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Because the rates of many biological processes are af-fected by temperature, it would be advantageous for any animal to be able to control its body temperature.

However, the high heat capacity and heat conductivity of water make it diffi cult for most fi shes or aquatic amphibians to maintain a temperature diff erence between their bodies and the surrounding water.

Air has both a lower heat capacity and a lower heat conductivity than water, however, and the body tem-peratures of most terrestrial vertebrates are at least partly independent of the air temperature.

Many terrestrial vertebrates and some aquatic verte-brates do have body temperatures substantially above the temperature of the air or water around them.

Maintaining those temperature differences requires Nitrogenous Wastes Diff erences in how nitrogenous

wastes are excreted are partly a matter of the avail-ability of water and partly the result of diff erences among phylogenetic lineages. Most vertebrates elimi-nate nitrogen as ammonia, as urea, or as uric acid.

Excreting nitrogenous wastes primarily as ammonia is called ammonotelism , excretion primarily as urea is ureotelism , and excretion primarily as uric acid is uricotelism . Most vertebrates excrete a mixture of ni-trogenous waste products, with diff erent proportions of the three compounds.

Ammonotely Bony fi shes are primarily ammonotelic and excrete ammonia through the skin and gills as well as in urine. Because ammonia is produced by deami-nation of proteins, no metabolic energy is needed to produce it.

Ureotely Mammals are primarily ureotelic, although they excrete some nitrogenous wastes as ammonia and uric acid. Normal values for humans, for example, are 82 percent urea and 2 percent each ammonia and uric acid. The remaining 14 percent of nitrogenous waste is composed of other nitrogen-containing compounds, primarily amino acids and creatine. Urea is synthe-sized from ammonia in a cellular enzymatic process called the urea cycle . Urea synthesis requires more en-ergy than does ammonia production, but urea is less toxic than ammonia. Because urea is not very toxic, it can be concentrated in urine, thus conserving water.

Uricotely Reptiles, including birds, are primarily urico-telic, but here again all three of the major nitrogenous compounds are present. The pathway for synthesis of uric acid is complex and requires more energy than synthesis of urea. The advantage of uric acid lies in its low solubility: it precipitates from the urine and is excreted as a semisolid paste. The water that was released when the uric acid precipitated is reabsorbed, so uricotely is an excellent method of excreting nitrog-enous wastes while conserving water. Some species of reptiles change the proportions of the three com-pounds depending on the water balance of the animal, excreting more ammonia and urea when water is plen-tiful and shifting toward uric acid when it is necessary to conserve water.

4.5 Responses to Temperature

Vertebrates occupy habitats that extend from cold polar latitudes to hot deserts. To appreciate their adaptability, we must consider how temperature af-fects an aquatic vertebrate such as a fi sh or amphib-ian that has little capacity to maintain a diff erence

thermoregulatory mechanisms, and these are well de-veloped among vertebrates.

Poikilothermy and Homeothermy Vertebrates were com-monly described as poikilotherms (Greek poikilo = variable and therm = heat) and homeotherms (Greek homeo = the same) through the middle of the twentieth century. Poikilotherms were animals with variable body temperatures, and homeotherms were animals with sta-ble body temperatures. Fishes, amphibians, and reptiles were called poikilotherms, and birds and mammals were homeotherms. Th is terminology has become less appro-priate as our knowledge of the temperature-regulating capacities of animals has become more sophisticated.

Poikilothermy and homeothermy describe the variabil-ity of body temperature, and these terms cannot readily be applied to groups of animals.

For example, some mammals allow their body tem-peratures to drop 20°C or more from their normal lev-els at night and in the winter, whereas many fi shes live in water that changes temperature less than 2°C in an entire year. Th at example presents the contradictory situation of a homeotherm that experiences 10 times as much variation in body temperature as a poikilotherm.

Ectothermy and Endothermy Complications like these make it very hard to use the words homeotherm and poi-kilotherm rigorously. Most biologists concerned with temperature regulation prefer the terms ectotherm and endotherm . These terms are not synonymous with the earlier words because, instead of referring to the vari-ability of body temperature, they refer to the sources of energy used in thermoregulation.

Ectotherms (Greek ecto = outside) gain their heat largely from external sources—by basking in the sun, for example, or by resting on a warm rock. Endotherms (Greek endo = inside) largely depend on metabolic pro-duction of heat to raise their body temperatures. The source of heat used to maintain body temperature is the major diff erence between ectotherms and endo-therms because their body temperatures are quite sim-ilar. Terrestrial ectotherms (like lizards and turtles) and endotherms (like birds and mammals) have activ-ity temperatures between 30°C and 40°C.

