The innate basis of food-hoarding decisions in grey squirrels: evidence for behavioural adaptations to the oaks

Rodents selectively cache acorns of red oak species (subgenus Erythrobalanus; RO) over those of white oaks (subgenus Quercus; WO) because of perishability of early-germinating WO acorns. On occasion, at least three species of tree squirrels also will cache white oak acorns, but only after they excise acorn embryos, thereby preventing germination. Failure to observe embryo excision of acorns in several other mammal species led us to hypothesize that tree squirrels may have an innate tendency to perform embryo excision and possibly the differential caching of RO and WO acorns. To test this, we evaluated the hoarding decisions of eight captive-reared (naïve) eastern grey squirrels, Sciurus carolinensis, with no previous experience with acorns and four wild-caught squirrels. Squirrels were presented individually with pairs of dormant RO and nondormant WO acorns (two species) and pairs of dormant and nondormant RO acorns over a 2-month period. Both naïve and wild-caught squirrels selectively cached dormant RO acorns more often than those of one WO species (Quercus alba), but showed a much weaker response to other pairs of dormant and nondormant acorns. Naïve squirrels also removed the radicles of germinating acorns, and on occasion, attempted to excise embryos of germinating acorns. We conclude that there is a strong innate basis for some of the hoarding decisions made by tree squirrels, strengthening an emerging argument that tree squirrels show specific behavioural adaptations to the oaks.