Adaptations to different temperatures allow terrestrial life to exist around the planet

On Earth, land temperatures can reach as high as 58°C in northern Africa 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 they 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 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 3.18, this exchange of heat can occur through four processes: radiation, conduction, convection, and evaporation.

Figure 3.18 Sources of heat gain and loss. The Sun is the original source of almost all heat. The heat from the Sun is exchanged among objects across the landscape. In the case of the cactus, heat is gained by direct sunlight, scattered sunlight, and reflected sunlight. Heat can be lost by evaporation of water vapor to the atmosphere. Heat can also be gained or lost by radiation from surrounding objects such as rocks, by conduction where the cactus comes into contact with the soil, and by convection as winds move hot or cold air over the surface of the cactus and disrupt its boundary layer.

Radiation

Radiation is the emission of electromagnetic energy by a surface. The primary source of radiation in the environment is the Sun. As objects in the landscape are warmed by solar radiation, they emit more 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. So, for example, we can consider the heat radiation of two small animals such as a mouse and a lizard. If the mammal 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 than the lizard.

The relatively high amount of heat radiation produced by animals with a higher body temperature than their external environment 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.

Conduction

Conduction is the transfer of the kinetic energy of heat between substances that are in contact with one another. For example, lizards often lie flat on hot rocks to warm their bodies by conduction. Water, because it is so much denser than air, conducts heat more than 20 times faster. 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.

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 curl up in a ball to lessen the amount of exposed surface when they are trying to stay warm on a cold night. An organism’s resistance to heat transfer 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 conductance. Indeed, this is why you choose to wear insulated boots rather than walking barefoot in snow. Finally, the rate of heat loss is higher when there are large differences between the temperature of the organism and that of the environment. As we will see in Chapter 4, this feature of conductance 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.

Convection

Convection is the transfer of heat by the movement of liquids and gases: molecules of air or water next to a warm surface gain energy and move away from the surface. In still air, a boundary layer of air forms over a surface of organisms. Having a thicker boundary layer will tend to slow heat transfer between an organism and its environment. When the environment is colder than the organism, the organism tends to warm this boundary layer, effectively insulating itself against heat loss. If there is a current of air, it tends to disrupt the boundary layer so that heat can be carried away from the body by convection. This 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 feel as cold as you would on an even colder day without wind. 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.

In the same way, air movement can add heat to an organism if the boundary layer is cooler than the surrounding air. If you were to stand in a hot desert, and your boundary layer were cooler than the air, for example, 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 water from a liquid to a gaseous state with the input of heat energy. Evaporation removes heat from a surface. The evaporation of 1 g of water from a body’s surface removes 2.43 kilojoules (kJ) of heat when the temperature of the surface is 30°C. 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.

As summarized in Figure 3.18, all of these sources of heat gain and loss can occur simultaneously. 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 the air compared to the temperature of the organism. Finally, organisms that experience evaporation can lose heat because evaporation requires heat energy.

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 then 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 an organism increases as the square of its length (L), but the volume (V) of an organism increases as the cube of its length:

SA = L²

V = L³

In short, as an organism grows larger, its volume grows faster than its surface area.

Of course, organisms are not shaped like boxes, but the same principles apply to the shapes that organisms do have. Because an organism’s metabolic needs are related to its volume and volume increases faster than surface area, an organism’s metabolic needs increase faster than the surface area that exchanges energy and materials between the organism and its environment.

The relationship between surface area and volume is particularly relevant when considering heat exchange. Because large organisms have a low surface-to-volume ratio, larger individuals lose and gain heat across their surfaces less rapidly than smaller individuals. In general, larger sizes and lower surface-to-volume ratios make it easier for 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. However, very large animals can 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

The ability of an organism to control the temperature of its body is known as thermoregulation. Some organisms, known as homeotherms, maintain constant temperature conditions within the cells. Maintaining a constant internal body temperature allows an organism to adjust its biochemical reactions to work most efficiently. In contrast, poikilotherms do not have constant body temperatures. These terms tell us whether an organism’s temperature is constant or variable, not whether the temperature changes of the body are controlled internally or externally.

Ectotherms

Ectotherms have body temperatures that are largely determined by their external environment. Ectotherms tend to be organisms with low metabolic rates—such as reptiles, amphibians, and plants—or small body sizes—such as insects—that cannot generate or retain sufficient heat to offset heat losses across their surfaces.

Although ectotherms have body temperatures that match the temperature of their environment, they are not powerless to alter their body temperature. Indeed, many species of ectotherms adjust their heat balance behaviorally by moving into or out of shade, by changing their orientation with respect 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 widespread among reptiles and insects (Figure 3.19). 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. Some larger species of ectotherms, such as tuna, can generate a substantial amount of heat by exercising their massive muscles. This flexing of large muscles allows such fish to stay warmer than their external environment, making it possible for them to swim and feed in relatively cold waters.

