1
\n4.4 Adaptations to different temperatures allow
\nterrestrial life to exist around the planet
\nOn Earth, land temperatures can reach as high as 57\u00b0C in Death Valley, California, and as
\nlow as \u221289\u00b0C in Antarctica. These extremes can limit the occurrence of life. To understand
\nhow organisms are affected by temperature and the adaptations that have evolved to deal
\nwith different temperatures, we first need to examine how the organisms gain and lose heat.
\nSOURCES OF HEAT GAIN AND LOSS
\nBecause body temperature impacts physiological functions, organisms must manage heat gain
\nand heat loss carefully. The ultimate source of heat at the surface of Earth is sunlight, most of
\nwhich is absorbed by water, soil, plants, and animals and then converted to heat. Objects and
\norganisms continuously exchange heat with their surroundings. When the temperature of the
\nenvironment exceeds the temperature of an organism, the organism gains heat and becomes
\nwarmer. When the environment is cooler than the organism, the organism loses heat to the
\nenvironment and cools. As illustrated in Figure 4.14, this exchange of heat can occur through
\nfour processes that can occur simultaneously: radiation, conduction, convection,
\nand evaporation.
\nFIGURE 4.14 Sources of heat gain and loss. Radiation from the Sun can occur as direct
\nsunlight, as well as sunlight that has been scattered as it interacts with gas molecules in the
\natmosphere or is reflected from clouds and the ground. Plants and animals in contact with rocks,
\nsoil, and each other can conduct heat to or from these objects, depending on whether their body
\ntemperatures are warmer or colder than the surrounding objects. As winds move the air past the
\norganisms, there can be an additional exchange of heat, depending again on the temperature of
\n2
\nthe air compared to the temperature of the organism. Finally, organisms that experience
\nevaporation can lose heat because evaporation requires heat energy.
\nYellow and orange wavy arrows depict sunlight in three different paths: Direct sunlight has no
\nbreak in the wave path, reflected sunlight has a brake and changed direction in the wave path,
\nscattered sunlight has multiple brakes in the waves path. Each sunlight path is directed toward
\nthe desert flora at the arrows end. A black wavy arrow points from the flora to the atmosphere is
\nlabeled, Heat loss from evaporation. A red wavy arrow from the land points to fauna and is
\nlabeled, Conduction. Two other wavy arrows pointing to the flora from the atmosphere and stone
\non the ground are labeled, Thermal radiation from the atmosphere and thermal radiation from
\nobjects on the ground, respectively. A white wavy arrow, initiating from wind, points toward the
\narea across the landscape is labeled, Convection.
\nRadiation
\nRadiation is the emission of electromagnetic energy by a surface. The Sun is the primary source
\nof radiation in the environment. As objects in the landscape are warmed by solar radiation, they
\nemit lower-energy radiation in the form of infrared light. The temperature of the radiating
\nsurface determines how rapidly an object loses energy by radiation to colder parts of the
\nenvironment. We measure in units of Kelvins (K), also known as absolute temperature, where
\n0\u00b0C = 273\u00b0K. The amount of heat radiation increases with the fourth power of absolute
\ntemperature. For example, we can compare the heat radiation of two small animals, such as a
\nmouse and a lizard, as they lose heat to the environment. If the mouse has a skin temperature of
\n37\u00b0C (310\u00b0K) and the lizard has a skin temperature of 17\u00b0C (290\u00b0K), the difference in heat
\nradiation between the mammal and the lizard is
\nThis means that by having a 20\u00b0C higher body temperature, the mammal radiates 30 percent
\nmore heat to the environment than does the lizard.
\nThe relatively high amount of heat radiation produced by animals with a higher body
\ntemperature than their external environment, such as endotherms, has been used by ecologists in
\na variety of research endeavors, including estimates of population sizes. When biologists need to
\ncount the number of moose living in remote regions of Alaska, for example, planes equipped
\nwith infrared cameras fly over these regions in the winter and the warm bodies of the moose
\nstand out as a bright signal of infrared radiation against their cold, snowy background. Similar
\nefforts have also been conducted to detect warm birds against a cold background environment,
\nsuch as the penguins shown in Figure 4.15.
\n3
\nFIGURE 4.15 Infrared images. Thermal cameras can detect warm animals radiating heat
\nagainst a cold background, such as these king penguins (Aptenodytes patagonicus) in the
\nBlijdrop Zoo, Rotterdam, Netherlands.
