Tannins and Partial Consumption of Acorns: Implications for Dispersal of Oaks by Seed Predators

Abstract

-A common assumption in studies of seed predation is that seeds survive attack and are dispersed only 'when animals fail to find seeds, drop undamaged seeds or fail to recover seeds after they are cached. This study, however, suggests that many acorn consumers consistently eat only a portion of the cotyledon of several species of acorns and thereby permit embryo survival. Several vertebrates [gray squirrels (Sciurus carolinensis), common grackles (Quiscalus quiscula) and blue jays (Cyanocita cristata)] were observed to consume only 30-60% of the cotyledon from the basal portion (cap end) of willow oak (Quercus phellos) acorns. Gray squirrels exhibited a similar preference for the basal end of acorns of several other species of red oaks (Q. rubra, Q. laevis, Q. nigra, Q. palustris and Q. coccinea) from a wide geographic region. In addition, acorn weevil larvae (Curculio sp.) were observed significantly (>35%) more often in the basal portion than in the apical end of Q. alba acorns. Chemical analyses of acorns from two tree species revealed that the concentration of protein-precipitable phenolics (primarily tannins) was 12.5% (Q. phellos) and 84.2% (Q. laevis) higher in the apical portion of the seeds where the embryo is located. Moreover, germination experiments revealed equal or greater germination frequencies for partially consumed acorns than for intact acorns. We suggest that the higher tannin levels may render the apical portion less palatable, and thereby increase the probability of embryo survival after attack by seed consumers.