Figure 3.19 Basking. Ectotherms such as these painted turtles (Chrysemys picta) commonly lie in the sun to increase their body temperature.

Photo by George Grall/National Geographic Stock.

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 3.20). 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.

Figure 3.20 Skunk cabbage. Using mitochondria to generate heat, the skunk cabbage can elevate its temperature by more than 10°C above the environmental temperature. The elevated temperature melts a path through the snow in early spring, making the skunk cabbage one of the first plants to sprout and attract pollinators to its flowers.

Photo by JAPACK/age fotostock.

In considering the various adaptations of ectotherms, we should note that they are not necessarily poikilotherms; their internal temperature may not vary a great deal. While it is true that their temperature is determined by the environment, the environmental temperature may not be highly variable. For example, fish living in the polar oceans experience very cold waters with little temperature variation. These fish are certainly ectotherms, but their body temperatures are nearly homeothermic.

Endotherms

Endotherms are organisms that can generate sufficient metabolic heat to raise body temperature higher than the external environment. Most 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. These organisms gain the benefit of accelerated biological activity in colder climates, where they may be better able to forage, escape predators, and compete with individuals other than ectotherms.

Sustaining internal conditions in organisms 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. This is because at 20°C the outside temperature is 20°C lower than its body temperature, whereas an outside temperature of 30°C is only 10°C lower than its body temperature. To maintain a constant body temperature, endothermic organisms must replace heat lost to their environment either by generating metabolic heat or by gaining heat through other means such as solar radiation, conduction, or convection. 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.

Adaptations of the Circulatory System

You have probably noticed that when walking on a cold day your hands and feet are the first body parts to become cold. Similarly, because the legs and feet of most birds do not have feathers, these extremities would be major potential sources of heat loss in cold regions if they were held at the same temperature as the rest of the body. Exposed extremities lose heat rapidly, due to their high ratio of surface area to volume. The conduction of heat, particularly from exposed extremities, works against the maintenance of a constant warm body temperature. Ectotherms and endotherms have evolved a number of adaptations to minimize the impact of chilled extremities and thereby to help maintain a warm temperature in the core of the body where many vital organs are located.

One prominent adaptation is blood shunting. Blood shunting occurs when specific blood vessels can be shut off—at locations called precapillary sphincters—so that less of the animals’ warm blood flows out to the cold extremities such as the forelimbs and hindlimbs. Instead, much of this blood is redirected into the veins before it ever reaches the extremities. From the veins, it returns to the heart, as shown in Figure 3.21. By sending less blood to nonvital areas such as the limbs, the blood experiences less cooling and allows the core of the animal’s body to maintain a constant internal temperature while expending less energy.

Figure 3.21 Blood shunting. In cold environments some animals can close certain blood vessels at their precapillary sphincters. This reduces the circular flow of blood from the arteries to the extremities and back to the veins, which limits the amount of chilled blood that returns to the heart.

Another adaptation to cold extremities is countercurrent circulation. In Chapter 2 we saw that fish maximize oxygen uptake by this mechanism; the blood in their gills flows in the opposite direction from water. A similar arrangement occurs with the position of veins and arteries in the extremities of many animals; arteries that carry warm blood away from the heart and toward the extremities are positioned alongside veins that carry chilled blood from the extremities back to the heart. Figure 3.22 shows an example. When a gull stands on ice or swims with its feet in frigid water, it conserves heat by using countercurrent circulation in its legs. Warm blood in arteries leading to the feet cools as it passes close to veins that return cold blood to the body. Rather than being lost to the environment, heat is transferred from the blood in the arteries to the blood in the veins. The feet themselves are kept only slightly above freezing, which minimizes heat transfer to the environment. The muscles used in swimming and walking are in the upper part of the leg, insulated by feathers that keep the upper legs close to the core body temperature.

Figure 3.22 Countercurrent blood circulation. The arteries in a gull’s leg that carry warm blood from the heart to the feet are positioned next to the veins that carry chilled blood away from the feet and back to the heart. This positioning of the arteries and veins allows the birds to transfer heat from arteries to the veins.

The diverse range of adaptations for life on land is a fascinating testament to the ability of natural selection to favor those traits that improve the fitness of organisms. Whether we consider the adaptation of plants to obtain water and nutrients, the variety of scenarios for conducting photosynthesis under different environmental conditions, or the ability of animals to balance water, salt, nitrogen, and heat, these adaptations have evolved to help make the transition from life in water to life on land. Throughout this book, we will return to many of these adaptations as we seek to understand the ecology of communities and ecosystems.

