The deer’s big appetite for highly digestible, nutrients-rich forage apparently drove its evolution. The growth of large antlers demands such forage, particularly in large-bodied species; antlers grow with proportion to the body. Generally, the larger the body of a species the larger is its antlers. The bone-forming antlers demand calcium and phosphate in abundance. To support antler growth, male deer even raid their skeletons for these minerals… The counterpart to antler growth in the female is multiple births of large, fast-growing young fed by rich milk.
White-tailed deer and mule deer are opportunistic concentrate feeders, with overlapping food habits. Rocky Mountain mule deer have been found to feed on 484 species of forbs, 202 species of shrubs and trees, 84 species of sedges and grasses, and 18 species of lower plants
From Google books: