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Thermoregulation of Polar Vertebrates

Info: 4685 words (19 pages) Essay
Published: 11th Nov 2021 in Physiology

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Thermoregulation is the ability for an organism to maintain its own body temperature, regardless of the ambient temperature. While many organisms can regulate their own body temperature, the environment can be vastly different. With this in mind, how each species regulates their body temperature will differ. An organism living in the hot and humid Amazon rainforest will have to deal with different circumstances than an organism living in the cold and dry arctic.

This paper will be focusing on the latter of these two environments. Vertebrates found at the two ice caps such as, penguins, seals, and polar bears will be compared and contrasted on how they regulate their body temperatures.

Before the organisms can be looked at, an overview of their environment needs to be done. The Arctic pole is warmer than the Antarctic region. This is because more water is present, which retains more heat keeping the Arctic warmer (Bada,2018). The north pole has temperatures that range from -32 to 40 degrees Fahrenheit and the south pole has temperatures ranging from -18 to -76 degrees Fahrenheit (Bada,2018). Organisms in these two climates must deal with frigid temperatures.

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There is also little precipitation in both places. Antarctica only gets 7.87 inches of precipitation a year, classifying as a polar desert climate (Bada,2018). A polar desert is classified as an area that gets less than 9.84 inches of precipitation a year and its warmest month has an average temperature of fewer than 10 degrees Celsius (Bada,2018). Polar climates also have a permanent ice sheet. When temperatures never reach a point warm enough for the ice to melt, an ice sheet is formed. Because of this, ice sheets have been able to build up over millions of years and have become extremely thick (Bada,2018). Some ice sheets are several kilometers thick.

Plant species cannot survive due to the harsh climates that ice sheets need to be formed. Like most animal species, humans cannot survive these harsh conditions (Bada,2018). There are no permanent human settlements. However, temporary research stations can be found in polar climates. Polar climates and tundra climates share some similarities. The two climates also exhibit distinct differences. For example, tundra climates typically have a time period where the average temperature rises above the freezing point (Bada,2018). This is not found in polar climates. The warmer period also allows the ice to melt and not build up as well.

This enables more plants and animals to survive the harsh climate (Bada,2018).

Mammals can live in a bunch of harsh environments with morphological, physiological, or behavioral changes in response to their environment. Polar bears control their body temperatures through behavioral and physiological approaches based on their level of activity. In the journal "Thermoregulation in Resting and Active Polar Bears," by Robin Best, a study was done comparing how internal temperatures of polar bears differ when they are resting versus moving around, and what factors contribute to this. In the study, Best (1982) discussed differences polar bears have compared to other bears:

Unlike other North American bears, which generally spend the winter months in a state of reduced metabolic activity (Folk et al. 1972), the polar bear (Ursus maritimus) may be active throughout the year (Lono 1970). It thus shares with other arctic mammals the ability to maintain homeothermy in an environment with an annual range of ambient temperatures exceeding 80 °C. (p. 1)

Unlike other bears, polar bears do not hibernate and remain active all year around. The study found that 36-67% of the energy created by metabolism was lost as heat and lost about 8% to respiration. This means that the convective heat losses range from 33-64%. Walking caused the polar bears to become even cooler, and their bodies would have to compensate for this. The cooling of the surface of the bear caused by convection lead to heat loss of roughly 75% of the produced heat. There was only a 13-22% radiative loss, however. If wind speeds increased, the more this had an effect (Best,1982).

Polar bears have developed several was to combat heat loss by conduction and convection. Polar bear's fur has a high level of thermal conductivity. High thermal conductivity means that it can lose heat quickly. Polar bears also have two semi-circular, striated muscles that are highly vascularized in their latissimus region (Best,1982). Polar bears maintain different postures based on the current temperature. The pelt also has some unique features. The hair of a polar bear can either heat or help cool itself using light from the Sun. The hair of a polar bear is asymmetric, and this asymmetry is where the heating and cooling affect is derived from. If the UV light enters the hair perpendicular to its base, it will enter its excited state. This causes luminescence and will cause the heat to move towards the surface of the skin. However, if light moves toward the hair parallelly it will be absorbed by the tip of the hair, and the light will not provide much heat (Best, 1982). The absorption of the UV light is the cause of the yellow color their pelts can be observed as having. If a polar bear is photographed in UV light, it will cause the polar bear to appear black because of this effect (Best,1982).

