Squirrels Check Structural Integrity of Nuts to Determine Which to Bury and Which to Eat

Abstract

Squirrels (and chipmunks, and likely other rodents) examine nuts to determine which to bury and which to eat.

Question

Why do squirrels bury nuts? Why do they prefer in-shell nuts to shelled nuts?

Background

Living in a townhouse for quite a few years, Melody Soroudi has had the opportunity to be up close and personal with squirrels (primarily black and grey), as well as other related rodents such as chipmunks. She has been feeding them shelled nuts for years and later added in-shell nuts, or what David Suzuki calls “five-dollar bills”.

Feeding them in-shell nuts created an interesting behavior in the squirrels: they began to bury the nuts much more often than when they were getting only shelled nuts. Why?

Obviously they bury them in order to store them for later use (like the Aesop’s Fable about the Ant and the Grasshopper), specifically for use in the spring when plants are dead or dried up and not fruiting. But surely they cannot bury all of the nuts, they must eat during the summer as well.

The fact is that they do, but they are selective and discerning about which nuts they eat and which they bury.

Observations

If you watch squirrels closely, the behavior becomes apparent. When they get a hold of a nut, they turn it over and over in their hands while examining it with their mouths. If they detect significant cracks or broken or missing pieces of shell, they rip it open and eat the nut(s) inside. If they detect the shell to be structurally intact, they bury it. They do not eat nuts whose shells have a small crack along a seam which are still “closed”, only those they deem to be “compromised”.

This is a logical move because a nut that is compromised is like a coffin with a hole, it will be invaded by bugs and worms and the like and not only get covered in soil, but decompose, or even germinate and start to grow. In fact, nuts that squirrels have lost and forgotten is one method by which some plants reproduce; the shell eventually dissolves and the nuts inside (assuming that they are not roasted) begin to grow. Squirrels are essentially farmers who plant seeds.

Followup Question

Why do squirrels prefer in-shell nuts to shelled nuts? Obviously an in-shell nut provides less energy and nutrients than shelled nuts.

The ones that have the shell only contain two (or occasionally three, but sometimes even just one) nut. They also take up more space and thus can be packed less efficiently, wasting room on shells and air. Also, they require extra work to remove the shell.

Shelled nuts on the other hand require no extra work to get at the valuable part and can be packed better to put more food in less space.

Clearly shelled nuts are more optimal than in-shell nuts. So why do squirrels and other rodents prefer the shells? Why do they prefer to store food for the future than to use it now like some animals do?

This is simple to explain. Larger animals like bears and camels have the physical space to store the energy for future use by plumping up and storing fat in their bodies, however, small rodents do not have that physical capacity, so they must store the food externally in their homes or in the ground.

In addition, large animals tend not to move too much, and thus consume less energy while foraging than small animals like rodents do. Squirrels and chipmunks travel long distances from their nests to look for nuts and run up and down trees and such and thus burn a lot of energy just to get food. Therefore they need to accumulate and store a lot of food for future use.

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