\nA thermal gradient scale to the right of the photo shows the temperatures ranging from 5.7
\ndegrees Celsius at the bottom (black) to 31.3 degrees Celsius at the top (white), with labels 10,
\n20, and 30 degree Celsius. The gradient colors in between include dark blue from approximately
\n7 to 12 degrees Celsius, purple from approximately 13 to 17 degrees Celsius, red from
\napproximately 18 to 20 degrees Celsius, orange from approximately 21 to 25 degrees Celsius,
\nand yellow from approximately 26 to 29 degrees Celsius. The thermal photo reveals the penguin
\nheads and the upper portions of the wings and joints to be the hottest parts of the body, while the
\ntips of the wings and the legs are shown to be the coldest. The penguin bodies are approximately
\n17 degrees Celsius and lower.
\nConduction
\nConduction is the transfer of heat between objects that are in contact with one another, with heat
\nmoving from the warmer object to the cooler object. For example, lizards often lie flat on hot
\nrocks to warm their bodies by conduction. The rate at which heat moves by conduction between
\nan organism and its surroundings depends on three factors: its surface area, its resistance to heat
\ntransfer, and the temperature difference between the organism and its surroundings. An
\norganism\u2019s surface area helps determine its rate of heat conduction because a greater amount of
\nexposed surface allows a greater surface for the energy transfer to take place. This is why many
\nanimals, including people, curl up in a ball to lessen the amount of exposed surface when they
\nare trying to stay warm on a cold night.
\n4
\nAn organism\u2019s resistance to heat transfer via conduction is just another way of saying how much
\ninsulation the organism has. Thick layers of fat, fur, or feathers have a high resistance to heat
\ntransfer and therefore slow the rate of heat loss due to conduction. Indeed, this is why you
\nchoose to wear insulated boots rather than walking barefoot in snow. Water is so much denser
\nthan air, and it conducts heat more than 20 times faster than air. As a result, you would lose body
\nheat much faster if you stood in 10\u00b0C water than if you stood in 10\u00b0C air.
\nFinally, the rate of heat loss due to conduction is higher when there are large differences between
\nthe temperature of the organism and that of the environment. This feature of conduction is why
\nsome hibernating animals lower their body temperatures during the winter. A lower body
\ntemperature results in less heat loss to the cold external environment. We will discuss
\nhibernation in much more detail in Chapter 5.
\nConvection
\nConvection is the transfer of heat by the movement of liquids and gases. For plants and animals,
\na boundary layer of air forms over a surface of organisms when the air is not moving. This is
\nsimilar to the boundary layer of water that surrounds aquatic plants that we discussed in Chapter
\n3 and the boundary layer of hairy leaves that we discussed earlier in this chapter. Having a
\nthicker boundary layer will tend to slow heat transfer between the air surrounding an organism
\nand the air moving past an organism. When the environment is colder than the organism, the
\norganism tends to warm this boundary layer, which in turn reduces the animal\u2019s heat loss. If
\nwind passes by the animal, the current disrupts the boundary layer and heat can be carried away
\nfrom the body by convection. To combat this problem, birds can fluff their feathers and
\nmammals can raise their hair to make a thicker boundary layer.
\nThe convection of heat away from the body surface is the basis of the wind chill factor we hear
\nabout in winter on the evening weather report. Wind on a cold day makes you colder. For
\nexample, wind blowing at 32 km per hour in an air temperature of \u22127\u00b0C has the cooling power of
\nstill air at \u221223\u00b0C. This is the same reason that standing in front of a fan makes you feel cooler in
\nthe summer.
\nIn the same way that air movement can remove heat from the surface of a warm organism, air
\nmovement can also add heat to an organism if the boundary layer is cooler than the surrounding
\nair. For example, if you were to stand in a hot desert and your boundary layer was cooler than the
\nair, a hot wind would disrupt the boundary layer between your skin and the air and make your
\nbody even hotter.
\nEvaporation
\nEvaporation is the transformation of a liquid to a gas with the input of heat energy. Because
\nevaporation removes heat from a surface, it has a cooling effect on an organism. You have likely
\nexperienced this effect; when you are outside on a hot day, your body sweats and the evaporation
\nof your sweat has a cooling effect. In a similar way, kangaroos living in the hot deserts of
\nAustralia can experience very high temperatures. One way that they cope with such high
\n5
\ntemperatures is by licking their legs, such that their saliva can evaporate and have a cooling
\neffect. Just underneath the skin of their legs are blood vessels, so as the skin cools from
\nevaporation, the kangaroo\u2019s blood also cools and lowers the animal\u2019s body temperature.