ECOLOGY TODAY CONNECTING THE CONCEPTS

THE CHALLENGE OF GROWING COTTON

Cotton plants. Cotton has numerous adaptations that help it cope with hot and dry conditions.

Photo by David Nance/ARS/USDA.

Throughout this chapter we have examined the challenges of the terrestrial environment and the adaptations that organisms have evolved to deal with these challenges. For plants, there are the challenges of extracting water and nutrients from the soil and then photosynthesizing at the cost of water lost due to transpiration. At the same time, plants need to balance their water and salts while also frequently coping with heat gains and losses that can place organisms at the limits of their temperature tolerance. To understand how an organism handles all of these challenges, we turn to the common cotton plant.

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Cotton is a major crop throughout the world because of its many uses that range from clothing to cottonseed oil. Cotton is typically grown in regions of the world that have warm temperatures with either high or low amounts of precipitation. In the United States, nearly half of the cotton is grown in the panhandle of Texas. Like many arid regions, the Texas Panhandle experiences droughts every few years. In 2011, Texas experienced one of the worst droughts ever recorded, and this made it a tough place to grow cotton.

Although C₃ plants like cotton are typically associated with moister habitats, cotton can also grow well in relatively dry places. However, extended droughts often cause farmers to irrigate cotton fields because young cotton plants have relatively small and shallow root systems that have a hard time acquiring water. As we saw with cacti and other succulent plants, shallow root systems can be effective at taking up water from brief rain events, but only if a plant can store excess water in its tissues. However, cotton has a limited ability to store water.

The soils in the Texas Panhandle range from loam to clay. Recall from our earlier discussion and Figure 3.4 that such soils have a moderate to high field capacity. However, they also have a moderate to high wilting point, which makes it hard for cotton to extract water from these soils. In addition, the shallow watering can lead to soil salinization.

In Texas, cotton is traditionally planted in early May and harvested in July or August. Because July and August can be very dry months, farmers have been experimenting with earlier planting dates. When cotton is planted in April, the cotton plants bloom in late June and produce a larger crop, in part because the plants avoid the drier months of July and August. Plants that are stressed by a lack of water can abort the development of their flowers, and flowers are the source of the cotton fibers. In addition, cotton planted earlier matures in June, when days are longer and the plants have more daylight hours to conduct photosynthesis. In 2010, researchers reported that while an earlier planting date is effective, it only works if the seedling plants are irrigated to supplement scarce natural rainwater. Without irrigation, these seedling plants that have a small root system cannot survive.

To help cotton farmers achieve larger crop yields, scientists have conducted a great deal of research on how to make cotton more drought resistant. Researchers have recently found that when cytokinin, a natural plant hormone, is sprayed onto young cotton plants, it induces the plants to grow larger root systems. By growing more roots that can penetrate deeper into the soil, the cotton is less prone to experience the effects of a water shortage. Cytokinin also stimulates the cotton plants to build a waxy outer coating, which we know makes plants less susceptible to water loss. These two responses provide a 5 to 10 percent increase in crop yields under drought conditions.

Cotton plants also have to deal with hot temperatures. Researchers have found that the ideal temperature for growing cotton is a daytime high of 28°C. However, temperatures where cotton is grown can easily exceed 38°C. At these higher temperatures, the enzyme Rubisco does not work as well. As we discussed, Rubisco is a key enzyme for C₃ photosynthesis; poorly performing Rubisco results in a lower rate of photosynthesis and, in turn, lower cotton yields. Plant breeding efforts have developed varieties of heat-tolerant cotton with variations of the Rubisco enzyme that continue to perform well under higher temperatures.

Other varieties of cotton have been bred to transpire higher amounts of water vapor out of their stomata, which improves the plant’s ability to cool itself through the process of evaporation. These varieties of cotton can only transpire more water vapor out of their stomata if they have an abundant supply of water coming in through their roots and must therefore be grown in soils that are either naturally moist or irrigated.

The story of cotton illustrates how adaptations allow a species to live under a range of challenging environmental conditions on land. It also illustrates how a knowledge of these adaptations can help farmers adjust their practices and help scientists cultivate varieties of plants that can better perform in the face of these challenges and produce higher crop yields. However, not all challenges faced by agricultural crops can be solved through plant breeding. When this happens, we are left to plant the crops in regions of the world where their adaptations are better suited to the environmental conditions.

SOURCES: Pettigrew, W. T. 2010. Impact of varying planting dates and irrigation regimes on cotton growth and lint yield production. Agronomy Journal 102: 1379–1387.

Salvucci, M. E., and S. J. Crafts-Brander. 2004. Inhibition of photosynthesis by heat stress: The activation state of Rubisco as a limiting factor in photosynthesis. Physiologia Plantarum 120: 179–186.