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Bergmann's rule, which proposes a heat-balance explanation for the observed latitudinal gradient of increasing animal body size with increasing latitude, has dominated the study of geographic patterns in animal size since it was first proposed in 1847. Several critical reviews have determined that as many as half of the species examined do not fit the predictions of Bergmann's rule. We have proposed an alternative hypothesis for geographic variation in body size based on food availability, as regulated by the net primary production (NPP) of plants, specifically NPP during the growing season, or eNPP (ecologically and evolutionarily relevant NPP). Our hypothesis, "the eNPP rule," is independent of latitude and predicts both spatial and temporal variation in body size, as well as in total population biomass, population growth rates, individual health, and life history traits of animals, including humans, wherever eNPP varies across appropriate scales of space or time. In the context of a revised interpretation of the global patterns of NPP and eNPP, we predict contrasting latitudinal correlations with body size in three distinct latitudinal zones. The eNPP rule explains bodysize patterns that are consistent with Bergmann's rule, as well as two distinct types of contradictions of Bergmann's rule: the lack of latitudinal patterns within the tropics, and the decline in body size above approximately 608 latitude. Both types of contradictions of Bergmann's rule are consistent with the eNPP rule, as are a wide range of other phenomena. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]