\nEvaporation can come at a substantial cost of water loss. As plants transpire and animals breathe,
\nwater evaporates from their exposed gas exchange surfaces, especially at higher temperatures. In
\ndry air, the rate of evaporation nearly doubles with each 10\u00b0C increase in temperature. Under
\nhumid conditions with a lot of moisture in the air, evaporation occurs more slowly, which is why
\nhumid environments feel much hotter than dry environments when both are experienced at the
\nsame temperature.
\nBODY SIZE AND THERMAL INERTIA
\nMost exchanges of energy and materials between an organism and its environment occur across
\nbody surfaces. Therefore, the volume and surface of an organism affect the rate of these
\nexchanges. As an example, let\u2019s consider the differences between the body sizes of a mouse and
\nan elephant. The elephant obviously has a much larger volume and it takes much more energy to
\nmeet its metabolic needs each day. However, relative to its volume, the elephant has a smaller
\nsurface area than the mouse. This relationship becomes more apparent if we make the
\nsimplifying assumption that all organisms are shaped like a box with sides of equal length. In
\nthis case, the surface area (SA) of one side of the box increases as the square of its length (L).
\nSince a box has six sides, the total surface area of the cube is six times the surface area of one
\nside. In contrast, the volume (V) of an organism increases as the cube of its length:
\nIn short, as an organism grows larger, its volume grows faster than its surface area. You can see
\nthis graphically in Figure 4.16. Of course, organisms are not shaped like boxes, but the same
\nprinciples apply.
\n6
\nFIGURE 4.16 Increases in surface area and volume with length. If we represent organisms as
\nboxes with equal sides, the volume of an organism increases with length faster than its surface
\narea increases with length. This relationship has important effects on heat loss and gain for
\norganisms of different sizes.
\nThe horizontal axis represents length of one side of a cube (in centimeters) and ranges from 0 to
\n200, in increments of 20 centimeters. The vertical axis represents surface area (in centimeters
\nsquared) or volume (in centimeters cubed) and ranges from 0 to 8 million, in increments of 1
\nmillion. The positive curve for surface area starts at (0, 0), remains unchanged till (110, 0), and
\nthen slightly increases to end at (200, 0.2 million). The positive curve for volume starts at (10, 0),
\nremains unchanged till (40, 0), and then gradually increases to end at (200, 8 million). All data
\nare approximate. An illustration of a mouse is present on the bottom left and of an elephant is
\npresent at the top right.
\nThe relationship between surface area and volume is particularly relevant when considering heat
\nexchange. Because large organisms have a low ratio surface area to volume, larger individuals
\ntransfer heat across their surfaces less rapidly than smaller individuals. In general, this makes it
\neasier for larger organisms to maintain constant internal temperatures in the face of varying
\nexternal temperatures. The resistance to a change in temperature due to a large body volume is
\nknown as thermal inertia. Although thermal inertia can be an important advantage in cold
\nenvironments, in hot environments, it causes moderately large individuals to have a harder time
\nridding themselves of excess heat. For this reason, large individuals run a greater risk of
\noverheating. To cope with this risk, elephants have evolved large ears fed by numerous blood
\n7
\nvessels. Flapping their large ears cools their blood and removes some excess heat from their
\nbody. Some very large animals can actually benefit from thermal inertia under hot environmental
\nconditions because their bodies heat up more slowly. We saw an example of this in the case of
\nthe dromedary camels, whose very large bodies slowly added heat during the day but then
\nreleased this heat during the night.
\nTHERMOREGULATION
\nAs we saw in Chapter 3, organisms can thermoregulate by maintaining a constant body
\ntemperature (homeotherms) or variable body temperatures (heterotherms and poikilotherms). We
\nalso discussed the idea that endotherms generate sufficient metabolic heat to raise body
\ntemperature higher than the external environment whereas ectotherms tend to have lower
\nmetabolic rates, so their body temperatures are largely determined by their external environment.
