Posts Tagged ‘squirrels’

Squirrels and Blue Jays vs Acorn Tannins

December 12, 2020

For over 10,000 years acorns were the most important source of food for Native Americans wherever oak trees were common.  Acorns are an important source of food for animals too for everything from mice to bison.  However, acorns contain tannins, a substance difficult to digest and even toxic for some animals.  For example horses that eat too many acorns may die. Oak trees rely on animals to spread their seed, but if too much of their seed is consumed, their populations will decline.  The nutritional value in acorns attracts hungry animals, but the tannins act as a semi-deterrent.  Acorns from species in the white oak family evolved a different strategy for coping with acorn predation than species in the red oak family.  White oak acorns contain less tannins and are more palatable, so squirrels and jays prefer these and spread them throughout the landscape, but they germinate as soon as they are buried in the fall.  When a squirrel or jay tries to retrieve them later, it is too late.  White oaks only lose acorns that are consumed immediately.  Acorns from oaks in the red oak family are high in tannins, but the tannins are concentrated in the bottom half of the acorn.  Squirrels gnaw on the top half and abandon the bottom half.  If enough of the bottom half is left, the acorn can still germinate, though red oak acorns don’t need to germinate until spring because squirrels and other animals don’t want to eat the part of the acorn with such an high concentration of tannins. This year the sand laurel oaks (Quercus hemispherica), the most common oak species in my neighborhood, are producing a bumper crop of acorns, and the squirrels are gnawing the tops of them but leaving the bottoms.

Squirrels eat the tops of acorns from oak trees in the red oak family.  These acorns, found in my backyard, are from a sand laurel oak. also known as Darlington oak.  The top part of acorns have less tannins which are hard for most animals to digest.  Nevertheless, squirrels risk death from the 8 cats that live in my backyard to exploit this food source.  Oaks can germinate from acorns with the tops gnawed off.

Squirrels fed a diet of just red oak acorns in an experiment ate less.  Blue jays fed a diet of red oak acorns in an experiment actually lost weight.  Squirrels living in a location with mostly red oaks must vary their diet with other foods such as white oak acorns, nuts, fungi, berries, and insects.  Blue jays fed a diet of red oak acorns and acorn weevil larva maintained their weight, showing how blue jays can survive in the wild on a diet of mostly red oak acorns because the infestation rates of acorns by weevil larva are high.  Incidentally, oak trees were able to quickly colonize New England and southern Canada following deglaciation at the end of the Ice Age because of the acorns that were spread by blue jays.

Tannins posed an obstacle for hungry Native Americans as well.  According to Euell Gibbons, Native Americans processed the acorns by boiling them in water to leech out the tannins.  It’s necessary to periodically change the water–a tedious process.  I tried this years ago with the sand laurel oak acorns in my yard.  After dumping the water out and replacing it 8 times, I got tired of the process and gave up.  The acorns were becoming less bitter, but still not palatable enough to eat in a satisfying quantity.  Native Americans with no modern day supermarkets were most persistent from necessity.

References:

Chang-Macoubrey, A;  A.E. Hagerman, and R. L. Kirkpatrick

“Effects of Tannins on Digestion and Detoxification Activity in Gray Squirrel (Sciurus carolinensis)”

Physiological Zoology 20 (3) 1997

Johnson, W., Libby Thomas, and Curtis Adkinson

“Dietary Circumvention of Acorn Tannins by Blue Jays: Implications for Oak Demography”

Oecologia 99 (2) 1993