It is interesting how one thing can lead to another. While reading about acorns in conjunction with my previous post (“Selective Food Storage” 11-28-16), I learned additional new facts about acorns.
Tannins are plant polyphenols that bind and precipitate proteins and interfere with an animal’s ability to metabolize proteins. Fleck and Layne writing in the “Journal of Chemical Ecology” (1990) noted that acorns of the red oak group (Erythrobalanus) have more tannins in their embryos than acorns of the white oak group (Leucobalanus). Steele et. al. (“American Midland Naturalist”, 1993) showed that in some red oaks the apical end of the acorns (opposite the cap and containing the embryo) has as much as 82.4% more tannins than the basal (cap) end.
Often tree squirrels will eat only about 30 to 60 % of the basal end of a red oak acorn and ignore the remaining portion of the acorn. By selectively eating the portion of the acorn that has a lesser concentration of tannins the squirrels are eating the more palatable end and avoiding the high concentrations of deleterious polyphenols.
Experiments show that the partially consumed acorns have a germination rate higher than or equal to intact acorns. It is suggested that the high tannin levels at the apical end of the acorn may render the embryo less palatable and as a result increase the probability of germination even after the acorn is attacked by squirrels or other mammals.
Other experiments (Fleck and Woolfender, “Journal of Chemical Ecology” 1997) suggest that higher tannins in acorns might also have a harmful effect on insect larvae in acorns.
These California black oak (Quercus kelloggii – a red oak) and Oregon white oak (Quercus garryana – a white oak) acorns were growing (in August) along the Lower Hat Creek Trail (Shasta County CA).