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Archive - December 2016
Survival can be difficult for animals in climates like we have in Illinois, particularly in the winter when their natural food supply is greatly reduced or not available. Water may be hard to find in liquid form in winter. Some animals overcome these problems by hibernating.
What is Hibernation?
Many animals use varying degrees and lengths of dormancy to conserve energy and survive periods of limited food supply. Dormancy is a period of time when growth, development and (in animals) physical activity are stopped.
There are several types of dormancy. They include diapause, aestivation, brumation and hibernation.
Diapause is used often by insects that stop developing from fall until spring. For instance, they may overwinter as a larval (caterpillar) stage. When environmental conditions are improved, they continue to develop. Some mammals use diapause, too, to delay development of their young.
Aestivation is a form of dormancy used by animals that need to escape very hot and/or dry conditions. They become inactive in a place where they are cooler and/or can stay moist.
Brumation is a type of dormancy in reptiles. It starts in late fall and continues until the outside conditions are favorable for the reptile and its food source to be active. During the brumation period, reptiles wake often to drink water then return to their dormant state.
Other animals may become inactive for a part of every day to reduce their energy requirements. Ruby-throated hummingbirds (Archilochus colubris) and little brown bats (Myotis lucifugus) are examples of such animals.
True hibernation involves greatly reducing an animal’s bodily functions and activity. Body temperature drops dramatically, as do heart and breathing rates. From a normal body temperature of 95°F, the temperature of a hibernating mammal may be as low as 36°F. A normal heart rate of more than 100 beats per minute may drop to only four or five beats per minute, and breathing may slow to less than one breath per minute. True hibernation can only occur in animals that generate their body heat internally. They are able to maintain a fairly constant body temperature at all times. Birds, mammals and some fishes fit this definition, but fishes cannot hibernate, although some may become dormant.
Not all activity stops during hibernation, though. Scientists have discovered that even during hibernation there are periods of wakefulness that become more frequent as the hibernation period comes to an end. External temperature is a factor in these periods of sporadic activity. For each species there is a critical temperature above which they will waken, and all will waken temporarily if the temperature drops so low that they are in danger of freezing. Wakening allows mammals to move to a deeper, warmer chamber or to warm up a little—by shivering or moving around—until the temperature moderates.
Which Animals Hibernate?
Some mammals hibernate. Have you ever heard of a bird that can hibernate? Well, there is a bird species that lives in the western United States that appears to do so. Do some research and find out which Illinois birds are related to it. In Illinois, there are 11 species of hibernating mammals. They are listed below along the amount of time they usually hibernate. The hibernating bats in Illinois generally do so in caves or abandoned mines. The other mammals on this list hibernate in underground chambers.
big brown bat (Eptesicus fuscus) - six months
woodchuck (Marmota monax) - five to six months
southeastern bat (Myotis austroriparius) - six months
eastern small-footed bat (Myotis leibii) - six months
little brown bat (Myotis lucifugus) - six months
northern long-eared bat (Myotis septentrionalis) - six months
Indiana bat (Myotis sodalis) - six months
tri-colored bat (Perimyotis subflavus) - six months
Franklin’s ground squirrel (Spermophilus franklinii) - six to seven months
thirteen-lined ground squirrel (Spermophilus tridecemlineatus) - six months
meadow jumping mouse (Zapus hudsonius) - six to seven months
Other mammals may become inactive in bad weather. Skunks and raccoons, for instance, “den up” during extremely cold weather, remaining in their burrows and living off excess body fat, while not actually hibernating. Their body temperature may drop a few degrees but not like that of a true hibernator.
How Do They Know When to Hibernate?
The answer to this question is not definitely known. Some animals prepare for hibernation while the conditions around them are still fine for obtaining food. They may be using the decreasing amount of daylight, reduced available food supplies and overall cooler temperatures to help them decide when to hibernate. They seem to have a chemical in their blood that detects these changes and starts hibernation. Other animals may wait until environmental conditions become bad, and then hibernate. This method can be helpful to the animal by allowing it to feed and be active longer than animals that started hibernating while food supplies were still good.
How Do They Prepare for Hibernation?
Woodchucks are an example of a true hibernator. Prior to the hibernation period, these mammals accumulate a thick layer of excess body fat that supplies them with the energy needed to survive. A hibernating mammal may lose as much as one-third of its total body weight during hibernation.
Others gather and store a supply of food to eat during brief periods of wakefulness.
When Do They Come Out of Hibernation?
Hibernation may last several days, weeks or months depending on the species, air temperature, time of year and individual's health. As spring approaches the air warms, food supplies are once again sufficient and the hibernating mammals return to normal activity.