assignment

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4.4 Adaptations to different temperatures allow terrestrial life to exist around the planet On Earth, land temperatures can reach as high as 57°C in Death Valley, California, and as low as −89°C in Antarctica. These extremes can limit the occurrence of life. To understand how organisms are affected by temperature and the adaptations that have evolved to deal with different temperatures, we first need to examine how the organisms gain and lose heat.

SOURCES OF HEAT GAIN AND LOSS

Because body temperature impacts physiological functions, organisms must manage heat gain and heat loss carefully. The ultimate source of heat at the surface of Earth is sunlight, most of which is absorbed by water, soil, plants, and animals and then converted to heat. Objects and organisms continuously exchange heat with their surroundings. When the temperature of the environment exceeds the temperature of an organism, the organism gains heat and becomes warmer. When the environment is cooler than the organism, the organism loses heat to the environment and cools. As illustrated in Figure 4.14, this exchange of heat can occur through four processes that can occur simultaneously: radiation, conduction, convection, and evaporation.

FIGURE 4.14 Sources of heat gain and loss. Radiation from the Sun can occur as direct sunlight, as well as sunlight that has been scattered as it interacts with gas molecules in the atmosphere or is reflected from clouds and the ground. Plants and animals in contact with rocks, soil, and each other can conduct heat to or from these objects, depending on whether their body temperatures are warmer or colder than the surrounding objects. As winds move the air past the organisms, there can be an additional exchange of heat, depending again on the temperature of

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the air compared to the temperature of the organism. Finally, organisms that experience evaporation can lose heat because evaporation requires heat energy. Yellow and orange wavy arrows depict sunlight in three different paths: Direct sunlight has no break in the wave path, reflected sunlight has a brake and changed direction in the wave path, scattered sunlight has multiple brakes in the waves path. Each sunlight path is directed toward the desert flora at the arrows end. A black wavy arrow points from the flora to the atmosphere is labeled, Heat loss from evaporation. A red wavy arrow from the land points to fauna and is labeled, Conduction. Two other wavy arrows pointing to the flora from the atmosphere and stone on the ground are labeled, Thermal radiation from the atmosphere and thermal radiation from objects on the ground, respectively. A white wavy arrow, initiating from wind, points toward the area across the landscape is labeled, Convection.

Radiation

Radiation is the emission of electromagnetic energy by a surface. The Sun is the primary source of radiation in the environment. As objects in the landscape are warmed by solar radiation, they emit lower-energy radiation in the form of infrared light. The temperature of the radiating surface determines how rapidly an object loses energy by radiation to colder parts of the environment. We measure in units of Kelvins (K), also known as absolute temperature, where 0°C = 273°K. The amount of heat radiation increases with the fourth power of absolute temperature. For example, we can compare the heat radiation of two small animals, such as a mouse and a lizard, as they lose heat to the environment. If the mouse has a skin temperature of 37°C (310°K) and the lizard has a skin temperature of 17°C (290°K), the difference in heat radiation between the mammal and the lizard is

This means that by having a 20°C higher body temperature, the mammal radiates 30 percent more heat to the environment than does the lizard.

The relatively high amount of heat radiation produced by animals with a higher body temperature than their external environment, such as endotherms, has been used by ecologists in a variety of research endeavors, including estimates of population sizes. When biologists need to count the number of moose living in remote regions of Alaska, for example, planes equipped with infrared cameras fly over these regions in the winter and the warm bodies of the moose stand out as a bright signal of infrared radiation against their cold, snowy background. Similar efforts have also been conducted to detect warm birds against a cold background environment, such as the penguins shown in Figure 4.15.

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FIGURE 4.15 Infrared images. Thermal cameras can detect warm animals radiating heat against a cold background, such as these king penguins (Aptenodytes patagonicus) in the Blijdrop Zoo, Rotterdam, Netherlands.

A thermal gradient scale to the right of the photo shows the temperatures ranging from 5.7 degrees Celsius at the bottom (black) to 31.3 degrees Celsius at the top (white), with labels 10, 20, and 30 degree Celsius. The gradient colors in between include dark blue from approximately 7 to 12 degrees Celsius, purple from approximately 13 to 17 degrees Celsius, red from approximately 18 to 20 degrees Celsius, orange from approximately 21 to 25 degrees Celsius, and yellow from approximately 26 to 29 degrees Celsius. The thermal photo reveals the penguin heads and the upper portions of the wings and joints to be the hottest parts of the body, while the tips of the wings and the legs are shown to be the coldest. The penguin bodies are approximately 17 degrees Celsius and lower.

Conduction

Conduction is the transfer of heat between objects that are in contact with one another, with heat moving from the warmer object to the cooler object. For example, lizards often lie flat on hot rocks to warm their bodies by conduction. The rate at which heat moves by conduction between an organism and its surroundings depends on three factors: its surface area, its resistance to heat transfer, and the temperature difference between the organism and its surroundings. An organism’s surface area helps determine its rate of heat conduction because a greater amount of exposed surface allows a greater surface for the energy transfer to take place. This is why many animals, including people, curl up in a ball to lessen the amount of exposed surface when they are trying to stay warm on a cold night.

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An organism’s resistance to heat transfer via conduction is just another way of saying how much insulation the organism has. Thick layers of fat, fur, or feathers have a high resistance to heat transfer and therefore slow the rate of heat loss due to conduction. Indeed, this is why you choose to wear insulated boots rather than walking barefoot in snow. Water is so much denser than air, and it conducts heat more than 20 times faster than air. As a result, you would lose body heat much faster if you stood in 10°C water than if you stood in 10°C air.

Finally, the rate of heat loss due to conduction is higher when there are large differences between the temperature of the organism and that of the environment. This feature of conduction is why some hibernating animals lower their body temperatures during the winter. A lower body temperature results in less heat loss to the cold external environment. We will discuss hibernation in much more detail in Chapter 5.

Convection

Convection is the transfer of heat by the movement of liquids and gases. For plants and animals, a boundary layer of air forms over a surface of organisms when the air is not moving. This is similar to the boundary layer of water that surrounds aquatic plants that we discussed in Chapter 3 and the boundary layer of hairy leaves that we discussed earlier in this chapter. Having a thicker boundary layer will tend to slow heat transfer between the air surrounding an organism and the air moving past an organism. When the environment is colder than the organism, the organism tends to warm this boundary layer, which in turn reduces the animal’s heat loss. If wind passes by the animal, the current disrupts the boundary layer and heat can be carried away from the body by convection. To combat this problem, birds can fluff their feathers and mammals can raise their hair to make a thicker boundary layer.

The convection of heat away from the body surface is the basis of the wind chill factor we hear about in winter on the evening weather report. Wind on a cold day makes you colder. For example, wind blowing at 32 km per hour in an air temperature of −7°C has the cooling power of still air at −23°C. This is the same reason that standing in front of a fan makes you feel cooler in the summer.

In the same way that air movement can remove heat from the surface of a warm organism, air movement can also add heat to an organism if the boundary layer is cooler than the surrounding air. For example, if you were to stand in a hot desert and your boundary layer was cooler than the air, a hot wind would disrupt the boundary layer between your skin and the air and make your body even hotter.

Evaporation

Evaporation is the transformation of a liquid to a gas with the input of heat energy. Because evaporation removes heat from a surface, it has a cooling effect on an organism. You have likely experienced this effect; when you are outside on a hot day, your body sweats and the evaporation of your sweat has a cooling effect. In a similar way, kangaroos living in the hot deserts of Australia can experience very high temperatures. One way that they cope with such high

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temperatures is by licking their legs, such that their saliva can evaporate and have a cooling effect. Just underneath the skin of their legs are blood vessels, so as the skin cools from evaporation, the kangaroo’s blood also cools and lowers the animal’s body temperature.

Evaporation can come at a substantial cost of water loss. As plants transpire and animals breathe, water evaporates from their exposed gas exchange surfaces, especially at higher temperatures. In dry air, the rate of evaporation nearly doubles with each 10°C increase in temperature. Under humid conditions with a lot of moisture in the air, evaporation occurs more slowly, which is why humid environments feel much hotter than dry environments when both are experienced at the same temperature.

BODY SIZE AND THERMAL INERTIA

Most exchanges of energy and materials between an organism and its environment occur across body surfaces. Therefore, the volume and surface of an organism affect the rate of these exchanges. As an example, let’s consider the differences between the body sizes of a mouse and an elephant. The elephant obviously has a much larger volume and it takes much more energy to meet its metabolic needs each day. However, relative to its volume, the elephant has a smaller surface area than the mouse. This relationship becomes more apparent if we make the simplifying assumption that all organisms are shaped like a box with sides of equal length. In this case, the surface area (SA) of one side of the box increases as the square of its length (L). Since a box has six sides, the total surface area of the cube is six times the surface area of one side. In contrast, the volume (V) of an organism increases as the cube of its length:

In short, as an organism grows larger, its volume grows faster than its surface area. You can see this graphically in Figure 4.16. Of course, organisms are not shaped like boxes, but the same principles apply.

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FIGURE 4.16 Increases in surface area and volume with length. If we represent organisms as boxes with equal sides, the volume of an organism increases with length faster than its surface area increases with length. This relationship has important effects on heat loss and gain for organisms of different sizes.

