Rodents' Annoying Gnawing Habit

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August 1, 1997

Rats and mice are classified within the mammalian order Rodentia: the gnawing mammals, an appropriate description considering their zealous gnawing habits. Though rats and mice also chew their food, they gnaw on many different objects both in the wild and within our buildings, often wreaking havoc for humans. Just recently, in the autumn of 1996, gnawing rats in San Francisco shut down the Internet — quite a historic event in rodent pest management, don’t you think?

In fact, as we move ahead in technology, there can be no doubt about how increasingly dependent we are becoming on electronics, computers and all the associated circuitry and cabling, which is now measured in millions of miles of linear wireways. This ties us even closer to the rodents in our environments and leaves us even more vulnerable. Consider for example the power, the potential, and the wide-scale use of electronic information today, not to mention its potential as we enter a new millennium. Recognizing this trend, it’s important for PCOs to understand why rodents gnaw, and the impact that rodent gnawing has on our lives.

RODENT GNAWING ANATOMY. A prominent single pair of incisors on their upper and lower jaws morphologically characterizes rodents. (Rabbits are not characterized as rodents, in part because they contain two pair of incisors in the upper jaw). Basically, the incisors serve as tools for the rodent and are critically important for the rodent’s survival. Rodents use their incisors for gathering food and water, defense and aggression, climbing and for gaining access to harborage. Obviously, tools of such importance need to be well designed. In the case of rodents, they are.

First, the incisor teeth do not contain any roots. As a result the incisors grow continuously at the rate of about 0.4 millimeters per day. Thus, if incisor tips should break off or the edges become worn from overuse, new tooth surfaces are being constantly produced to replace the old.

Second, the teeth are very hard, allowing the rodent to gnaw on many different objects efficiently and repeatedly. Measured on Moh’s scratch hardness scale, the rat’s lower incisors rank 5.5, which exceeds even the hardness of metals such as aluminum, copper, lead and iron.

Third, the incisors are razor sharp. They are kept sharp and filed down by the rodent’s grinding the upper and lower pairs against each other. This is supplemented perhaps by gnawing on other hard objects such as rocks and various hardwoods. But despite the popular misconception, rodents don’t need to gnaw on our buildings and objects to manage the length of their incisors.

Finally, the rodent’s jaws, which house the hard and sharp incisors, are also incredibly strong. Rats have been found to exert pressures of up to 7000 psi, and to bite repeatedly up to six bites per second.

THE HUNT FOR RESOURCES. Being opportunists, rodents have learned that by exploring on a regular basis, not only with their senses of smell and taste, but also with their incisors, resources can be found and gathered. To this end, rodents spend about 2% of their daily activity gnawing. By gnawing on and through wood, rodents create openings into tree trunks and hollow logs, or cut and clear through the thick roots of trees to hollow out nest cavities. Around and inside our buildings, rodents gnaw through doors, floors, windows and walls for the same purpose: to gain entrance to a possible harborage and protective site.

In the wild, rats can obtain food and moisture by gnawing on various types of plant stems, seeds, nutshells and tree bark. Larger food items can be chiseled into easy-to-handle sizes prior to grinding with the cheek teeth. The bases of tall plant stems often contain insect larvae and moisture, which the rodent can access by slicing the seam of the stem open with their incisors. It is likely the rodent can smell, hear and feel the presence of insect larva as the larva chew and feed within the stems. It may be the same with the subtle trickling of water through plant stems or hollow logs. When felled via gnawing (much in the same way a beaver fells trees), the tops of plant stems can reward the rodent with nutritious seed heads. And the stem itself or the leaves growing along the stem can be gathered and used as nesting material.

But why do rodents gnaw on so many human objects (wires, wooden studs, etc.) which offer no food, harborage or nesting material? Of particular concern and interest is the rodent’s seemingly excessive attraction to our linear objects. Rodents attack utility wires, computer wires, the wires of vehicles, and a wide range of electrical, gas and water conduit lines of different shapes, sizes and functions, creating all types of shutdowns and havoc. What is it about wires and conduits that attract the rodents gnawing behaviors? Little research exists on this subject. Perhaps, in part, it is because the linear objects visually appear to rodents as the very familiar shape and diameter as plant stems and twigs, which is so prevalent in their natural world and to which they constantly opportunistically gnaw on.

Finally, there is one last aspect of the "design" of the rodent’s teeth which should be mentioned. The incisors are separated from the molars (the chewing teeth) by a gap called a diastema. The gap is relatively large and allows the rodent to form a plug in its mouth behind the incisors by drawing in the sides of its lips behind the incisors. In this way, the rodent does not ingest inedible parts of indigestible items such as plastic, wood or soft metal. Additionally, the rodent often positions the tongue behind the cheek plug during gnawing, resulting in the rodent not even tasting the object it is gnawing upon. This is why it has been so difficult to develop an effective chemical taste repellent to prevent rodents from gnawing through building components and cables and wires.

Gnawing is a natural and necessary survival behavior of the rat and mouse. When they move into our buildings, transportation vehicles, utility rooms, and electrical equipment, and gnaw on the various objects that seemingly offer them no nutritional advantage, rodents are simply following the instinctive and opportunistic behaviors they have employed for thousands of years. The rodent has learned it has little to lose, and usually much to gain by regularly gnawing on the many objects encountered during its daily explorations. Furthermore, some researchers believe gnawing may also serve as a means of communication, as well as a means of territory marking among rodent colonies. All in all, more research is certainly needed to address the complexities of gnawing behavior.

Dr. Robert Corrigan is a contributing editor to PCT magazine.