Another polar vertebrate is penguins. Penguins are one of a few types of birds that does the possess the ability to fly. They can be found on the Southern hemisphere. They are most commonly found in Antarctica but can make their way to the tip of Africa. Penguins are from the family Spheniscidae and are considered aquatic birds (Chappell,1988). 17 species of penguins currently exist. They are not bound to cold environments. Some species, like the Galapagos Penguin, can be found at the equator. The biggest species of penguin is the emperor penguin. They weigh roughly 35 kg and are about 105 cm tall (Maho, 1976). The larger the species of penguin, the further south they are found normally found. Penguins spend about half of their lives in the water and spend the other half on land. The largest penguin ever was the same size as an average man (Maho, 1976).

Penguins have an internal temperature ranging from 37.8°C to 38.9°C. Since penguins live in such a cold environment, they require more energy to keep these temperatures (Pinshow, 1976). Overlapping feathers can be found in some species that create a surface that protects them from water and cold wind. In Antarctic water can be frigid, getting as low as -2.2ºC. 80 to 84% of thermal insulation for penguins are caused by tufts that can trap air (Pinshow, 1976). Penguins also increase their body temperature by absorbing the light from the sun using the dark feathers found on their backs. Penguins can also save heat by shivering in a similar fashion to humans. The downside to shivering is the penguins must expend energy to shiver. Penguins are also known for huddling into groups to stay warm (Maho, 1976).

Emperor penguins are a unique species of penguin. They are most known for their huddling technique to stay warm, and they are also the only penguin that breeds during the winter in Antarctica. Their huddling technique is not as simple as thousands of penguins standing in a large huddle. As one penguin moves, the rest must move to fill the spot it left. For emperor penguins, the male incubates the egg. Research has been done, and it shows that a penguin takes one step every 30 to 60 seconds (Maho, 1976). Each step is ranges from 2 to 4 inches. What has baffled researchers is how they are all able to move together as if they were a single entity. Using time-lapse camera footage, they have made mathematical models to gain a better understanding of the huddles. The models have shown that the huddle is like a wave that begins when an individual penguin moves. When several waves converge on each other, they merge instead of passing by each other.

A gap of only 0.8 inches will cause a penguin to move and begin one of these "w It is not understood why penguins move so often or why they take such small steps (Pinshow, 1976). A possible idea is that the penguins use the small movements to rotate the egg so one part of the egg does not get to cold. The huddles move in a spiral rotation and not in a straight line. The penguins use this technique for two main reasons. It provides them a source of wind protection, and it creates a warmer environment temperature. As a penguin gets colder, they move towards the center of the huddle. If a penguin gets to warm, they move towards the edge to cool down and allow other penguins to warm up.

A group of 5-10 penguins have a 39% reduced metabolic rate when compared to a penguin on its own. 32 of the 39% is due to the wind protection the huddle can provide (Maho, 1976). The benefit of having the amount of energy needed to maintain their internal body temperature is gained because of the reduction of exposed body surfaces and creating a warmer climate within the group. For every degree Celsius that the emperor penguin is able to maintain outside its body, creates a 7 to 17% reduction of lost energy (Maho, 1976). Emperor penguins use this huddling process to help maintain their internal body temperature, incubate their egg, and save energy throughout the winter. In the article "Thermoregulation in fasting emperor penguins under natural c by Maho, Declitte, and Chatonnet(1976), looks into how they are able male emperor penguins are able to survive the incubation time. Their study found that:

The males incubate the single egg while fasting for up to 4 mo and losing some 20 kg of their body mass. Fasting captive birds under outdoor conditions lost from 0.145 to 0.434 kg day -1. Mean resting metabolic rate, 49.06 W for 24.8 kg body mass, is 7 and 27%, respectively, higher than predicted from general metabolic equations for birds. Minimal thermal conductance, 1.31 W m-2 degrees C-1, is within the range for other birds. The lower critical temperature is about -10 degrees C; this can be related to large body size (20-40 kg) and to body shape, giving a smaller relative surface area than for other birds. Rigidity of the feathers explains why winds of moderate speed (up to 5 m s-1) have little effect on heat loss. At very low temperatures the behavior of huddling close together is essential in reducing metabolic rate. Without this behavior, survival during the long fast (up to four mo) at winter temperatures would be impossible. (p.1)

The study looked into how the penguins thermoregulate and what the exact effects on their bodies are.

Moving off the ice and into the water, there are several species of whales that are also found in the polar regions. Water conducts heat 25 times more than air. This makes it hard for whales to maintain a temperature of 37°C. Whales use blubber as their first line of defense to help insulate their body. Whales can have a layer of blubber that is up to 50 cm thick (Kasting, 1989).

