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Rodent population dynamics are predicted to respond positively to the masting of acorns, but diverging results have been published. This study tested the hypothesis that population responses to acorn masting vary depending on differences in the tolerance to tannins of different rodent species. The effects of acorn abundance on the rodent population densities were analyzed using a dataset obtained in Hokkaido, Japan. Specifically, population fluctuations of three rodent species (Apodemus speciosus, A. argenteus, and Myodes rufocanus) and the abundance of Quercus crispula acorns have been monitored since 1992. Acorn production in previous years had a positive effect on the annual population growth rates of A. speciosus; however, this trend was not clear in the other two species. Tannin tolerance, assessed by body weight changes in an acorn feeding experiment, exhibited clear differences among …