ADAPTATION OF GRAY SQUIRREL BEHAVIOR TO AUTUMN GERMINATION BY WHITE OAK ACORNS.

Abstract

The oaks (Quercus L.) are divided into two subgenera, the white oaks (Leucobalanus Engelmann) and the red oaks (Erythrobalanus Spach). Like most midlatitude plants, red oaks have dormant seeds which germinate in spring after being dispersed in autumn. Acorns of many species of white oaks germinate in autumn soon after they fall, producing a large taproot which stores much of the acorn's food reserves over winter. Live (evergreen) oaks are taxonomically regarded as red oaks (Williams, 1939), but resemble white oaks in many important seed and life history traits, including early germination (Sargent, 1905; Lewis, 1911; Trelease, 1924; USDA, 1974). Red oaks and white or live oaks typically occur together as associates and potential competitors throughout North America. They will be referred to collectively as WO and RO; WO by implication includes live oaks insofar as their seed biology is convergent with white oaks. Unabbreviated common names refer to single species for which the scientific name is given at least once. The seedling taproot of white oak (Q. alba L.), probably the most common WO of eastern North America, grows rapidly in autumn and perhaps also grows more slowly through the winter, drawing upon the food reserves of the cotyledons. The seedling taproot is remarkably thickened on seedlings of Q. alba and Q. virginiana Mill. (Fig. 1A; Lewis, 1911) and appears to serve as an overwinter storage organ. A white oak acorn in a sense buries itself, and the rapid germination and root engorgement may serve as an escape from various seed predators (D. H. Janzen, pers. comm.). This paper reinterprets autumn germination in the WO as a post-dispersal escape from tree squirrels, although the trait may have other adaptive values as well. It also reports that gray squirrels (Sciurus carolinensis Gmelin) kill acorns of WO before caching them, by cutting out (excising) the seed 'embryo' (Fig. 1B, C). The adaptive value suggested for this behavior is preventing 'escape' and loss of that fraction of the acorn which is transferred into the seedling taproot between the time an acorn is cached in autumn and the time when a squirrel returns to utilize the acorn in winter. A brief description of oak seed and seedling morphology will be helpful (see also USDA, 1974 p. 698). The mature acorn, technically a fruit but here called a seed, consists of a pericarp (shell) enclosing two seedling leaves (cotyledons) which are filled with food reserves and occupy most of the seed. The cotyledons are connected by petioles to the tiny seedling (hereafter improperly but less ambiguously called the embryo when referring to ungerminated acorns). The embryo is located very near the apex or pointed end of the acorn and is folded between the cotyledons. In germination the pericarp splits at the apex and the radicle (seedling root) emerges first. Later, the entire seedling is thrust out, as the cotyledonary petioles arch out and downwards and lengthen to 2-5 cm. The radicle grows into a stout taproot in autumn, but the epicotyl and hypocotyl (the presumptive leaves and stem) grow scarcely at all until spring.