The horizontal axis represents length of one side of a cube (in centimeters) and ranges from 0 to 200, in increments of 20 centimeters. The vertical axis represents surface area (in centimeters squared) or volume (in centimeters cubed) and ranges from 0 to 8 million, in increments of 1 million. The positive curve for surface area starts at (0, 0), remains unchanged till (110, 0), and then slightly increases to end at (200, 0.2 million). The positive curve for volume starts at (10, 0), remains unchanged till (40, 0), and then gradually increases to end at (200, 8 million). All data are approximate. An illustration of a mouse is present on the bottom left and of an elephant is present at the top right.

The relationship between surface area and volume is particularly relevant when considering heat exchange. Because large organisms have a low ratio surface area to volume, larger individuals transfer heat across their surfaces less rapidly than smaller individuals. In general, this makes it easier for larger organisms to maintain constant internal temperatures in the face of varying external temperatures. The resistance to a change in temperature due to a large body volume is known as thermal inertia. Although thermal inertia can be an important advantage in cold environments, in hot environments, it causes moderately large individuals to have a harder time ridding themselves of excess heat. For this reason, large individuals run a greater risk of overheating. To cope with this risk, elephants have evolved large ears fed by numerous blood

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vessels. Flapping their large ears cools their blood and removes some excess heat from their body. Some very large animals can actually benefit from thermal inertia under hot environmental conditions because their bodies heat up more slowly. We saw an example of this in the case of the dromedary camels, whose very large bodies slowly added heat during the day but then released this heat during the night.

THERMOREGULATION

As we saw in Chapter 3, organisms can thermoregulate by maintaining a constant body temperature (homeotherms) or variable body temperatures (heterotherms and poikilotherms). We also discussed the idea that endotherms generate sufficient metabolic heat to raise body temperature higher than the external environment whereas ectotherms tend to have lower metabolic rates, so their body temperatures are largely determined by their external environment.

Ectotherms

Many species of terrestrial ectotherms adjust their heat balance behaviorally by moving into or out of shade, by changing their orientation to the Sun, or by adjusting their contact with warm substrates. When horned lizards are hot, for example, they decrease their exposure to the ground surface by standing erect on their legs. When they are cold, they lie flat against the ground and gain heat both by conduction from the ground and from direct solar radiation. This behavior, known as basking, is common among reptiles and insects (Figure 4.17). Endotherms can show similar behaviors; you may recall in our discussion of camels that they face into the Sun to present a smaller profile and reduce their heat intake. Ectothermic animals that bask in the radiation of the Sun can effectively regulate their body temperatures. Indeed, their temperatures may rise considerably above that of the surrounding air, well into the range of birds and mammals.

FIGURE 4.17 Basking. Ectotherms such as these painted turtles (Chrysemys picta) commonly lie in the sun to increase their body temperature.

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Some plants can occasionally generate enough heat to make their tissues substantially warmer than the external environment. The skunk cabbage (Symplocarpus foetidus), for example, is a foul-smelling plant that lives in wet soils in eastern North America (Figure 4.18). The odor attracts insect pollinators such as flies that typically feed on dead, rotting organisms. The skunk cabbage sprouts new leaves in early spring, even when snow still covers the ground. The plant’s mitochondria generate enough metabolic heat in its tissues to raise its temperature more than 10°C above the external environment. This incredible achievement requires a great deal of energy, but it provides a number of substantial benefits, including earlier flowering in the spring, more rapid development of flowers, and protection from freezing temperatures. In one species of skunk cabbage, scientists have discovered that generating heat also improves the rate of pollen germination and pollen tube growth in the flowers. The heat also benefits the pollinators, which can absorb some of the heat produced by the plant. Collectively, heat generation in plants can be very beneficial to both the plants and their pollinators.

Endotherms

As discussed in Chapter 3, endotherms such as mammals and birds maintain their body temperatures between 36°C and 41°C, even though the temperature of their surroundings may vary from −50°C to 50°C. Maintaining a higher body temperature allows endotherms to occupy

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environments that ectotherms are not able to occupy. However, sustaining internal conditions that differ significantly from conditions in the external environment requires a lot of work and energy. Consider the costs to birds and mammals of maintaining constant high body temperatures in cold environments. As air temperature decreases, the difference between the internal and external environments increases. Recall that heat is lost across body surfaces in direct proportion to this temperature difference. Consider, for example, an animal that maintains its body temperature at 40°C. At an outside temperature of 20°C, it loses heat much faster than it does at an outside temperature of 30°C. The greater the difference between an animal’s body temperature and the outside temperature, the greater the heat loss. One way for endotherms to gain heat is from solar radiation, conduction, or convection. Another way to gain heat is by generating metabolic heat. The rate of metabolism required to maintain a particular body temperature increases in direct proportion to the difference between the temperature of the body and the temperature of the environment. This means that while endotherms can perform well across a wide range of temperatures, they have to consume much more food than ectotherms do to maintain their thermoregulation.