Squirrels, Acorns, and Tannins

Abstract

Squirrels, but also birds such as jays, bury acorns in the ground to cache them as winter food. By this behavior, they also disperse the acorns and start the germination process by exposing the acorns to soil moisture. Burying protects acorns from surface-feeding competitors such as deer, ruffed grouse, and turkeys. It also prevents other squirrels from pilfering the stores, provided the “owner” remembers where he had buried the seeds, and other squirrels are not attracted by the odor of the buried seeds.

An acorn is technically a fruit, although customarily called a “seed.” It consists of a shell (pericarp) that encloses two seedling leaves (cotyledons) which in turn contain food reserves. The cotyledons connect to a tiny seedling by petioles. The seedling is also (technically incorrect) called the “embryo.” This embryo is located near the tip (distal end) of the acorn and is folded between the cotyledons. When germination starts, the pericarp splits at the tip. The seedling root (radicle) appears first. Then the entire seedling emerges, as the petioles of the cotyledons arch out downward and stretch to 2–5 cm. The radicle develops into a substantial taproot, while the presumptive leaves (epicotyl) and stem (hypocotyl) will grow little until spring.

Squirrels certainly do know their taxonomy: they identify acorns as either storable or less suited for underground storage. They bury red oak (Quercus rubra) acorns as winter caches, but eat immediately those of white oak, Q. alba (Smith and Follmer 1972; Smallwood and Peters 1986). White oak germinates already in fall, while red oak germinates later, and therefore “keeps better” in the ground. White oak sends a thickened taproot deep into the ground before winter starts. It thus “buries itself” and is thought to escape seed predation this way (Fox 1982). To prevent white oak acorns from germinating, gray squirrels kill these seeds by excising the seed “embryo” before caching them. Mature squirrels practice this technique more often than juveniles (Fox 1982). The squirrels discard the embryo.