How do Cold Climate Conditions Affect the Physiology and Lifestyles of the Animals that Live in Them?
All animals need to keep their body temperature regulated. Enzymes are proteins that are critical for the body to function and they work within an optimum range. If the body temperature becomes too high, the hydrogen bonds holding the specific tertiary structure of the proteins break and the protein unravels, and the enzyme becomes denatured; it cannot work any more. If the body temperature becomes to low, the enzyme cannot function efficiently because it is out of it's optimum temperature.
As well as enzymes need the temperature kept constant, if the temperature drops significantly cell membranes become rigid (Campbell and Reece, 2008), and regular cell processes cannot occur. –The fluidity of the cell membrane allows for transportation into and out of cells.
If the temperature were to become very low (below freezing) then ice crystals would form in blood. This would result in blood cells expanding as they freeze and bursting. This would be fatal as the blood would not be able to carry oxygen to the heart and around the body.
There are many different adaptations that have evolved in both ectothermic and endothermic animals. Endothermic animals are those which rely on their own metabolic processes to keep their body temperature up; ectothermic animals must rely on the environment.
Fur, hair or feathers are all good means of insulation found in endothermic land animals as they trap a layer of warm air next to the animal's skin (Campbell and Reece, 2008). As the simple diagram in figure 1 shows, individual hairs (or feathers) can be erected. Each hair has a muscle attached to it, which involuntarily contracts in the cold, to pull the hairs erect so that air becomes trapped between them, and the heat from the animal's skin warms this air up. Only a small amount of heat is lost to the atmosphere as this blanket of trapped, warm air covers the skin.
Animals living in wet, cold conditions need to try to stay dry, as water would significantly reduce the insulating capacity of the animal's fur/feathers. Some animals have oil glands located under the skin near to the root of each hair or feather. Oil is hydrophobic which means that it cannot mix with water therefore when an animal secretes oil it repels the water, which will just run off the oily fur/feathers.
Subcutaneous fat is a very common method of insulation that has evolved in endothermic animals to keep them warm. It exists just under the skin as adipose tissue and acts as an energy store. Marine animals in particular have a very thick layer of fat, called blubber (Campbell and Reece, 2008). Blubber is mainly specific to marine animals as the waters that they live in can be very cold (as low as -2˚C in the Antarctic). Fur, feathers, or hair would not effectively insulate an animal when it is in water as the hairs would be pushed flat by the water and would not be able to trap a layer of air.
Insulation is not the only adaptation in endothermic animals to keep warm; they have also evolved several circulatory adaptations. Vasoconstriction is the term for when capillaries that pass near the skin constrict (their diameter decreases) and less blood is then able to flow through them. This results in less blood flowing near to the skin surface and therefore less heat is lost to the environment by the blood. This is also illustrated in figure 1.
Another circulatory adaptation is the countercurrent heat exchange system. Arteries and veins are located next to each other and arteries carry blood towards extremities and veins carry blood away from extremities. Figure 3 shows an artery and a vein passing next to each other, and how the blood in them flows in opposite directions. The blood in the artery is warm because it is coming from the core body, but as it passes into the extremities, which are more exposed to the cold air/water, it gets cooled and would then flow through the vein and into the body as cool blood which could have fatal consequences (i.e. enzymes cannot function). The countercurrent heat exchange system tackles this issue as heat from the arterial blood passes into the venous blood that is heading back into the body; the blood that ultimately enters the extremities is cooler and therefore very little heat can be lost from it. Then as this cooler venous blood passes back towards the body it is heated by arterial blood and enters the body at almost core body temperature. Because the heat exchange system is countercurrent (i.e. the exchange occurs as blood flows in opposite directions), this allows the heat exchange to occur along the entire length of the artery and vein system. If they were next to each other and the blood was flowing in the same direction, the system would be very inefficient as figure 4 shows. The coldest blood flowing from the extremity towards the body in the vein is exchanging heat from the warmest arterial blood that is just leaving the body, flowing towards the extremity. Once the arterial and venous blood reach the same temperature, no more heat can be exchanged and warmer blood must now flow through the extremities which would result in cooler blood entering the body.
