The body size of white-tailed deer (Odocoileus virginianus) increases with latitude and thus exhibits the pattern predicted by Bergmann's rule on the basis of surface to volume ratios and heat loss. This pattern is more simply explained by the distribution of food available per individual animal, which is driven by two factors, the net primary production (NPP) of plants and deer population density. Food availability is often overlooked as a cause of an increase in body size in large terrestrial herbivores in temperate latitudes because of a fundamental misconception about the global distribution of plant productivity. Within a small latitudinal range, white-tailed deer body size as evidenced by modern deer and Holocene paleozoological remains is inversely related to population density and directly related to food availability. Food availability per animal is a product of plant productivity and population density, and is correlated with both local and regional body size variability. These local and regional food-body size patterns are consistent with recent analyses of global NPP datasets which show that ecologically relevant NPP is highest in the north temperate latitudes where white-tailed deer attain their largest body size.