Thursday, January 8, 2015

Snow and Survival - Animal Adaptations in Winter

After a mild December, Winter has arrived in Mid-Michigan with a vengeance.  Projected temperatures for the next twenty-four hours are set to dip down to -1 degree Fahrenheit (-18 Celsius).

In addition to the cold temperatures, over the past week Mid-Michigan has also received several inches of snow and a day of freezing rain.  The snow and cold pose survival challenges for animals that remain in Michigan during the Winter.

Life during the Winter seems to occur at a much slower pace than it does during the other eight or nine months of the year.  Most of the birds that called the region home during the warmer months have migrated to warmer climates.  Insects seem to have disappeared - some lie dormant as adults, others spend the cold months as eggs, larvae, or pupae.  Reptiles, amphibians, and a few mammals lie still in hibernation until the weather improves.  Even most of the trees and other plants sit dormant and await the return of Spring.

For each animal that remains active here during the Winter survival comes down to two factors; can it find enough food (and avoid becoming food for another animal) and can they stay warm.  While part of staying warm is staying sheltered from wind and moisture, a big part of staying warm is being able to metabolize at a high enough rate to stay warm.  So in reality the ability to survive Winter in the northern United States comes down to finding enough food (and avoiding avoiding becoming food).

The challenges of Winter survival are very different depending on the species.  Large animals can typically survive a short period of bad weather.  They normally have enough body fat stored up that they can keep their metabolism fired up even if the weather prevents them from finding food for several days.  However, if the weather remains bad for a long time their size becomes a disadvantage.  Large animals like White-tailed Deer require a large amount of food.  A long Winter with deep snow means that many deer face potential starvation as they deplete available food sources.

Two White-tailed Deer bedded down in snow

There is a flock of Wild Turkeys that call the north end of Alma home.  Here is a portion of that flock last Friday (02 JAN 2015).

Foraging Wild Turkeys

This is the same flock on Monday (05 JAN 2015).  Note how the turkeys are mostly sitting with the heads and necks tucked under their wings.  Because their head are mostly bare of feathers this helps them conserve body heat.

Wild Turkeys at rest

Cold, snowy weather doubles the difficulty of surviving.  Animals such as Wild Turkeys need more food to fuel their internal furnace if they are to remain active, but snow makes it harder for them to find food.  The turkeys in the first photo were able to forage by sight.  The snow in the second photo is not deep, but there is also a layer of ice (provided by a day of freezing rain) just above ground level.

Add snow and ice to the ground and now the turkeys must expend extra energy scratching aside snow in their search for food.  Now their food is not only harder to find, but they also use more energy searching for it.

For smaller animals such as squirrels and rabbits snow also presents a great challenge.  Because they have much smaller stores of body fat, they have a much smaller margin for error.  A deer or turkey may be able to survive a week or two with limited food, but a squirrel or rabbit would likely not survive that length of time.  Fortunately, both animals require much less food than a larger animal such as a deer. 

Leaves and snow turned over by squirrels searching for food.

While much of a squirrel's food is foraged from the ground, they are much less limited in their search for food by their ability to climb.

A Fox Squirrel eating maple seeds

Rabbits survive the Winter largely on a diet of tree bark and buds.  As the the level of snow rises, they are able to walk on top of the snow and forage higher on trees and shrubs.  In fact you can often measure the height of the previous winter's snowfall by the scars that rabbits left on plants.  To read more about rabbit survival in winter please check out this post from January 2014.

Rabbit sign

So is snow bad for the survival of all animal species that remain active through the Winter? 

No.  The smallest species of mammals such as mice and voles can actually survive better with a healthy coating of snow.  Because their bodies have a high surface area to volume ratio their bodies lose heat at a much higher rate than larger animals - this means their metabolic rate must be much higher than that of a deer just to maintain body temperature.  A deep layer of snow acts as an effective insulator.  Normally there is a small gap between the snow and the surface of the ground.  This gap is created by vegetation that supports the snow and also by melting cause by the release of heat from the earth.  This space under the snow is known as the subnivean zone.  Because of the insulating properties of snow, temperatures in the subnivean zone can be as much as 30 degrees (Fahrenheit) higher than temperatures above the snow.  This means that small mammals in the subnivean zone don't have to work as hard to maintain the body temperature as they would if there were cold temperatures with zero snow cover.  In addition, deep snow protects these small animals from larger predators that would normally be able to see them as they forage.

For more information on the subnivean zone please look here.

For information on how small birds survive during the winter please check out this article on chickadee survival tactics.

A gateway to the subnivean zone

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