THERMOREGULATION IN MAMMALS
Over the course of their evolution, mammals have developed a sophisticated nervous system by which they can control their internal temperature. Thermoregulation allows mammals to keep their internal temperatures constant. This can occur despite external temperature changes in their environment. It offers mammals a degree of freedom from their environment. However, it requires a very large expenditure of energy. This process is centered within the hypothalamus. The hypothalamus is a part of the brain responsible for other subconscious functions like breathing and blinking. By using nervous signals sent from temperature-sensing nerves in the skin, the hypothalamus can activate mechanisms in the body to regulate the body’s internal temperature.
The most basic process for temperature regulation is simply to change the rate of heat production. The hypothalamus sends signals to skeletal muscles, which causes them to move. This movement requires muscle cells to metabolize substances to produce energy and, in doing so, create heat. A mammal involuntarily shivers when it is cold because the hypothalamus is ordering the muscles to move and produce heat. Conversely, the hypothalamus can also stop unnecessary muscle movement and slow metabolic heat production to cool a mammal.
The hypothalamus can also trigger vasodilation and vasoconstriction in blood vessels near the surface of the skin. Using these processes, the amount of heat lost to the environment can be controlled to a degree. When a mammal enters a cold environment, the blood vessels near the skin surface constrict. This lessens the amount of blood that flows into those regions. As such, heat loss resulting from the flow of blood can be reduced. The opposite process, vasodilation, can be used to help cool the body by increasing the flow of blood to vessels near the surface of the skin.
Terrestrial mammals - those that live on land - can also regulate their temperature by controlling the amount of moisture released by the skin.
When released moisture collects on the skin of a mammal, it eventually evaporates into the air around it. The evaporation process creates a cooling effect on the surface of the skin, which can be used to regulate temperature. This is common among humans, who sweat when they enter warm environments. Some mammals lack the ability to produce sweat. However, they can still use evaporative cooling methods to regulate their body temperature. Rodents, for instance, may use saliva to wet their heads. Bats use their own saliva and urine to cool themselves. Evaporative cooling of the skin coupled with vasodilation can quickly reduce a mammal’s body temperature.
Finally, mammals may simply relocate to regulate their body temperature. In desert regions, mammals gather underneath the shade of trees to avoid harsh sunlight. During winter, many mammals go into hibernation - a deep sleep in which many body functions slow considerably. Hibernating mammals survive the winter months by conserving energy and dedicating it to sustaining a constant temperature. The hibernation process relies on reserves of energy stored as fat, which are burned over the winter to produce heat. Thanks to hibernation, many mammals can inhabit extremely cold regions and survive the winter.
All of these thermoregulatory processes are controlled by the hypothalamus. Some mechanisms, such as hibernation or relocation, may require a conscious effort on behalf of the organism. However, even those processes employ parts of the hypothalamus. At the core of any thermoregulatory process is one of two parts of the hypothalamus: the heating center or the cooling center. As the name suggests, the heating center senses temperature changes and triggers certain mechanisms when the mammal needs to conserve or produce heat. Alternatively, the cooling center senses excessively warm conditions around the mammal. It then triggers responses to reduce the body temperature. The hypothalamus is the core of thermoregulation.
Without these thermoregulatory devices, mammals would be at the mercy of their environment. Like reptiles, mammals would have to bask in the sun to be able to move or function. Thermoregulatory devices allow mammals to travel to regions inaccessible to reptiles and continue to function in spite of weather. This comes at the cost of energy consumption, which is much higher than for most reptiles. To maintain a steady temperature in an environment of twenty degrees Celsius, a human male has to metabolize 1800 kilocalories a day. Meanwhile, a typical alligator only metabolizes sixty kilocalories per day. The alligator, however, has to wait for adequate heat in its environment before it can move. Furthermore, alligators and most other large reptiles are forced to remain in environments that stay warm year-round. Thanks to thermoregulation, mammals are mobile and independent from their environments.