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The extinction of the Irish elk Megaloceros giganteus has traditionally thought to have been caused in one way or another by the enormous antlers of the males. Recently, a popular hypothesis for the Irish elk extinction has been their inability to cope with the nutritional demands of growing such large antlers during worsening habitat conditions. However, this hypothesis is weakened by several previously unaddressed and biologically unreasonable assumptions. We discuss these assumptions and conclude that, because antler mass is expected to have been evolutionarily labile, nutritionally sensitive, and ontogenetically variable and male mortality is expected to have had limited impact on population growth, the large antlers of Irish elk probably had little to do with the extinction. We focus on the reproductive energetics of females as a possible contributor to extinction, and model the nutritional demands of producing precocial cursorial young. Our model shows the reproductive output of females being reduced by 50% due to changes in the length of the growing season at the end of the Pleistocene when most populations of Irish elk went extinct. The model was validated with parameters from the extant wapiti, which was predicted to maintain high levels of reproduction during the Pleistocene climatic deterioration. Thus, nutritional stress on reproductive females is likely to have contributed more to the Irish elk extinction than nutritional stress on large-antlered males.