When antlers grew too large.

With antlers that can spread 2 meters, moose are the giants of the modern deer universe, but these big-nosed browsers pale in comparison with the extinct Irish elk. An ice age inhabitant of Europe and Asia, the Irish elk evolved antlers reaching 3 meters across and weighing 40 kilograms--too big for the animals' own good, according to a new analysis of antler growth.

"People have always been interested in the Irish elk because they had the largest antlers of any deer. And they have always guessed what limited antler growth and why did they go extinct when the other deer didn't," says Ron A. Moen of the University of Minnesota-Duluth.

Moen and his colleagues used physiological information about moose to analyze antler growth in Irish elk. They calculate that males deposited more than 60 grams of calcium and 30 g of phosphorus daily into the growing antlers during midsummer. Diet would have provided most of these nutrients, but not all. The remainder would have been leached out of the animal's skeleton during summer and replaced later, the researchers report in the February EVOLUTIONARY ECOLOGY RESEARCH.

Some of the largest and last Irish elk lived in Ireland itself. They went extinct there during the final phase of the ice age, a 1,000-year-long cold snap called the Younger Dryas. At that time, glaciers covered much of the island, and tundra replaced forests.

According to the physiological model constructed by Moen and his coworkers, Irish elk ran into nutritional problems during the Younger Dryas. Their diet would have provided enough calcium and protein but not enough phosphorus and total calories. The males were able to grow big antlers during summer and thus could mate, but ones with the largest racks would have suffered later in the year when they could not restore their bone density or their fat reserves, says Moen.

Paleontologists have previously suggested that food limitations may have harmed Irish elk, but the new study is the first to quantify the nutritional requirements of the extinct species, says Adrian M. Lister, a paleontologist at University College London who studies Irish elk.