Planet Earth

The Allure of Big Antlers

The LoomBy Carl ZimmerSep 3, 2008 5:09 AM


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I can't help finding the Irish elk wonderfully ridiculous. The reaction probably has something to do with the fact that we are all familiar today with deer, moose and other animals that look for the most part like the Irish elk, except for that extravagant rack. Irish elk grew the biggest antlers ever recorded, stretching over ten feet across and weighing about 90 pounds. Of course, for the people who lived alongside the Irish elk in Europe and Asia before its extinction 7000 years ago, it probably didn't seem terribly ridiculous at all--no more ridiculous than an orangutan or a river dolphin look to us today. And the way things are going, orangutans and river dolphins may not be long for this world. After they're gone, people will look back at pictures of them the way we look at pictures of the Irish elk, and imagine they were just made up by a bored zoologist. The antlers of the Irish elk have long obsessed zoologists, bored or otherwise. In the late 1800s, many scientists believed that evolution proceeded in straight lines, with too much momentum to be steered off course by natural selection. The Irish elk was a prime example of this straight-line evolution known as orthogenesis. Evolving from small ancestors, they grew to be giants. Their antlers grew larger still. While they may have been useful at first for keeping away predators and such, orthogenesis could not be stopped. The poor Irish elk ended up burdened with massive headgear that got them stuck in thickets and drove them extinct. (For more, see this essay by Stephen Jay Gould.) Orthogenesis fell apart in the early 1900s, as scientists began to learn how genes make natural selection possible. The Irish elk was not a victim of some mysterious inner momentum. Its antlers simply evolved to be bigger at a faster rate than its body. A big body helped Irish elk survive, and the bigger antlers were just an unavoidable--but manageable--inconvenience. When Gould wrote his essay in the mid-1970s, scientists were just beginning to recognize that antlers had important functions of their own beyond just warding off a wolf. They were signals. Big antlers in living deer are a sign of a high-quality male--a male that can fight off other males for territory, and that may be able to attract females. Today there's a lot more evidence supporting the idea that antlers are driven by sexual selection, not natural selection or orthogenesis. The funny thing about this story is that hypotheses about the extinction of Irish elk have come back to those doomed antlers again. Playing the role of orthogenesis now is the relentless power of sexual selection. The antlers got bigger and bigger as the Irish elk stags advertised their quality. Each year they lost these monstrous growths and regenerated them the following year. As the shrubby woodlands in which the Irish elk lived began to shrink at the end of the last Ice Age, the animals could no longer get enough food to fuel those giant antlers, and they died. Sexually selected suicide, in other words. This always seemed a bit odd to me, but I didn't see much research from scientists that took a critical look at the idea. Recently, though, a University of Florida graduate student named Cedric O'Driscoll Worman noticed this picture of an Irish elk proudly displayed on the front page of my web site. It turns out he is a devotee of Megaloceros giganteus as well, and he sent me his new paper with Tristan Kimbrell in the journal Oikos. There, they take aim at the notion of the doomed antlers. Worman and Kimbrell argue that even with the advantages that some with a fancy set of antlers, the Irish elk could have easily shrunk them if they didn't have enough food. They point to red deer that have adapted to living on predator-free islands, where they shrank to half the size of their mainland ancestors. Their antlers shrank far more. Antlers can get small even without an evolutionary shift--if red deer suffer overcrowding and run low on food, their antlers can lose almost a third their weight. Feeding stags extra food can double the size of their antlers. If an Irish elk stag responded to a lack of food the way red deer do today, their antlers would shrink down to the size of moose antlers or less. Obviously moose have managed to survive with antlers that size, so Irish elk should have as well.Worman and Kimbrell argue that even if stags with big antlers died off, there would have been plenty of males with smaller antlers to take their place. And in a species where a few males probably fathered most of the offspring each generation, a lot of stags would have to die off before the species as a whole began to suffer. Hunters today show this to be the case--populations of deer can handle a lot of killed stags. But the does are another matter. And it's the does that Worman and Kimbrel think we should think about when we think about why the Irish elk are gone. The long, graceful legs of Irish elk suggest that they were fast runners that could cover long distances. The females needed a lot of energy during pregnancy and nursing, so their offspring would be ready to run with the herd from the get-go. Fast running may have been the only way to survive the dire wolves and other predators that hunted Irish elk. Other deer could respond to the limited food on small islands by getting small. But the predators Irish elk faced did not allow that strategy. The females were stuck, trying to produce big offspring with less food. As a result, Worman and Kimbrel propose, their numbers dwindled. In some places, humans may have hastened their death with hunting, but the Irish elk vanished from Ireland long before humans arrived. If Worman and Kimbrel are right, their disappearance had little to do with their antlers. Those giant signals may actually be a beautiful distraction from the real cause of their extinction. Not sexual suicide, but just the struggle to produce healthy young.

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