Interpretation of Intraspecific Life History Patterns: Evidence from Columbian Ground Squirrels

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Abstract
In populations of Columbian ground squirrels in the Rocky Mountains of southwestern Alberta [Canada] life history patterns consistently differ at different elevations. At present, these differences are not appropriately explained by the traditional models of life history evolution, r- and K-selection and bet-hedging. Nor do more-recent models based on the interspecific scaling of body weight appear to be appropriate. To explain different life history patterns of the ground squirrels, we present a simple qualitative hypothesis that invokes phenotypically plastic responses to the limitation of demonstrably influential environmental resources (food, and its availability) and to other environmental factors that may influence the acquisition and expenditure of resources. The principal strength of this resource-limitation hyothesis (and other hypotheses based on environmental factors influencing patterns of life history within species) is that it does not invoke genic differences among populations, though it need not preclude the influence of such differences. Because evidence indicates substantial plasticity in the life history patterns of ground squirrels and other vertebrates, different life history patterns within species might result from different phenotypic expressions of similar genotypes, either with or without different genotypic adaptations to different environments.