When the air grows icy and
winter sets in, New Jersey’s
animals have many ways of coping with colder temperatures and a lack of
food. Some, like birds, migrate. Warblers and swallows fly south to find
active insects, sparrows fly south to find uncovered seeds, and water birds fly
south to find open water. Other animals,
like insects, spiders, reptiles, and amphibians will also migrate—deeper into
the earth. Salamanders and earth worms
will burrow beneath leaf litter and toads will burrow under the soil, below the
frost line. Snow provides an insulating
layer for these animals.
Some animals grow warm,
thick winter coats, such as white-tail deer, whose winter coats are made of
hollow hairs to trap extra body heat.
Birds that stick around for the winter, such as goldfinches and
redpolls, grow more feathers and fluff them for insulation. Animals like deer, birds, squirrels, rabbits,
and beavers stay active during the cold months.
Even in the coldest places, staying active will keep an animal alive
through the winter. For example, the
arctic fox in the Arctic and the emperor
penguin in the Antarctic will survive the cold because of their appropriate
insulation, considerable energy reserves, and ability to successfully compete
for continuing food sources.
Several of New Jersey’s animals have another adaptation
for surviving the cold winter months, however.
They will either enter a state of torpor or prolonged torpor, otherwise
known as hibernation. To enter into a
state of torpor is to enter into a state of hypothermia, accompanied by
inactivity and lowered metabolism, body temperature, and heart rate. There is a fall in oxygen consumption and
breathing rates and a restriction of blood flow to the main organs. Why would an animal adapt to enter such a
state? This type of deep sleep is a form
of energy conservation. Animals that
live in cold climates develop such sleeping patterns when food gathering
becomes more difficult and it would take too much energy and body heat to
search for food.
Torpor is driven by ambient
temperature and food availability. Many
of New Jersey’s
mammals, such as the black bear, chipmunk, raccoon, and skunk, enter states of
torpor to make it through the cold months.
To prepare for the winter, black bears accumulate body fat throughout
the summer and autumn. In late fall,
when food becomes scarce, they make their dens and enter them for the
winter. Black bears lose about 25
percent of their body weight during the winter, but they stay in good physical
condition, and they will awaken and leave their dens periodically, especially
if food is available. Female black bears
give birth during the winter, and their fat stores from the summer and autumn
provide enough nourishment to suckle their young. Even in the arctic and sub-arctic, bears are
not true hibernators. They will simply
experience seasonal lethargy and periods of torpor to get through the winter.
Animals that maintain very
high metabolic rates will often go into daily torpor. This allows them to sleep through times when
it is difficult to find food. For
example, bats will enter torpor during the day, when it is difficult to find
insects. Their bodies therefore use less
energy, and food will last longer in their bodies. They will wake up again when it is time to
search for more food. Bats take about
two hours to reduce their metabolic rates, drop their body temperatures, and
enter into a state of torpor. When they
are ready to awaken, it will take about an hour. They warm up by shivering violently and
contracting their muscles. Frogs will
enter torpor during the night because nighttime air temperatures are too cool
for them. They therefore use less food
and energy to keep warm during the night.
Other animals that enter daily torpor include hummingbirds, swifts,
nightjars, nighthawks, poor-wills, and goatsuckers. Doves and pigeons enter a shallow state of
torpor when they are deprived of food.
Hibernation is a sustained
state of torpor. Entry into and exit
from hibernation are governed both by internal signals such as hormone changes
and external cues such as day length and temperature. Whether in a state of torpor or a state of
hibernation, animals have the ability to wake spontaneously, despite the
temperature. Animals hibernate for
several months, though they will occasionally awaken throughout the
winter. They will remain in a state of deep
torpor for several weeks at a time, and then awaken for several hours before
entering into a deep state of torpor again.
These short periods of arousal and activity may serve to maintain
organs, tissues, and cells; animals are susceptible to parasitic infections
while hibernating, so waking up occasionally may boost their immune systems.
Very
few of New Jersey’s
mammals are true hibernators. Only the
jumping mouse, woodchuck (or groundhog), and little brown bat hibernate through
the winter. For these animals, foraging
for food and maintaining their normal core body temperatures during the winter
months are energetically too costly.
Like animals who enter short periods of torpor during the winter,
animals that enter hibernation will also prepare in the summer and autumn by
eating more food than usual and building fat stores. An animal in hibernation will not lose any of
its muscle; it will just lose its stored body fat. The longer the animal is in a deep state of
torpor, the thicker its layer of fat will be.
When hibernating animals awaken, they will be much thinner, but they
will have maintained their muscle. As in
torpor, hibernating animals slow their heartbeat, breathing, and
metabolism. Animals that hibernate drop
their body temperatures lower than those who enter torpor. Their body temperatures will drop so low that
they will match that of the ambient air temperature! But hibernating animals protect themselves in
sealed dens that remain above freezing temperature because of insulation and
geothermal heating. During the depth of
winter, the length of time that animals stay in their deep state of torpor
lengthens. As spring comes closer, the
periods of torpor decrease and the periods of activity increase until one day,
arousal marks the end of hibernation.
Temperature and the instinct to mate are other factors that call animals
out of hibernation.
The
groundhog doubles in weight from May to September to prepare for a four to five
month period of hibernation. On
Groundhog Day, when we call the groundhog out of its den and look for its
shadow, we are awakening an animal deep in sleep. Depending on factors such as temperature and
photoperiod, the groundhog may reenter a state of prolonged torpor to make it
through the remaining winter months. As
humans, though, we don’t have that option; we will have to continue emerging
from the warm dens of our beds, bundling up, and staying active throughout the
cold season.
References:
"Animals at the
Extremes: Hibernation and Torpor." Open Learn: The Open University. Web. 27 Dec. 2011. http://openlearn.open.ac.uk/mod/oucontent/view.php?id=398616&direct=1.
Harris, Steve. "How to
Tell Torpor from Hibernation." Discover Wildlife | Wildlife and Photography at Its Best with BBC
Wildlife Magazine. 15 July 2010. Web.
27 Dec. 2011. http://www.discoverwildlife.com/british-wildlife/how-tell-torpor-hibernation.
"Hibernation." ThinkQuest.
Web. 27 Dec. 2011. http://library.thinkquest.org/TQ0312800/hibernate.htm.
"Mammalian
Hibernation." University
of Calgary, 26 Feb. 1999.
Web. 27 Dec. 2011. http://people.ucalgary.ca/~kmuldrew/cryo_course/cryo_chap12_1.html.
"Torpor." ThinkQuest.
Web. 27 Dec. 2011. http://library.thinkquest.org/TQ0312800/torpor.htm.
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