The proximate basis of the oak dispersal syndrome: Detection of seed dormancy by rodents

Abstract

Previously we h'ave shown how a range of physical and chemical characteristics of acorns influences the behavioral decisions of food-hoarding rodents which in turn affects the dispersal, establishment and spatial arrangement of oaks. One such behavior involves the selectiveaching of acorns of red oaks (subgenus: Erythrobalanus) over those of white oaks (Quercus) because of reduced perishability that results from delayed germination of acorns in the red oak group. In this study, we sought to identify the specific proximate cues (visual and olfactory) that eastern gray squirrels (Sciurus carolinensis) use when making these decisions. In two series of field experiments, we presented individual, free-ranging animals with pairs of experimentally altered acorns (that differed with respect to a single ' chemical or visual characteristic) and recorded their feeding and caching responses. Squirrels cached artificial acorns with pericarps (shells) of red oak acorns and ate those with shells of white oak regardless of the internal chemical composition of either type of acorn. Only when the shells of artificial acorns were first soaked in acetone (to remove potential chemical odors) did animals eat artificial acorns made with the shells of red oak acorns. Squirrels also ate one-year old red oak acorns that had broken dormancy, even when they exhibited no signs of germination. We argue that a chemical cue in the shell of acorns is important in the detection of seed dormancy and the decision to cache acorns, and that such a'cue might ultimately contribute to the differential dispersal of red and white oaks by rodents.