The Ultimate Basis of the Caching Preferences of Rodents, and the Oak-Dispersal Syndrome: Tannins, Insects, and Seed Germination1

Abstract

SYNOPSIS. We have shown that eastern gray squirrels and other animals consistently prefer to store intact acorns from the red oak group rather than those from the white oak group. We hypothesized that the ultimate advantage to this behavior comes from the dormancy of red oak acorns. Acorns of the white oak group germinate early in the autumn; thus, we hypothesize that avoiding germination is the primary selective advantage to the preference for caching red oak acorns. Here, we test two alternative hypotheses to explain the benefits of this caching preference: 1) storing red oak acorns allows the high tannin concentrations in red oak acorns to decline (making them more palatable), and 2) storing red oak acorns minimizes losses to insects, presuming they are less infested with insects. We also test the effect of germination schedule on squirrel caching preferences directly, by presenting them with dormant red oak acorns, and red oak acorns about to germinate. We find no evidence that tannin concentrations in red oak acorns decline, although tannin levels did decline in our white oak acorns. We found high losses to insect infestations in one white oak species, but a second white oak species lost very little mass to insects. Finally, we found that germination schedule directly affects squirrel caching preferences: red oak acorns that are near germination are treated like white oak acorns. We conclude that the primary advantage to the preference for caching red oak acorns is that they are less perishable, due to their dormancy. We discuss the effects of this preference on the dispersal of red and white oak acorns, and the subsequent effects of differential dispersal on the ecology and evolution of oaks.