Whales also have a unique vascular adaptation that differs from other marine mammals. Whales regulate blood circulation that allows them to control the amount of heat lost. The heat exchange system they use is counter current (Kasting, 1989). In a counter current system, the fluids flow in opposite directions. It is an efficient system. It allows the most possible heat to be transferred because the difference in temperature will be as high as it can be. The counter current networks are normally found in the extremities because they are poorly insulated. It works by having one artery wrapped by multiple veins (Kasting, 1989). Using heat recovery and air exchange, the arteries transfer the heat to the veins, so the heat can travel back to the body before they reach the capillaries in the extremities. This means the blood is as cold as it can be once it reaches the capillaries, so a minimal amount of heat is lost to the water. With the blood being rewarmed by the arteries before it reaches the internal organs, it will not lower the internal body temperature. This system can also be used to cool the body. The central artery has the ability to dilate and crush the surrounding veins (Kasting, 1989). This limits the amount of blood in the veins decreasing the amount of heat that can be exchanged between the artery and veins. Some whales have arteriovenous anastomosis. These are canals that connect a vein and artery (Kasting, 1989). They can only be found in the blubber and give the whale the ability to pass blood directly between the two, bypassing the capillaries. This is another way they can maintain their body heat since the blubber is a good insulator. Whales also have the ability to bypass these canals causing more blood to rush to the capillaries. This will decrease their body temperature and also cause their skin to turn a pinkish color (Kasting, 1989).

The shape of a whale's body creates a higher volume to surface area ratio that helps in heat exchange. If a warm spherical object is placed in cold water, the surrounding water and the sphere will attain thermal equilibrium (Kasting, 1989). Thermal equilibrium occurs when they both reach the same, intermediate temperature. Heat exchange has occurred at the surface of the sphere, at a rate that depends on the surface area of the object. If the diameter of the sphere is doubled, the time needed to reach equilibrium is increased. Most importantly, the time it takes to reach that equilibrium point is more than doubled. The amount of heat the sphere can transfer to the water depends on the volume of the sphere, while the surface is the determining factor for the rate the heat is transferred between the sphere and water. If the same experiment is run using a different shape that has the same volume, it will lose heat faster. A sphere minimizes surface-tovolume ratios. Independent of the shape, a larger object loses heat at a slower rate than a smaller object. For a specific volume, the more spherical the object is, the slower the rate that heat will be lost. This helps demonstrate why arctic whales are so large. Some being the largest to ever live on Earth, the blue whale and the fin whale, for example. Whale species that live in colder waters are typically bigger than those living in temperate/tropical environments. While their shape is not spherical, they do have a more round or spherical shape compared to other animals. They cannot be spherical because it would be impossible for them to swim effectively, since hydrodynamics requires a different shape compared to heat conservation. The shape of whales falls between the optimal shapes for heat conservation and streamlining (Kasting, 1989).

Seals can also survive these cold waters. When seals dive, they experience a few changes. These are called dive reflexes. The heart rate of a seal drops when it dives. This occurs because as the seal dives the blood vessels found in the periphery become restricted (Hind,1998).

The restriction of the blood vessels helps protect the seal from heat loss since less blood is located near the skin of the seal. An interesting fact is that, seals also stop breathing out of water, similarly to when they dive. Their heart-rate changes in a similar fashion. Oxygen used in metabolism is used similarly for all mammals. Oxygen will attach itself to the hemoglobin in red blood cells and will attach itself to myoglobin in muscle. It is also found in the lungs. When a seal dives, its lungs are nearly empty (Hansen,1995). Humans have a 7% blood by weight ratio, while seals have a 22% ratio. They also have 33% more hemoglobin in their blood compared to humans. The myoglobin in a seal's muscle is eight times denser than a human (Hansen,1995).

While a seal's lungs are empty when they dive, their body is still carrying a large supply of oxygen. On a typical dive, a seal will be able to use most of the oxygen in its body. These abilities allow them to dive and only resurface for a few seconds before going back under. A seal can dive to a depth of 3,000 feet on average. When in the ocean, a seal will spend 90% of its time under the surface (Hansen,1995).

