Predictable empirical patterns of variation in body size along spatial and environmental gradients have been documented within many species of mammals. Four main hypotheses, heat conservation, heat dissipation, primary productivity and seasonality, have been proposed to explain these patterns of variation in body size. In this study, we reported an analysis of geographic variation in body size of Richardson's ground squirrels Urocitellus richardsonii, a North American hibernating, burrowing mammal. Firstly, we evaluated whether a Bergmannian size pattern was exhibited by Richardson's ground squirrels. Secondly, we used an information-theoretic approach to test which of the four main hypotheses best explain(s) geographic variation in body size of Richardson's ground squirrels or to assess whether, as proposed by McNab's ‘resource rule’ or Huston and Wolverton's ‘eNPP rule’, the primary productivity hypothesis is the only explanation. Richardson's ground squirrels exhibited a pattern of increasing body size towards the colder areas, that is, more productive and seasonal central Alberta and foothills of the southern Alberta Rocky Mountains, indicative of a Bergmannian size pattern. Plant productivity and seasonality in plant productivity were likely the primary underlying factors generating the observed pattern of geographic variation in body size. Thus, our results supported primary productivity and seasonality hypotheses. From these results, we see that McNab's ‘resource rule’ or Huston and Wolverton's ‘eNPP rule’ (i.e. spatial variation in food availability) is an explanation for a Bergmannian size pattern in Richardson's ground squirrels, but not the only explanation.