Go Nuts for Acorns!

By Mary Jane Busch, Pennsylvania Forest Steward, Class of ‘22

One of the most recognized seeds is the rotund acorn. Like brilliant fall foliage, oak nuts are associated with autumn and are introduced to young children in their science studies for the seed’s role in nourishing wildlife and propagating oak trees. For most people, their knowledge about acorns probably concludes with these simple facts, and the importance of these oak gifts is little comprehended or appreciated.

Acorns are the fruits or seeds of oak trees, but not all acorns are created equally. The nuts are species-specific and vary in size, shape, color, cupule (cap), and nutrients. For instance, in the United States, the red oak and black oak have hybridized so extensively that experts argue the two species cannot be identified by their leaves, but their acorn sets them apart. In the black oak acorn, the 0.5-1.0-inch body is covered by half or more of the acorn’s cupule. The red oak’s 0.75-1.25-inch fruit has a cap that sits like a beret on top and covers only a quarter of the acorn.

Oak species are categorized into two main groups: the white oak group and the red oak group. The latter is recognized by the soft bristles on the tips of the lobes of the leaves; the former has no bristles on their leaves. Also, species in the white oak group produce acorns every year, whereas the red oak group’s acorns mature every two years. In Pennsylvania, the most abundant white oaks include the chestnut oak and the white oak. The most abundant red oaks include the red oak, black oak, pin oak, and scarlet oak.

Each acorn is one seed (rarely two) protected by a hard outer shell and cupule. The fruit has a high nutrient content with large amounts of protein, carbohydrates, and fats and critical dietary minerals such as calcium, phosphorus, potassium, and niacin. Acorns also contain bitter, toxic tannins which vary by species.

To make them palatable, Native Americans gathered acorns and leached the bitter tannins from the nuts. Sometimes the ripe acorns were first ground into flour using a mortar and pestle and then leached. Some tribes roasted the seeds to kill the acorn weevil larva that often metamorphosizes inside the nuts. Each tribe had their species preference and depended on the variety of acorns available in their environment. Even though they are generally smaller, acorns from the white oak group were often preferred because they are less bitter than fruit from the red oak group.

Mast is the fruits, seeds, and nuts of trees and shrubs that are eaten by wildlife. Soft mast includes fruits and berries, and hard mast includes nuts such as acorns, hickory nuts, hazelnuts, and beech nuts. Oak trees produce more hard mast (by weight) than all other nut trees combined. An older oak can drop up to 10,000 acorns in a year!

Particularly heavy acorn yields are dubbed mast year events. One hypothesis for mast bumper crops is that the huge seed quantities overwhelm the seed eaters, and thus more seeds escape for germination. Additionally, the significant impact of these acorn bonanzas is evident in nature’s food web. More than 100 species of vertebrates consume acorns in the United States. Mammals feasting on or caching the nuts include white-tailed deer, squirrels, chipmunks, bears, mice, voles, rabbits, foxes, raccoons, opossums, and wild hogs.  Birds that feed on acorns include turkeys, bobwhite quail, woodpeckers, crows, jays, wood ducks, and mallard ducks. Obviously, many vertebrates have digestive systems which counteract the toxins in acorns; however, the nuts eaten in large enough quantities are lethal to horses, cattle, and dogs.

Animals that cache acorns have unique sensory abilities—sniffing and shaking—and can differentiate between acorns from the white oak group and the red oak group. Squirrels identify the acorns by smell, a definite advantage when it comes to storing acorns. Acorns from the white oak group germinate immediately after falling and will spoil if cached. To preserve some of the oak nut, the rodents chew out and eat the germinating embryo and then store the rest of the seed. Acorns from the red oak group are left intact as they do not germinate until the following spring. The rodents also shake the nuts to determine if they are inhabited by a consuming acorn weevil; occupied acorns are today’s meal and uninhabited seeds are keepers.

Because of the relatively large size of the oak seed, transporting acorns for winter stash would be a major, energy-depleting chore if it were not for the cheek pouch or the gular pouch of some vertebrates. The pudgy cheeks of the chipmunk and the ground squirrel are a dead giveaway that the rodents are carrying acorns. (Tree squirrels do not have cheek pouches.) Blue Jays have a gular pouch, a sac which opens under the bird’s tongue, enabling it to transport up to five acorns at a time. Because of this physiological capacity, Blue Jays are known to disperse acorns a half-mile or more away from the mother tree.

Acorn stashers are labeled as either scatter-hoarders (a large number of small hoards, such as gray squirrels and Blue Jays) or larder-hoarders (a single, large hoard, such as chipmunks and red squirrels). Individuals that do not survive the winter, amass excess food, or “forget” where they buried the food may have inadvertently planted an oak tree.

Chipmunks can gather up to 165 acorns in a day, an amount equal to half its winter needs, so uneaten acorns may survive to sprout into an oak. Woodpeckers are renowned larder-hoarders. A pest control worker found over 700 pounds of woodpecker-stashed acorns in the walls of a home in California!

There are about 450 species of oak trees (600 species if hybridized varieties are included) found in the northern temperate zone and the high altitudes of the tropics. The United States harbors about 90 species. As proven by these nutty facts, these hardwood giants feed millions of animals via their fruits. The significance of this food source and the intricate and substantial food chain relationships of acorns to fauna should impel landowners to encourage the growth of oak trees. Remember, also, that the lowly acorn is the seed source for oak regeneration which continues this life cycle. Like the many animals that go nuts for acorns, so should homo sapiens!