When temperatures get colder and the days get shorter, many mammals settle into a period of hibernation. But what effect will climate change — and the associated changes in weather patterns — have on this annual behavior?
What do you think of when you hear the term hibernation? Do you imagine a bear asleep in its den, curled tightly into a ball? Though hibernation is often thought of as a time when certain animals go to sleep for months at a time, the actual act of hibernation is a bit more complex. Hibernation refers to a long-term state during which an animal’s body temperature is dramatically decreased (in some cases by as much as 60 degrees Fahrenheit!)and its metabolism slows significantly.
The two major steps in preparing for hibernation include prepping a den (sometimes called a hibernacula) and building up fat reserves. Prepping a den typical involves finding a safe location and lining the den with insulating materials such as leaves, grasses, or mud. Building up fat reserves requires eating large amounts of nutrient-rich food to form a layer of fat that will both insulate the body and provide energy to run metabolic processes. In addition, the fat reserves serve as a source of water to keep the animal hydrated while it is hibernating.
Once an animal enters hibernation, its body temperature lowers to a set point, its heart rate lowers to as low as 2.5 percent of the normal rate, and its breathing rate decreases by at least 50 percent. The animal’s insulating layer of body fat provides the fuel for these metabolic activities. Although a hibernating animal does not produce any fecal matter, it does produce urea. However, while hibernating, the animal’s body is able to recycle the urea by breaking it down into amino acids, thus preventing the animal from needing to urinate.
The three main triggers that activate hibernation behaviors in an animal include changes in air temperature, food availability, and day length (also called photoperiod). Researchers are finding that changes in a region’s climate are affecting hibernation behaviors around the world.
For example, in Spain, scientists have found in recent years that a European brown bear species that inhabits the Cantabrian Mountains no longer hibernate during the winter months. Mother bears (sows) and their cubs have remained active due to warmer conditions and the ability to find abundant food resources year-round. In England, dormice, which typically hibernate for a six-month period, are now hibernating five weeks fewer than they did 20 years ago.
In the United States, a long-term research project at the Rocky Mountain Biological Laboratory (RMBL) in Gothic, Colorado, indicates that marmots are emerging from their dens around 30 days earlier than they did in the mid 1970s. Scientists think warmer air temperatures are responsible for this change in behavior. One problem with this change in behavior is that, even though the air temperatures indicate spring is underway, the areas where the marmots live are still typically covered in snow. Because of this, the marmots do not have immediate access to food resources. Instead, they must rely on their remaining fat reserves to tide them over until the spring thaw occurs. Because rousing from hibernation requires a relatively large amount of metabolic energy, the animals typically don’t have much fat reserves to spare. The additional stress on their bodies can have deleterious results and may lead to death.
However, all is not bad news when it comes to hibernation and climate change — at least in the case of Rocky Mountain marmots. For those marmots that do make it through the spring thaw, the earlier emergence is associated with a larger gain of weight throughout the growing season. The marmot population at the RMBL has tripled since 2000, and the percentage of the marmot population that survives the hibernation period has risen from 70 to 80 percent in recent years.
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