to tip, the antlers of the extinct Irish elk (Megaloceros giganteus) were 鈥� but it is no longer completely clear why they grew to such an astonishing size.
About 50 years ago, influential biologist Stephen Jay Gould suggested a simple explanation for the Irish elk鈥檚 enormous headgear. He compared shoulder height and antler length across several species of 鈥� a group including the Irish elk 鈥� and concluded that antler size and body size are closely linked. More specifically, Gould proposed that when deer evolve larger bodies, the antlers males grow evolve to be larger too, but at a faster rate. Biologists call this 鈥溾�� and Gould said it predicted that the Irish elk, which stood 2 metres tall at the shoulder, .
But at the University of Oslo in Norway and his colleagues say it isn鈥檛 quite that simple. 鈥淪ome of the data and methods Gould used were really poor by standards of today,鈥� says Hansen. For instance, some of Gould鈥檚 figures on deer shoulder height seem to be based on estimates given in a . Hansen says it is also simplistic to use antler length as a measure of their size: antler volume is better.
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Hansen and his colleagues have now run their own analyses using better data and methods. They still found Gould鈥檚 positive allometry signal across the cervine deer, but it predicted the Irish elk should have an average antler volume of 17.5 litres. In reality, the ancient deer鈥檚 antlers were about 150 per cent as large, with an average volume of 25.5 litres.
The result came as a surprise, says Hansen. 鈥淚 would have loved to be able to confirm and strengthen Gould鈥檚 analysis,鈥� he says. 鈥淏ut it鈥檚 not quite how it turned out.鈥�
Other researchers are not willing to give up on Gould鈥檚 model yet, however. 鈥淓ven in the original Gould study, individual species do not fall exactly on the 鈥榩erfect鈥� allometric line but scatter around it,鈥� says at the Natural History Museum in London.
Hansen鈥檚 view is similar. 鈥淚 wouldn鈥檛 say Gould was entirely wrong,鈥� he says. 鈥淎llometry still plays an important role.鈥� But the new results indicate that additional factors drove the evolution of the Irish elk鈥檚 enormous antlers.
Unfortunately, it is challenging to definitively identify those factors because Irish elk has been extinct for about 7700 years ago in Siberia, which means we can only study their skeletons 鈥� many of which have been discovered in Irish peat bogs. For instance, males with big antlers probably benefitted during the breeding season, but we don鈥檛 know how because we can鈥檛 study the Irish elk鈥檚 behaviour. 鈥淚n some species, it鈥檚 important for males to have big antlers to fight and intimidate other males,鈥� says Hansen. 鈥淚n other species, large antlers are more important for displaying to attract females.鈥�
Nutritious diets could have been important too, says Hansen, to help males find the energy to grow their antlers every year 鈥� and habitat probably played a part, with males presumably living on open landscapes so that they didn鈥檛 continually tangle their antlers among tree branches.
Hansen speculates that this last point may explain another antler mystery. The modern true elk (Alces alces) 鈥� known as the moose in North American English 鈥� is similar in size to the ancient Irish elk, but its antlers are much smaller. 鈥淧erhaps it is much more of a forest animal,鈥� he says.
Evolutionary Biology
