1.0 introduction
Thermoregulation is basically temperature regulation. It is a homeostatic process where by the internal temperature of an organism is kept constant within a specific range despite fluctuations in the external temperatures. It is important to note that all homeostatic systems use negative feedback mechanism. This implies that whenever there is a change in external or surrounding temperature of an organism, consequently a corrective mechanism is initiated within or without the organism to counter the change. Not all animals can maintain a constant body temperature; this is possible only for birds and mammals. Such animals are referred to as endotherms or commonly termed as warm-blooded animals. All other animals have body temperatures which periodically vary with that of the surrounding. These animals are referred to as ectotherms or commonly called cold-blooded animals. The major distinction between the two categories of animals is that while endotherms rely mostly on internal corrective mechanism to cope with the temperature fluctuations, ectotherms rely on behavioral adaptations.
In the cold climates, the major challenge for the animals is to keep their body temperature warm enough. This is to prevent body fluids from freezing and thus eventual death of the organism. To achieve this, animals employ both behavioral adaptations and internal corrective mechanism regardless of whether they are ectotherms or endotherms. Thus, the main objective of this article is to examine some of the coping mechanisms of animals in the cold climate as far as thermoregulation is concerned. These measures include but are not limited to hibernation, huddling, physiological adaptations i.e. being larger than their counterparts in temperate and moderate climatic zones, having thick subcutaneous layers and a time they possess anti-freeze mechanisms.
2.0 Thermoregulation in cold climates
Animals in cold climates depend on physiological capabilities to perform there thermoregulation functions. For instance, fish in the cold regions have a high concentration of glycerol in their blood compared to their counterparts in tropical and temperate climates. In addition to this, these animals have thicker subcutaneous layers and more feathers (fur or hair). This dense growth of fur, hair or feathers accordingly beefs up the insulation to the body hence minimizing heat loss to the surroundings. These adaptations are meant to minimize heat loss to the environment and thus help regulate body temperatures within a required range. Mammals can prevent heat loss through varying the pilo-erection. In this case, the angle between the hair follicle and the skin is varied through muscle contraction. Pilo-erection is also responsible for making the hair “stand” in most mammals and birds which reduces heat loss. This is commonly seen in terms of goose bumps. Birds achieve this better than mammals. They adopt a mechanism referred as feather fluffing which is an advanced and more efficient form of pilo-erection. In addition to this, birds also possess the uropygial gland whose main function is to lubricate their feathers. This ensures that the feathers do not get wet and hence insulate the body from heat loss.
Glycerol specifically is an anti-freeze gel and thus it reduces the subzero temperatures and hence preventing the bodily fluids from freezing. Glycoprotein is another antifreeze compound present in the bodies of animals in cold climates. Apart from the mentioned above coping mechanisms, some insects in cold climates have nucleating agents. These basically are hydrophilic protein molecules that stimulate and catalyze the formation of frost in the haemolymph. These frost particles increase the osmotic pressure of the haemolymph fluid thus forming hyperosmotic surroundings for the cell. This environment consequently buffers the body cells from heat loss to the cold environment. In addition to this, this hyper osmotic environment ensures that there is presence of water within the cells. This implies that the environment within the cell eventually has a higher osmolality lowering frost formation within intracellular fluid.
Also, animals especially endotherms living in cold climates have a higher concentration of mitochondria. This means that thus they have the capability to produce more heat and subsequently effectively counter the low environmental temperatures. Mitochondria are used in heat generation within the animal body. Heat from mitochondria is generally generated in three major ways. First, there is voluntary muscle movement (in human beings it is referred to as exercising). This is common in birds and some insects that flap their wings move around aimlessly just to achieve this fete. Second, there is involuntary muscle movement commonly referred to as trembling and shuddering. This adaptation mechanism is associated with all endotherms and some ectotherms, and it is more efficient because the animal does not have to move around to achieve this. Thus, less heat is lost to the surrounding environment through unnecessary contact with it. Shivering can either be low intensity or high intensity. In low intensity, the animal can shiver at minimal rates for a lengthy duration of time. While in high intensity, the animal shivers violently for a very short duration of time. The other difference between the two is that while low intensity shivering uses fat for heat generation, high intensity utilizes glucose. Lastly, this has to all to do with high concentration of mitochondria in the fat tissues (adipose tissues) rather than muscle movement. This fat is then metabolized to produce heat.