Just like whales, seals also use blubber for thermoregulation. While blubber is a type of fat, it contains collagen fibers and blood vessels that fat typically lacks. The blubber helps insulate their internal organs since seals swim in water that can reach temperatures slightly below freezing. Blubber increases the buoyancy of seals. It also helps the body adjust its shape slightly to help pass through the water more easily (Hind, 1998). Blubber can also be metabolized to create energy. By being able to metabolize blubber, seals can go long periods of time fasting. An adult male's weight is roughly 50% blubber before breeding season begins. Seals lose roughly, 40% of their weight by the end of mating season due to blubber being metabolized. In the journal, "Are there thermoregulatory constraints on the timing of pupping for harbor seals?" it goes in depth on how energy is conserved based on when the harbor seals have their pups. This study. The study by Hind and Gurney (1998) found the following:

To test this hypothesis, we used the harbour seals in the Moray Firth as our case study. The model does predict an energetic cost resulting from thermoregulation during haul-out for a mother and her pup in the Moray Firth. Taking the mother and pup as a unit, we estimate the minimum cost during lactation. This combined cost, which must be met by the female seal, is similar to the minimum metabolic rate during haul-out for the summer predicted from the model. In winter, the predicted minimum metabolic rate exceeds the lactation cost, and an additional cost of thermoregulation results. The model predicts the most energetically favourable time for lactation to be June and July, and this is coincident with the timing of pupping in this seal population. We suggest that for harbour seals in Scotland, the timing of pupping may be influenced by the thermoregulation costs of haul-out. (p. 1)

Seals have a very specific time frame where they can most efficient have their pups. Having them outside of this time frame can make it hard for the adults, and thus the pups, to survive.

Seals, just like whales, also use a countercurrent heat-exchange, the same way whales do.

The seals have arteries located next to veins in their extremities. Similar to whales, as the warm blood passes through the arteries, the blood is cooled while warming the blood in the surrounding veins (Tarasoff, 1970). Seals also have to deal with a problem on land when they need to remove excess heat from their body. They can do this through a couple of processes. One cooling process is to use their flippers to toss wet sand onto their body. This causes evaporation to occur on their skin and helps cool them down. It is a similar process to sweating. Seals will occasionally raise a flipper. This cools them because there is almost no blubber beneath their flipper so the cold air will have more of an effect on their internal body temperature. Seals will also lay on the wet sand near the water's edge to help cool them off (Hansen,1995). This is similar to flipping the wet sand onto their body.

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A second white mammal, though much smaller, is the artic hare. 20% of an arctic hare's body mass is fat used for insulation. This is a pretty small amount for an animal in such a cold climate. The lack of weight, however, allows them to reach speeds of around 40 mph to avoid predators. They use behavioral, physiological, and anatomical adaptations that allow them to survive in such a cold climate. Similar to other animals, when compared to their warmer climate relatives, arctic hares have smaller limbs, ears and a stockier build. They maintain a good volume to surface area ratio to minimize the amount of heat lost. The arctic hare's coat is thick and is a good insulator. The hares found further southern grow two different coats throughout the year. For most of the year their coat is white, but it turns into a blue-grey color during the summer to help blend in with the environment better. Hares that are found further north have a white coat for the entire year. Their summer coat is shorter than the winter coat. They also have fur on their paws to help spread their weight while walking on soft snow. It also helps insulate them from the cold and provide a good grip on slippery surfaces. They also have a way of making their own shelter to help avoid some of the cold weather. Arctic hares will tunnel beneath the snow allowing them to avoid the wind and get some insulation from the cold arctic wind. The temperature in their hole is still below freezing, it can still be substantially higher than on the surface. Arctic hares also live in groups during the winter. This allows arctic hares to huddle for warmth. They also use it to help avoid predators. This is known as "flocking". Arctic hares create groups ranging from a dozen or so individual hares to over 3,000. The flock disperses in the spring when breeding starts.

Arctic foxes have developed several strategies to survive the cold environment similar to other mammals. Arctic foxes are rather common in the Arctic, but they are not only confined to this area. Arctic foxes are small and only weigh 3-4 kg (Prestrud,1991). Similar to the arctic hare, it is either a blue or white color depending on the time of year. During the colder months, it has a white color and a bluish color during the warmer months to help camouflage themselves (Prestrud,1991). These foxes have physiological, behavioral, and morphological adaptations to the low temperatures they face. They have to cope with temperatures of under -20°C for long periods of time. Like most mammals, as the temperature drops, the activity levels of the arctic fox drop as well (Prestrud,1991).

The arctic fox has two choices for regulating its body temperature. It either has to change the temperature gradient between the air and its body or the conductance of its fur. The arctic foxes change the conductance of its fur to stay warm during the cold months. Like the polar bear and arctic hear, the arctic fox can change the length/thickness of its coat gradually over time (Prestrud,1991). As the temperature begins to drop, its fur will become longer and thicker. For an arctic fox, its coat typically doubles in length during the winter months. The areas that see the largest increase are the areas that make contact with the ground while walking or laying down.

Other parts of the fox may see minor changes, but typically stay about the same all year around.

