A Little Bit More
When we think of wildlife in winter, we tend to think of the animals we see on a regular basis: squirrels, rabbits, and deer. But what about the least obvious and most vulnerable? Small mammals such as mice, voles, and shrews have a proportionally greater surface area to volume ratio compared with larger animals. With this large surface area, they are unable to maintain adequate body heat in frigid temperatures and would soon freeze if left exposed in winter. When the snow falls, however, small mammals receive a "thermal blessing"
Once a snowy blanket covers the landscape, the warmth radiating from the ground is trapped beneath the snow. As the layer of snow immediately against the soil slowly sublimates (ice changes directly to water vapor), the water vapor moves upward into the layers of snow above. Gradually, an open space, which may be several inches high, forms at the ground level. These covered corridors provide a microenvironment that rarely goes below 20 degrees Fahrenheit and is usually within a few degrees of freezing. Small mammals can remain active there throughout the winter despite the frigid temperatures above; some species are even able to breed within this environment.
The icy tunnels provide protected access to food plus predator protection as well. The winter winds may howl, but in the tunnels all is quiet and safe - almost. Only one predator is able to move about through the tunnels: the weasel. Weasels are quite small and can move easily through the icy maze and pounce on unsuspecting prey, but as long as the snow cover remains intact, the small mammals are safe from all other predators above. Winter is actually the safest season of all for them!
Activities
Temperature Variation
Objectives: Demonstrate how temperatures can vary within a limited area
Materials: multiple thermometers, cup, masking tape and indelible markers
Every day, almost every one of us is bombarded with the current temperature. When we hear the temperature reading on the radio or TV each morning, we accept the number stated and use this information to help us plan our day. But the recording station that provided that temperature for the news media may be miles from our own home. Granted, temperatures are not going to be wildly different when listening to a "local" station, but temperatures do vary much more than most of us are aware.
Here is an activity for students of all ages that demonstrates just how temperature varies within a limited area such as a schoolyard, backyard, or city park.
Calibration - Calibrate your thermometers so all read the same. Standard thermometers are never perfectly accurate and may vary from one another by several degrees. Calibration does not guarantee accuracy, but does ensure that all your thermometers match. Place all of your thermometers (each should have a number or letter designation) in a glass of ice water for about 5 minutes and note the temperature for each. Now allow the thermometers to warm to room temperature and note the temperatures. Now place in a glass of warm water. After 5 minutes, again note each thermometer's temperature. If all thermometers agree, you're ready to begin. If some do not, simply note which ones are above or below the "typical" reading and by how much - be sure to remember to adjust any records accordingly at the end of the exercise for that particular thermometer.
Activity - Have the students take the temperature of a variety of outside areas. They may want to take the temperature in the middle of the lawn, at the base of a tree, on the swing set, next to the building foundation, on dark substrate, on light substrate, in the sun, under the snow, in the shade etc. The students will have fun finding "unusual" places to take their readings but that's OK. Each temperature and exact location should be written down. Before going outside, decide upon a reasonable length of time (two to three minutes) to allow all thermometers to reach a stable temperature when making each reading.
Have the students return with their readings and post the locations and temperatures each group has observed. Once the readings have been posted for all to see, you may want to have some discussion why there are variations in their readings. You may want to try this on different kinds of days (sunny, cloudy, rainy, icy, windy) to see how different days vary or don't vary in temperature differences. And be sure to have the students compare their findings with those from newspapers and/or TV.
Obviously, there are no "correct" answers to their readings. This exercise is a good way for students to begin to realize that many factors affect temperature that they may never have considered before.
Key Concepts
Adaptations and Diversity
Questions
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Posted by Elise Lockton on April 18, 2006 at 01:45 PM
Q: I have never clearly understood the concept of how small mammals have a greater surface area to volume ratio compared with larger mammals. It seems like it would be the opposite? How do large mammals (moos, caribou) endure the exposure of the AK winter?
A: I'm glad to see that you're thinking about this. It's not an easy concept to grasp fully, even when we have a physics explanation. But I want to try another way that may help explain this concept - at least it does for me.
Let's compare a mouse and an elephant. I like to work with extreme examples when trying to understand a new concept so I've chosen two very different-sized animals. Try to imagine this as I explain and see if it helps. First, picture a mouse sitting next to an elephant. Now picture in your mind the imaginary creation of a "new" elephant of exactly the same size, made up of lots and lots of those mice. It's going to take an awful lot of those mice but if we pack them all together in just the right way, we'll have a solid elephant. Some of those mice will be on the outside but many more will be on the inside of our imaginary elephant. Those many, many mice on the inside won't lose body heat directly to the air; only the relatively fewer mice on the outside will. So, although the new area on the outside is much much larger than on that original mouse, the total area that the imaginary elephant will lose heat from is less, by comparison. Larger animals certainly lose heat but not as quickly as the smaller animals.
One other way to look at this is to reverse this image. If I have a large bowl of peas that are too hot to eat, I can speed up the cooling by spreading out all of the peas on a large plate. This dramatically increases the surface area (ratio of surface to internal volume) and the peas cool much more quickly. So, larger animals in the north are much more capable of maintaining their own body temperature than are smaller animals. Of course, there are mice in the north but they don't live out in the open during winter; they're in burrows and under the snow where it isn't so cold.
I hope this helps and doesn't confuse you even more.