Producing this heat is not enough; the produced heat needs to be distributed within the animal body. This why there is the countercurrent heat flow mechanism within animals found in cold climates. In this case warm arterial blood travelling to the limbs passes cooler venous blood from the limbs. Consequently, heat exchange occurs hence the venous blood is warmed up at the expense of the arterial blood. This mechanism ensures that the less insulted external body parts of an animal are at lower temperature compared to the inner body parts. This thus minimizes heat loss to the environment. Arterioles carrying blood to superficial parts of the body constrict and hence are re-routed away from these parts of the body. Thus, blood is channeled towards the inner warmer parts of the body. This mechanism effectively prevents heat loss to the surrounding environment and also stops the body temperature from dropping further. This phenomenon is referred to as vasoconstriction. This implies that the appendage is always cool. This results to regional heterothermy, a thermal gradient along the extremity. In aquatic animals, the story is a little different. This is because blood flows outside the body surface specifically between the two subcutaneous layers. These two mechanisms are an example of the skin of animal playing a role in thermoregulation thus keeping the internal body temperature within a specific range.
Another adaptive mechanism in cold climates is for an animal momentarily reduce its metabolic rate. This effectively reduces the thermal gradient between the animal and its immediate environment thus minimizing heat loss. The reduced metabolic rate is also important in that less energy is used in sustaining the animal’s metabolism. Subsequently, the saved energy is used to help the animal survives the cold climate. This adaptive mechanism is referred to as hibernation. Hibernation is important because it not only helps the animals deal with the low temperature; it also helps them cope with the reduced food resource base. Thus, hibernators have vast reservoirs of brown fat and also have the capability to slow down all body processes. There are two types of hibernators: true and false. True hibernators can maintain their body temperatures at extremely low levels throughout the entire hibernation period. False hibernators on the other hand vary their body temperatures through the period by a times emerging briefly from their dens. It must also be mentioned that hibernation occurs mostly in underground burrows. This phenomenon is mostly associated with mammals. On a lesser scale, it can be termed as torpor. In this case, the animal temporarily drops body temperatures to survive a short ordeal of unfavorable weather conditions.
Lastly, animals in cold climates have been known to also apply behavioral mechanisms in thermoregulation. Some of these mechanisms include but are not limited to huddling and migration. Huddling refers to this phenomenon whereby animals cluster together in pairs or more. Accordingly, heat exchange occurs between or among the animals hence successfully minimizing heat loss to the environment. Migration is coping mechanism for animals that cannot adapt to the extremely cold climatic conditions. Thus they move to more favorable climates for survival. It has been discovered that as the temperatures decrease, animals of the same species subsequently increase in size (Bergmann’s Rule). This is a physiological thermoregulation mechanism. The large size is important in the following ways: one the metabolizing tissue is increased exponentially and thus the capacity to generate internal heat is also increased. Secondly there is a reduction in the surface area to mass ratio thus minimizing the heat loss to the environment. This implies the surface of the body exposed to the environment is relatively smaller (Allen’s Rule). Now we shall, briefly examine how some animals cope with the cold environment.
3.0 How specific animals cope with the cold environment
Penguins rely on several mechanisms to cope with the cold environment. For starters, penguins have a thick layer of fat that protects them against cold conditions while in water. However, while on land, they rely on their dense feathers to keep warm. It must be mentioned this feathers are not the typical large flat feathers. Instead, they have short feathers with a woolly underline. These are also good at shedding off water when penguins emerge from the sea. Penguins can also puff their feathers thus trapping more air for better insulation against heat loss. Also, the less insulated areas of a penguin are important in thermoregulation in that they act as heat sinks. Most importantly, the feet and flippers are remotely controlled. Their muscle are located in deeper warmer regions of the body thus it does not matter whether the feet and flippers are cold or warm.
The polar fox on the other hand has thick fur which turns white in winter and covers even the soles of the feet to keep warm. Polar bear also have a white thick coat which blends in with the snow. This important because they are false hibernators and thus need to camouflage to hunt for food when they re-emerge from their dens. It also has thick layer of fat to keep warm and for nourishment during hibernation. It has large, wide paws to enable it walk effectively in the snow while in false hibernation. As for marine animals such as whales and seals, they have a thick layer of blubber to prevent heat loss thus keeping them warm. As earlier mentioned, fish on the other hand have anti-freeze compounds (glycerol and glyco-proteins) to cope with the cold water environment. Some bird species such as swallows that cannot cope with winter are seen to migrate to more favorable climatic zones. Most cold blooded animals hibernate during cold climatic conditions e.g. amphibians (toads, newts and frogs), snakes and most reptiles lizards included.
References
Blatteis, C. M. (2001). Physiology and Pathophysiology of tempearture regulation. New Jersey: World scientific publishing company.
Hall, J. E., & Guyton. (2010). Hall textbook of medical physiology student consult online access (12th edition). Philadephia: Elsevier Saunders.
Hall, J., & Guyton, A. (2006). Textbook of Medical Physiology(11th edition). Philadephia: Elsevier Saunders.
Marino, F. E. (2008). Thermoregulation and Perfomance:physiological and biological aspects. Basel: Karger.
Romanvisky, A. (2007). Thermoregulation: fucntional architecture of a thermoregulatory system. Moscow.