When the temperature begins to go back up, it will shed some of its fur. Similar to other animals in this paper, the arctic fox will vasoconstrict its blood vessels near its skin during the winter months to lessen the amount of heat lost to the air through its skin (Prestrud,1991).

Being small, along with the arctic hare, comes with a disadvantage. The fur depth has to be shorter compared to a larger animal or it will drag across the ground, hampering its mobility. Neither have good volume to surface area ratios (Prestrud,1991). This means they must deal with losing more internal heat compared to a large animal like a whale or seal. They can counteract this disadvantage during extreme weather. If the temperature gets cold enough, the fox will curl into a ball maximizing its volume to surface area ratio. While in the ball, it strategically covers the areas that have less dense fur and only exposes the parts of its body that has large amounts of fur. The difference in temperature between the air and the fox can reach 52°C when the fox is in this state (Prestrud,1991). The arctic fox is one of the smallest mammals located in the arctic region. This is because small mammals do not have the ability to insulate effectively enough to survive the harsh conditions. Most small mammals have to leave or dig below the snow, like the arctic hare. The arctic fox is able to avoid having its paws freeze in temperatures below -30°C by increasing its blood flow through the capillaries in the pads of its paws. Unlike other canids, the arctic fox has fur cover the pads on their paws (Prestrud,1991). Typically, fur can be found between the pads but not covering them. They also utilize the counter-current heat exchange system in its extremities to minimize the amount of heat lost. Like the arctic hare, the arctic fox will make dens inside the snow to avoid the cold arctic wind (Prestrud,1991).

To conclude, there are many adaptations that help different species to help them survive the harsh polar climates. The counter-current heat exchange is very commonly found in animals living in the extreme cold. It is the most efficient way to maintain the internal body temperature using the blood. Blubber, a type of insulating fat, is found in animals that spend a lot of time in the water, specifically whales and seals. Polar bears, arctic hares, and arctic foxes all have the ability to adjust their fur coats depending on the ambient temperature. The arctic hare and arctic fox have the addition ability to tunnel under the snow to avoid the wind and have slightly warmer ambient temperatures due to their smaller size. Penguins and arctic hares both have the ability to huddle to stay warm, but it is done in vastly different ways between the two species.

There are many species in the polar region, and they all rely on thermoregulation to survive the harsh environment.

References

Bada, Ferdinand. (2018, November 13). What Are The Features Of A Polar Climate? Retrieved from https://www.worldatlas.com/articles/what-are-the-features-of-a-polar-climate.html

Best, R. C. (1982). Thermoregulation in resting and active polar bears. Journal of Comparative Physiology ? B, 146(1), 63–73. doi: 10.1007/bf00688718

Chappell, M. A., & Souza, S. L. (1988). Thermoregulation, gas exchange, and ventilation in Adelie penguins (Pygoscelis adeliae). Journal of Comparative Physiology B, 157(6), 783–790. doi: 10.1007/bf00691009

Hansen, S., Lavigne, D. M., & Innes, S. (1995). Energy Metabolism and Thermoregulation in Juvenile Harbor Seals (Phoca vitulina) in Air. Physiological Zoology, 68(2), 290–315. doi: 10.1086/physzool.68.2.30166505

Hind, A., & Gurney, W. (1998). Are there thermoregulatory constraints on the timing of pupping for harbour seals? Canadian Journal of Zoology, 76(12), 2245–2254. doi: 10.1139/cjz-76-12-2245

Kasting, N. W., Adderley, S. A. L., Safford, T., & Hewlett, K. G. (1989). Thermoregulation in Beluga (Delphinapterus leucas) and Killer (Orcinus orca) Whales. Physiological Zoology, 62(3), 687–701. doi: 10.1086/physzool.62.3.30157921

Maho, Y. L., Delclitte, P., & Chatonnet, J. (1976). Thermoregulation in fasting emperor penguins under natural conditions. American Journal of Physiology-Legacy Content, 231(3), 913–922. doi: 10.1152/ajplegacy.1976.231.3.913

Pinshow, B., Fedak, M., Battles, D., & Schmidt-Nielsen, K. (1976). Energy expenditure for thermoregulation and locomotion in emperor penguins. American Journal of Physiology-Legacy Content, 231(3), 903–912. doi: 10.1152/ajplegacy.1976.231.3.903

Prestrud, P. (1991). Adaptation by the Arctic Fox (Alopex lagopus) to the Polar Winter. Arctic, 44(2). doi: 10.14430/arctic1529

Tarasoff, F. J., & Fisher, H. D. (1970). Anatomy of the hind flippers of two species of seals with reference to thermoregulation. Canadian Journal of Zoology, 48(4), 821–829. doi: 10.1139/z70144

 

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