VERTEBRATE PESTS: Hibernation And Other "Sleeps"

Many parts of the United States are now well into the cold winter months. In some areas, several inches (or more) of snow and ice are likely covering the ground.

When it comes to certain mammal pests, such as chipmunks, woodchucks, ground squirrels and bats, client calls for managing these animals are virtually non-existent during the winter months. This is because many of these animals “go down” or “hibernate” during the winter.

But why is it that some mammals hibernate while others do not? For example, why don’t commensal rodents or tree squirrels hibernate? Or moles, voles and shrews living outside in the cold seemingly frozen ground? And what happens to species that typically hibernate but encounter mild winters?

This month we focus on “hibernation,” or more specifically, forms of dormancy in small mammals. For pest management professionals, understanding dormancy in mammals is important. Clients occasionally inquire about their previous season’s mammal pests and wonder whether or not they might be “hibernating” in their attics, crawlspaces, wood sheds or within some earthen burrow next to the foundation or below their slab floors.

Dormancy in animals may be of five different types: sleep, torpor, hibernation, winter sleep and estivation.

SLEEP. Although sleep has been studied extensively, it is still poorly understood, even in humans. In mammals, sleep is characterized with slight decreases in body temperature and extensive adjustments in various brain functions. During the hours mammals are active and awake, research has shown there is a build-up of sleep-inducing substances that accumulate in the central nervous system. However, the mode of action of these substances and the physiological benefits of sleep are not well understood. Still, most mammals require sleep on a regular basis to perform effectively at most other daily functions.

TORPOR. Torpor is a period of inactivity by an animal in response to cold or other environmental conditions that might unfavorably affect the animal. Generally, torpor occurs for short periods (hours to a day or so) and animals usually resort to torpor when their energy reserves are very low. When an animal isn’t feeding, perhaps due to the effects of winter, it is advantageous for the animal to allow its body temperature and metabolism to drop to a lower level. Some small mammals, because of their high rates of metabolism (e.g., shrews, moles, bats), are vulnerable to starvation during periods of inactivity when they are not feeding. During these periods, animals can avoid possible starvation by becoming torpid. Bats, for example, regularly become torpid on cold nights during the early spring and late fall, (i.e., just after or before they enter hibernation).

HIBERNATION. Hibernation is a period of deep torpor or winter dormancy that may last from weeks to several months, depending on the severity of the winter. The body temperature and metabolic rates of hibernating mammals drop significantly to low levels. Hibernators typically become fat prior to hibernating, as they must rely on the fat reserve as an energy source to carry them through the winter. Animals that fail to store enough fat are much less likely to survive, especially if the winter is particularly severe.

Most hibernators arise from their deep sleeps periodically to tend to their needs, such as emptying their bladders or avoiding dangerous drops in the ambient temperature that could freeze them. These activity periods explain why homeowners report animal sounds inside walls and within ceiling and floor voids every so often during the dead of winter.

Hibernation patterns vary considerably among mammals, even among those of the same genus. For example, ground squirrels, such as the thirteen-lined ground squirrel, the California ground squirrel and chipmunks, are hibernators. However, a few squirrels (especially those squirrels born the previous spring) of some colonies of the California ground squirrel, Spermophilus beechyi, may remain active throughout the winter, except on the coldest days. When property owners (and pest professionals) witness these individuals above ground during the winter, they incorrectly conclude that the ground squirrels in their area do not hibernate.

Moreover, ground squirrels and chipmunks may intermittently arouse from hibernation and feed on seeds and other foods they cached into their burrows during the autumn. Energized by a snack, these squirrels occasionally venture out, even on cold days, further adding to the misconception that certain species of ground squirrels are not “true hibernators.”

But why is it that some small mammals hibernate and others do not? (Even among the same order of mammals such as rodents?)

In addition to certain morphological and physiological traits, part of the answer also lies in how a particular species is adapted for foraging and food utilization. Species that are entirely dependent upon natural foods (plant seeds, berries, nuts, insects), are obviously at a strong disadvantage should they remain active during the cold, when snow may blanket the surface and eliminate their chances of finding their foods in a timely fashion.

Non-hibernating commensal rodents, on the other hand, are omnivorous and opportunists and can feed on many different types of food, allowing them a greater opportunity to survive the winter. They are also excellent hoarders and may store significant reserves of food inside burrows to get them through the winter.

Other non-hibernating small mammals, such as shrews, moles and voles, may use several strategies to survive the winter. Generally, these mammals become torpid from time to time and then forage when they can. Voles and shrews can forage for their foods in sub snow tunnels, protected by the insulative properties of the snow above them. Some moles are known to pack partially eaten earthworms on “shelves” within their tunnels and consume these worms periodically to generate enough energy to sustain them until conditions improve.
All this is not to say that hibernators and non-hibernators are always successful at winter survival. Harsh and prolonged winters are often deadly to many small mammals and serve as one of the most natural of limiting factors of many animal populations. The term “winter kill” is indeed appropriate.

WINTER SLEEP. In most cases, mid-sized mammals are best adapted for true hibernation, as they have the pre-requisite body size to body mass ratios to efficiently store sufficient reserves for extended hibernation. Large mammals cannot hibernate. Bears for example, which are widely referred to in storybooks and general conversation as hibernators, cannot hibernate. Bears and other large mammals during the winter enter what is referred to as “winter sleep.” Compared to hibernation, winter sleep does not involve a substantial drop in body temperature. Rather, winter sleepers select harborages protected from the elements (e.g., hollow logs, caves, underground culverts, etc.) and curl up tightly and using their pelage and fat layers to help conserve body heat. By remaining inactive, the large mammals can use stored fat reserves for heat and energy over a period of days and weeks. But winter sleepers are able to wake and become active quickly (as some panicked hunters periodically discover when encountering sleeping bears).

ESTIVATION. Finally, estivation is a physiological summer version of hibernation or a “summer sleep” that may last for a few days to a couple of weeks. Many of the same small mammals that hibernate (e.g., ground squirrels, woodchucks and chipmunks) also estivate. Mammals estivate as a response to threatening hot ambient temperatures and/or danger from desiccation. For some burrowing rodents, it is advantageous for them to rest down in their cool earthen ground burrows and conserve energy and metabolic water resources rather than to fight the hot and desiccating rays of the sun above ground.

CONCLUSION. Sometimes I think about all this hibernation and estivation stuff when I realize how difficult it is for me to get out of my warm bed at 5:30 a.m. on a dark, bleak, cold Indiana February morning or how I only want to nap inside an air conditioned room on a brutally hot and humid August afternoon. A warm burrow or a cool burrow…I’m just like a ground squirrel.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.