J. Rebecca Gould Calabro
August 8, 2005
Today was a lecture day. Our morning activities included listening to Stefan Andersson on cold hardiness in animals. This lecture goes along with the book Life in the Cold that we were assigned to read during this program. In this entry, I summarize the key points that I found interesting and that relate to the concepts that we study in the Boston Public Schools middle school science curriculum.
Animals survive in nature only if they are able to control their body temperature. Homoeothermic or endothermic animals such as mammals and birds are able to produce their own body heat. At night this type (of which human beings are included) have to be inactive. Another type of animal is an exothermic animal. These rely on external sources of heat, most commonly the sun. They are unable to produce their own body heat. Lastly, there are also heterothermic animals. These animals are able to sometimes produce their own body heat and they can also use external sources to produce heat as well.
Remember when we studied Water and Weather and discovered that heat is a form of energy? Where do human beings get this energy? In Human Body Systems, we produced heat (energy) by eating foods that our cells can break down into carbohydrates, proteins, and fats. After the cells broke down these foods, do you remember what the cells "gave" to our bodies? That's right. Energy.
Back to cold hardiness and adaptations in animals. A rodent who weighs 6 grams is required to consume at least 12 grams of food in 24 hours. A reptile, such as a baby snake, weighing 6 grams as well is only able to eat 3 grams of food once per week (that's 168 hours). Clearly it is necessary for endothermic animals to eat food to allow their bodies to create enough energy to survive in the cold.
Let's look at a larger animal and see how the animal has adapted to the cold climate. Everyone knows that polar bears live in the coldest of climates -- that of sea ice. We know that it is needs to find food to have enough fuel to produce its' own body heat. We also know that in the winter, there is not as much of an available food source as there is in the summer. Polar bears have some additional adaptations that help them stay warm. The polar bear has very thick fur -- it is used for insulation just as you might wear a winter coat in January. The polar bear's coat is also colorless. Furthermore, each hair is a tube-like structure that allows the sun to get straight through reaching the skin. The skin is black. This allows the sun to be absorbed into the skin. The sun's energy is then converted into heat that keeps the polar bear warm in the winter.
Bears, like all animals, have a choice when winter arrives. They can remain in the Arctic or they can leave. Bears choose to remain and as such begin what we call a winter sleep. They are only able to lower their body temperature slightly and they cannot change their metabolism. This is why bears do not hibernate as bats do. Hibernation is when an organism is able to dramatically decrease their body temperature and reduce their metabolic rate to almost nothing. This sudden decrease makes it very dangerous if the animal hibernating is woken up before winter is over. If the animal is inadvertently woken up, then the body needs much more energy and will deplete the animal's reserves quickly. A bear is able to wake up in the winter, walk outside, and if a lack of food is observed returned to sleep.
How do these animals remaining in the cold Arctic know when it is time to hibernate or to go to sleep? When the daylight hours decrease and food becomes too sparse to justify the use of stored energy, the animal makes a conscious decision to bunk down and either hibernate (if they are an exothermic) or sleep (if they are endothermic). These animals of the north do not generally resume activities until the daylight hours and the temperatures increase.
Some animals choose to avoid the harsh Arctic winter. Many birds return to their native tropical homes after having summered in the Arctic. The Arctic climate provides abundant food with little competition. When the cold comes, the birds leave. They came up to the Arctic to enjoy the lack of competition in finding food. When food becomes scarce, the birds migrate back to their original homes.
Still other animals, such as deer, caribou (reindeer), and moose relocate to a more protected, coastal part of the terrain. These animals remain active during the winter often using the removal from the high ridges to bear their calves. These animals have evolved over time a central nervous system. This helps them to constantly allow their brain to receive enough blood every minute to guarantee that heat is produced. These animals are able to lower the temperature in their extremities (fingers and toes) should it be very cold. This helps to ensure that the core temperature remains constant by creating less time for the blood to circulate the body.