\nEctotherms
\nMany species of terrestrial ectotherms adjust their heat balance behaviorally by moving into or
\nout of shade, by changing their orientation to the Sun, or by adjusting their contact with warm
\nsubstrates. When horned lizards are hot, for example, they decrease their exposure to the ground
\nsurface by standing erect on their legs. When they are cold, they lie flat against the ground and
\ngain heat both by conduction from the ground and from direct solar radiation. This behavior,
\nknown as basking, is common among reptiles and insects (Figure 4.17). Endotherms can show
\nsimilar behaviors; you may recall in our discussion of camels that they face into the Sun to
\npresent a smaller profile and reduce their heat intake. Ectothermic animals that bask in the
\nradiation of the Sun can effectively regulate their body temperatures. Indeed, their temperatures
\nmay rise considerably above that of the surrounding air, well into the range of birds and
\nmammals.
\nFIGURE 4.17 Basking. Ectotherms such as these painted turtles (Chrysemys picta) commonly
\nlie in the sun to increase their body temperature.
\n8
\nSome plants can occasionally generate enough heat to make their tissues substantially warmer
\nthan the external environment. The skunk cabbage (Symplocarpus foetidus), for example, is a
\nfoul-smelling plant that lives in wet soils in eastern North America (Figure 4.18). The odor
\nattracts insect pollinators such as flies that typically feed on dead, rotting organisms. The skunk
\ncabbage sprouts new leaves in early spring, even when snow still covers the ground. The plant\u2019s
\nmitochondria generate enough metabolic heat in its tissues to raise its temperature more than
\n10\u00b0C above the external environment. This incredible achievement requires a great deal of
\nenergy, but it provides a number of substantial benefits, including earlier flowering in the spring,
\nmore rapid development of flowers, and protection from freezing temperatures. In one species of
\nskunk cabbage, scientists have discovered that generating heat also improves the rate of pollen
\ngermination and pollen tube growth in the flowers. The heat also benefits the pollinators, which
\ncan absorb some of the heat produced by the plant. Collectively, heat generation in plants can be
\nvery beneficial to both the plants and their pollinators.
\nEndotherms
\nAs discussed in Chapter 3, endotherms such as mammals and birds maintain their body
\ntemperatures between 36\u00b0C and 41\u00b0C, even though the temperature of their surroundings may
\nvary from \u221250\u00b0C to 50\u00b0C. Maintaining a higher body temperature allows endotherms to occupy
\n9
\nenvironments that ectotherms are not able to occupy. However, sustaining internal conditions
\nthat differ significantly from conditions in the external environment requires a lot of work and
\nenergy. Consider the costs to birds and mammals of maintaining constant high body
\ntemperatures in cold environments. As air temperature decreases, the difference between the
\ninternal and external environments increases. Recall that heat is lost across body surfaces in
\ndirect proportion to this temperature difference. Consider, for example, an animal that maintains
\nits body temperature at 40\u00b0C. At an outside temperature of 20\u00b0C, it loses heat much faster than it
\ndoes at an outside temperature of 30\u00b0C. The greater the difference between an animal\u2019s body
\ntemperature and the outside temperature, the greater the heat loss. One way for endotherms to
\ngain heat is from solar radiation, conduction, or convection. Another way to gain heat is by
\ngenerating metabolic heat. The rate of metabolism required to maintain a particular body
\ntemperature increases in direct proportion to the difference between the temperature of the body
\nand the temperature of the environment. This means that while endotherms can perform well
\nacross a wide range of temperatures, they have to consume much more food than ectotherms do
\nto maintain their thermoregulation.<\/p>\n
——–<\/p>\n
1<\/p>\n
4.4 Temperature adaptations allow terrestrial life to exist all across the world.<\/p>\n
On Earth, land temperatures can reach as high as 57\u00b0C in Death Valley, California, and as low as 32\u00b0C in the Arctic Circle.<\/p>\n
Antarctica has temperatures as low as 89\u00b0C. These extremes have the potential to limit the occurrence of life. To comprehend<\/p>\n
how temperature affects organisms and the mechanisms that have evolved to deal with it<\/p>\n
With varying temperatures, we must first investigate how organisms gain and release heat.<\/p>\n
HEAT GAIN AND LOSS SOURCES<\/p>\n
Organisms must manage heat buildup because body temperature affects physiological functions.<\/p>\n
and heat loss with caution. The ultimate source of heat at the Earth’s surface is sunlight, which accounts for the majority of it.<\/p>\n
which is taken up by water, soil, plants, and animals and transformed to heat Items and<\/p>\n","protected":false},"excerpt":{"rendered":"
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