Another example of adaptation to the cold in endothermic animals is thermogenesis (heat production). Two examples of this are shivering and non-shivering mechanisms; shivering is when muscles in the body spasmodically contract to create heat through movement. Non-shivering thermogenesis occurs when hormones cause mitochondria to increase their metabolic rate and generate heat instead of ATP (Campbell and Reece, 2008). Brown fat is a type of adipose tissue that is abundant with mitochondria and for this reason can generate more heat via non-shivering thermogenesis than can white fat (Stryer et al, 2002). Brown fat is found most commonly in newborn babies and in hibernating animals (Gesta et al, 2007). These are both very vulnerable animals as neither can move around significantly to generate enough heat to keep warm, so it is an important evolutionary advantage.
Some endothermic animals hibernate through the cold seasons. As already stated, hibernating animals have brown fat which is abundant in mitochondria which are needed for non-shivering thermogenesis. While in hibernation the animal's body's thermostat is turned off as they no longer need to try to maintain a high body temperature as they require a lot less energy for just sleeping. If the animal were to stay conscious during very cold climates it would need to ‘spend' 30 times as many calories to maintain their body temperature as if they were hibernating (Campbell and Reece, 2008).
Ectothermic animals rely on the environment to keep their body temperature constant. An example of this would be lizards in the Sahara desert; although it can be very hot during the daytime (up to 50˚C), at night temperatures can plummet to below freezing (WorldWildlife.org, 2001). So in the morning when the sun is rising lizards have developed a behavioural adaptation, which is to bask in the sun to absorb its heat.
“In the early 1960s, Hernon Dowling, at the Bronx zoo in New York, documented this phenomenon for a female Burmese python. Dowling found that the snake maintained a body temperature roughly 6˚C above that of the surrounding air during the month when she was incubating eggs. They found that pythons, like mammals, can generate heat through spasmodic muscle contraction –in other words, shivering (Campbell and Reece, 2008).” These findings show that particular ectotherms can thermoregulate in a way that endotherms can. Is this just in snakes? Just in reptiles? This is not known but realising this suggests that ectotherms and endotherms are not as different as they seem. Thermoregulatory shivering is therefore not just an adaptation specific to endotherms, but to some reptiles too.
Reptiles are not the only ectotherms that can ‘shiver'. Many flying insects shiver before they fly, so that they warm up their flight muscles before take off in order to enable them to fly even in very cold conditions (Campbell and Reece, 2008).
In this essay I have only touched upon methods that animals have developed for keeping warm. Different species of animals use different methods and have different adaptations to keeping warm.
Endothermic animals have both physiological and behavioural adaptations to keeping warm but it seems they are better physiologically adapted than are ectotherms. This is because endothermic animals rely on their own metabolic processes to keep their bodies warm and therefore need to have specially adapted physiological variations. Ectotherms on the other hand must rely on the environment and external influences to keep their body temperature up. To exploit the environmental sources of heat, ectothermic animals need to be aware of their surroundings and how to use them efficiently. Therefore, ectothermic animal's lifestyles and behaviour are more affected and well adapted for maintaining body temperature than are endotherms. This, however, is not to say that ectotherms' physiology is not adapted at all to maintain body temperature, or to say that endotherms' lifestyles are not affected at all; but that these two different varieties of animal (ectotherms and endotherms) have adapted in different ways to cope with cold climates. It shows the diversity of animals and how through evolution they have developed different ways of keeping warm.
Campbell, N.R., Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B. (2008). Biology, Eight Edition. Benjamin Cummings, San Francisco, US.
Gesta, S., Yu-Hua, T., Ronald Kahn1, C. (2007). Developmental Origin of Fat: Tracking Obesity to Its Source, Cell. 131 (2): 242-256.
Berg, J.M., Tymoczko, J.L., Stryer, L. (2002). Biochemistry, Fifth Edition. W.H. Freeman and Company: 41 Madison Avenue, New York NY10010.
World Wildlife Fund © (2001). Accessed 13/02/2010. http://www.worldwildlife.org/wildworld/profiles/terrestrial/pa/pa1327_full.html .