Behavioral adaptation of Pallas's squirrels to germination schedule and tannins in acorns

Abstract

Using acorns from Castanea henryi (CH) and Quercus variabilis (QV) with contrasting germination schedule (related to food perishability) and tannin levels, we conducted field experiments in a subtropical forest in Southwest China to investigate how free-ranging Pallas's squirrels (Callosciurus erythraeus) utilize acorns as long-term storage based on the food perishability and high-tannin hypotheses. Though QV acorns germinated much earlier than CH ones, we found that Pallas's squirrels hoarded more high-tannin QV acorns over low-tannin CH ones, supporting the high-tannin hypothesis (but not the food perishability hypothesis). However, several other predictions derived from the food perishability hypothesis received sound support: nondormant QV acorns had their embryos removed with a higher probability (68.5%) than dormant CH ones (8.8%) and embryo-removed acorns had a much lower germination success but had a higher probability surviving as long-term storage. During the caching-recovery process, hoarding animals actively detected acorn state (dormant or not) and removed acorn embryos with an increasing probability in subsequent hoarding events. In addition, embryo-removed acorns could serve as long-term storage because they had a very low probability being infested by fungi as intact acorns did during the time of storage. We conclude that tree squirrels can differentially respond to acorn germination and remove the embryos of nondormant acorns more frequently as long-term storage, but tannin level is more important in determining whether a given acorn is hoarded or not. Combined with the studies from North America, our results suggest convergent evolution of acorn hoarding behavior in tree squirrels across different continents.