Antler growth and extinction of Irish elk

Adult male Irish elk (Megaloceros giganteus) grew the largest antlers of any extinct or extant cervid. These antlers have often been implicated in the extinction of the Irish elk, although the effects of antler growth on Irish elk physiology have not been analysed quantitatively. We used a simulation model of energy and mineral metabolism to compare nutritional requirement for antler growth in Irish elk and moose (Alces alces), the largest extant cervid. The model simulates intake, metabolism, deposition, and excretion of energy, nitrogen, ash calcium and phosphorus with mass balance for each of these nutrients on a daily time step. Predicted energy requirements for antler growth by moose are half as large as energy requirements for summer fat and protein deposition. In contrast, the predicted energy requirements for antler growth by Irish elk were about 75% as large as energy requirements for summer fat and protein deposition. Irish elk antlers weighing 40 kg at the end of velvet shedding would have contained 2.1 kg nitrogen 7.6 kg calcium and 3.8 kg phosphorus. The nitrogen requirements for antler growth were met by forage intake. The model predicts that, to grow 40 kg antlers in a 150 day period, more than 60 g of calcium and more than 30 g of phosphorus were deposited in antlers daily for 60 consecutive days when antler mineralization rate was highest in mid-summer. Simulated Irish elk depleted skeletal mineral reserves to support antler growth more than extant moose, even when hypothesized adaptations to reduce skeletal mineral resorption were implemented. Even though Irish elk fit the allometric relationship between antler size and body size in extant cervids, mineral metabolism does not scale allometrically in the same manner. About 6% of the calcium and 10% of the phosphorus in the antler were resorbed from the skeleton because dietary intake of minerals was insufficient to meet requirements for antler mineralization. The minerals resorbed from the skeleton in summer would have to be replenished by dietary intake over the following winter. Pollen records document a shift in plant species composition from a tall willow-spruce community during the Allerod interstadial to a tundra during the Younger Dryas cold episode with reduced forage density coincident with the extinction of the Irish elk about 10,600 years before present (B.P.). The reduction in forage density would have made replenishing calcium and phosphorus in the skeleton even more difficult, as well as making it more difficult for male Irish elk to replenish fat reserves depleted during the rut. Sexual selection pressures for larger antlers and larger body size were opposed by selection pressures for smaller antlers and smaller body size imposed by environmental change. We suggest that the inability to balance these opposing selection pressures in the face of rapid environmental change contributed to extinction of the Irish elk 10,600 years B.P.