A suite of characters describing digestive tract structure has been hypothesized to reflect the relative degree of specialization of the digestive system of rodent species along a continuum from a proteinaceous diet of seeds and insects to a cellulosic diet of vegetation. Similarly, it has been proposed that life history traits might reflect diet and digestive tract structure, with the most opportunistic species consuming the most energy-rich diets of seeds and insects. The five members of the rodent assemblage of agroecosystems of the Argentine pampa were found to be omnivores and varied in the relative proportions of seeds, insects, and vegetation consumed. On a gross level, diet reflected life history; the most opportunistic species (smallest body size, highest fecundity, preference for disturbed habitats) consumed the most energy-rich diet, while the least opportunistic species consumed the most vegetation. However, comparative digestive tract structure was generally converse to that predicted, based on diet. Failure to observe predicted correlations may be due in part to seasonal variability in diet, lack of evolutionary relevance of crop habitats, or flaws in the underlying hypotheses. Alternatively, the observed variation in digestive tract structure may reflect the spectrum of variation encountered within an omnivore rodent guild rather than the degree of food specialization.
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