Bobby Corrigan Offers Help on Understanding Rodent Behavior

Editor’s Note: The following is an excerpt from Bobby Corrigan’s book “Rodent Control: A Practical Guide for Pest Management Professionals.”

Rodents comprise the most common mammals in the world, both in numbers and in species, and are well represented in urban, suburban and rural areas. The three most economically important commensal species of rodents in the United States are the house mouse, the Norway rat and the roof rat, all belonging to the family Muridae.

CLASSIFICATION OF RODENTS. Rodents are classified within the animal kingdom in the class Mammalia. Mammals, as compared to all other animal groups, have two distinguishing characteristics: First, the females contain milk-producing mammary glands, and second, mammals have hair covering their bodies in varying degrees during some point in the animal’s life.

Rodents are classified in the Mammalian order Rodentia. The derivation of the word rodent comes from the Latin word rodere, which means “to gnaw.” Rodentology then is the study of rodents, and people who specialize in the study of rodents are rodentologists.

When pest management professionals (PMPs) think of rodents, the animals that most often come to mind are rats and mice. But Rodentia comprises the largest order of mammals.

There are approximately 1,700 species of rodents among 35 families. The largest rodent in the world is the capybara of South America, a pig-sized rodent weighing up to 110 pounds. The smallest rodent is the pygmy mouse, weighing only a few grams.

In general, the characteristics that enable rodents to be such successful pests include a simple-yet-highly-effective body structure; high reproductive potential; their ability to adapt to a wide variety of habitats and climates; and their opportunistic feeding, gnawing and harborage selection behaviors.

KEY CHARACTERISTICS. What are some of the important biological and behavioral characteristics of rodents as compared to other urban mammals? By understanding these characteristics, we can correctly identify rodents from other mammals.

It also provides us with an understanding from which to build management programs and an appreciation of why rodents are such persistent and economic pests.

The following discussion is only provided as a broad overview of rodents as a group and will of focus on their simple, yet highly effective, body structure.

THE RODENT BODY. The pest species of rats and mice are relatively small mammals, weighing less than 1 pound. Most are squat, compact animals with short limbs and bodies of varying lengths. They have visible eyes and ears, and the body is covered with fur.

Their tails also vary in length, shape and hairiness. In addition to the general shape of the rodent, it often is the teeth, the fur and the tail that people note and identify with rodents.

SKULL AND TEETH. The arrangement of the teeth in the skull of a rodent is one of the chief characteristics that separate the rodents from all other mammals. A prominent pair of incisors on the upper and lower parts of the skull characterizes rodents.

The roots of the incisors of many rodents remain open throughout their lives, and thus they grow constantly. To keep them sharp and filed down, rodents constantly chisel the opposing upper and lower incisors against each other.

In this way, the hard enamel (anterior surface) wears against the softer dentine (bulk of the tooth). This activity is supplemented by gnawing on various objects the rodents encounter in their activities such as twigs, wood, plastics and many man-made items. Despite popular belief, rodents do not need to gnaw on objects to keep their incisors filed down.

Rodents do not possess any canine and pre-molar teeth. Directly behind the incisors, and in front of molars, is a prominent space or gap called the diastema. Rodents often form a “plug” in this space by tucking the cheek fur into the gap. In this way, the rodent does not ingest any non-edible objects such as metal, plastic or wood. The diastema is also used to carry the rodent’s young and various objects.

The mammals most closely resembling the rodents are the rabbits and hares. The skull of these mammals at first glace looks similar to a rodent skull. But lagomorphs possess two pairs of upper incisors in the skull — a small pair located behind the primary pair — instead of the single pair found in the rodents.

Outdoors, rabbits and hares inflict gnawing damage to various trees and plants, which is nearly identical in appearance to that caused by rodents.

The skulls of Insectivores, such as the shrews and moles, can be distinguished from rodents by the presence of long rows of needle-like teeth on both the upper and lower jaw. These are used for chewing and grinding up insects, worms, slugs and other animals. There is no diastema behind the incisors, and the incisors are not as elongated or prominent as they are in the rodents.

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PELAGE. Like most terrestrial mammals, rodents have hair covering their bodies. Collectively, all the hairs covering the rodent’s body are referred to as the pelage or fur. The pelage serves the rodent in several important ways.

First, the outer coat of coarse, protective hairs serve as a coat to the animal, protecting it from cold and heat. This is especially important for mice and rats, as they lose body heat relatively quickly in the cold.

Second, the pelage contains specialized long, stiff hairs with innervated bases (possessing nerves). These hairs are called vibrissae and provide tactile (or touch) feedback to the animal. Vibrissae are most prominent and visible on the face (whiskers), but are also located on other parts of the body, such as the top of the head, back and sides. The vibrissae are clearly visible on most rodents.

Third, the pelage serves to protect the skin of the animal by keeping wetness, dirt, some parasites and other irritants (e.g., burrs, seeds, etc.) away from the skin.

Fourth, rodents use their pelage to communicate. When confronted by aggressors, rodents may bristle the guard hairs of the pelage to appear as large and threatening as possible in an attempt to frighten their enemies.

And finally, the pelage can help provide some rodents with camouflage. The dark pelage of most rats and mice is not readily seen at night when the animals are active.

TAIL. Most rodents possess an external visible tail, but the form and length of the tail may vary significantly from species to species. Some are furred, some are scaly and semi-hairy, and some are bare.

Some tails are short and barely visible, such as those of some voles. Others, such as the roof rat, have very long tails.

For certain species, the tail is an important asset to the rodent. First, it serves to assist the rodent in regulating body heat. When necessary, the tail can serve a cooling function as heat can be dissipated from the surface of the tail. Or the tail can be used to maintain warmth. The thick, bushy tail of the tree squirrel, for example, can be placed over the back on cold days, providing an additional coat.

The tail is also used by rats and mice for balance and leverage during climbing and traveling on narrow limbs and wires.

CONCLUSION. Being able to identify rodents from other mammals provides PMPs with an understanding from which to build management programs and an appreciation of why rodents are such persistent and economic pests.

The author is well-known rodentologist.