Acorns

Wasteful or necessary?

Chinese sawtooth oak, Cambridge, Massachusetts. Drawing by WRT.

            Oaks and acorns are so commonplace that we rarely think about what an acorn is.  The acorn is the fruit of the oak, ceremoniously offered up in a fancy bowl, the cupule.  The outside of the cupule is covered with small scales, usually hardly noticeable, but when they are long and shaggy, as in the Chinese sawtooth oak in front of Erika's dorm in Cambridge, they make the acorn look as though it were wearing a fur coat.  The nut begins as the ovary, initially completely enclosed by the cupule, but as it grows, it gradually protrudes ever farther, so that in some species, when fully ripe it looks like the nut is an oversized pudding served on a cupule platter.

            The cupule develops from the attachment of the flower to the plant. I like to pretend that I know a little botany, and I am guessing that the scales on the cupule are cataphylls, that is, leaves that have a protective function and are not photosynthetic (e.g. bud scales). So, in the acorn, it looks like the oak packs a lot of cataphylls really tightly around the base of the fruit (the nut part of the acorn).  Anyway, until a real botanist corrects my theory, I am sticking with it, as it is in keeping with the way plants modify their basic parts to serve so many purposes.  As I have noted several times before, evolution always modifies what is already there.  Just to hammer home this last point, in chestnuts and beeches, both members of the same family as oaks (Fagaceae), the cupule continues to enclose the entire fruit, finally splitting open when ripe to reveal the nut. In chestnuts, the scales (cataphylls) are modified into nasty spines, something you don’t appreciate when you are enjoying a roasted chestnut from a street vendor as a New York city tourist.  In beech fruits, the cataphylls are less nasty, but nobody roasts them for sale on the streets of NY or anywhere else in the USA. 

            Acorns store a lot of food, and although that food is obviously intended to nourish its own embryo and seedling, the number of animals that want to get in on this bonanza is large indeed.  Wherever oaks and humans occur together, i.e. the northern hemisphere, people have collected acorns as food.  But it isn't like eating peanuts, for oaks got wise to being eaten millions of years ago, and lace their acorns with bitter tannins, and these tannins bind protein, reducing the digestibility of acorns and whatever else the animal has eaten.  Thus, to be suitable as human food, the acorns of most species of oaks must be leached to remove the tannins, whereafter they can be processed into flour, meal, beverage, or porridge.

            But humans are latecomers to eating acorns, beaten millions of years ago by insects. No doubt, when oaks first emerged as a distinct entity from the ancestral plant pool, insects were already onto them.  Millions of years have elapsed in the contest between the exploiting insects and the defending oaks, a contest that will continue as long as there are oaks and insects. Every year, I watch the live oaks in Tallahassee optimistically producing hundreds of thousands of acorns, and every year, I see them abort hundreds of thousands of green, unripe acorns because a female acorn weevil has used her tiny little mandibles at the tip of that long proboscis (aka rostrum) to chew a tiny hole in the acorn and lay an egg or two in it.

A acorn weevil female on an acorn. Males have a shorter rostrum. The rostrum is an extreme forward elongation of the head/face including the mouthparts and antennae.

Here is an excellent film on the Life history of acorn weevils, Deep Look

The hatching weevil larvae chew furrows in the seeds, and the oak, accepting the inevitable, stops feeding the acorn, separates it from the cupule, and drops it to the ground.  Up to 95% of acorns may be lost to insects, and a goodly chunk of the rest goes to feed squirrels, jays, deer, chipmunks and occasionally, humans.  The good news is that jays and squirrels hide acorns, keeping a mental map of their caches, but their memory is not perfect, and sometimes a hawk gets the squirrel before the squirrel gets back to the acorn, allowing the acorn to sprout in peace, another ticket in the raffle to continue the germ line of the parent tree.

            Because the weevil larvae come already conveniently packaged in acorns, rearing them out is a nice science project addressing many questions regarding the life cycle of the weevil, its reproductive success, its parasites and predators, and its local abundance.  It also answers questions about differences in the rate of acorn loss in relation to location, tree size/age, effect of neighboring trees, tree identity, and many others.  The student who carried out this complex project, Zoe, in addition to getting the honors credit she sought, gained a deep appreciation of what oak trees are up against in their efforts to reproduce.  I hope she saw that the excess and profligacy of oaks were not wastefulness, but a necessity for successful continuation of a line.

Zoe Galitz tending her acorn weevil rearing project.

            Here is the painful reality of life--- in a stable population of oaks, a tree may live 200 to 300 years, producing many millions of acorns, and on average, all this reproductive effort will result in only one, ...just one... descendant.