Endothermy and ectothermy are not mutually exclu-sive mechanisms of temperature regulation, and many animals use them in combination. In general, birds and mammals are primarily endothermal, but some species make extensive use of external sources of heat.

For example, roadrunners are predatory birds living in the deserts of the southwestern United States and ad-jacent Mexico. On cold nights, roadrunners allow their body temperatures to fall from the normal level of 38°C or 39°C down to 35°C or lower. In the mornings they

10 organisms. (a) A hypothetical reaction for which the rate initially increases and then falls as temperature rises. Between 10°C and 20°C the rate doubles from 50 to 100, which is a Q ₁₀ of 2.0. Th e rate does not change between 20°C and 30°C, so the Q ₁₀ for this temperature range is 1.0. Th e rate drops from 100 units at 30°C to 50 units at 40°C, so the Q ₁₀ is 0.5. (b) Th e maximum swim-ming speed of a goldfi sh increases up to about 30°C and then falls. (c) Spontaneous activity by a goldfi sh peaks at around 20°C and falls as the temperature increases. (d) Activity of the enzyme lactic dehydrogenase from a lungfi sh increases slowly from 25°C to 35°C ( Q ₁₀ about 1.2), more steeply between 35°C and its maxi-mum at 40°C ( Q ₁₀ about 1.4), and declines rapidly between 40°C and 50°C ( Q ₁₀ about 0.4).

by myoglobin-rich swimming muscles located close to the vertebral column ( Figure 4–14 ). The temperature of these muscles is held near 30°C at water temperatures from 7°C to 23°C. Additional heat exchangers are found in the brains and eyes of tunas and sharks, and these organs are warmer than water temperature but some-what cooler than the swimming muscles.

Hot Eyes Th e billfi shes have a somewhat diff erent arrangement in which only the brain and eyes are warmed, and the source of heat is a muscle that has changed its function from contraction to heat pro-duction. The superior rectus eye muscle of these bill-fi shes has been extensively modibill-fi ed. Mitochondria occupy more than 60 percent of the cell volume, and changes in cell structure and biochemistry result in the release of heat by the calcium-cycling mechanism that is usually associated with contraction of muscles.

A related scombroid, the butterfl y mackerel, has a thermogenic organ with the same structural and bio-chemical characteristics found in billfi shes, but in the mackerel it is the lateral rectus eye muscle that has been modifi ed.

An analysis of the phylogenetic relationships of scombroid fi shes by Barbara Block and her colleagues suggests that endothermal heat production has arisen independently three times in the lineage—once in the common ancestor of the living billfi shes (by modifi -cation of the superior rectus eye muscle), once in the butterfl y mackerel lineage (by modifi cation of the lat-eral rectus eye muscle), and a third time in the com-mon ancestor of tunas and bonitos (by development of countercurrent heat exchangers in muscle, viscera, and brain, and development of red muscle along the hori-zontal septum of the body).

Th e ability of these fi shes to keep parts of the body warm may allow them to venture into cold water that would otherwise interfere with body functions. Block has pointed out that modifi cation of the eye muscles and the capacity for heat production among scombroids is related to the temperature of the water in which they swim and capture prey. The metabolic capacity of the heater cells of the butterfl y mackerel, which is the spe-cies that occurs in the coldest water, is the highest of all vertebrates. Swordfi shes, which dive to great depths and spend several hours in water temperatures of 10°C or lower, have better-developed heater organs than do marlins, sailfi shes, and spearfi shes, which spend less time in cold water.

Marine Mammals Th e temperature equilibration of blood and water that occurs in the gills is the primary obstacle to whole-body endothermy for fi sh. Counter-current systems allow them to keep critical parts of bask in the sun, raising the feathers on their backs to

expose an area of black skin. Calculations indicate that a roadrunner can save 132 joules per hour by using solar energy instead of metabolism to raise its body temperature.

Deviations from general patterns of temperature regulation go the other way as well. Snakes are nor-mally ectothermal, but the females of several species of pythons coil around their eggs and produce heat by rhythmic contraction of their trunk muscles. The rate of contraction increases as air temperature falls, and a female Indian python is able to maintain her eggs close to 30°C at air temperatures as low as 23°C. Th is heat production entails a substantial increase in the py-thon’s metabolic rate—at 23°C, a female python uses about 20 times as much energy when she is brooding as she does normally. Th us, generalizations about the body temperatures and thermoregulatory capacities of vertebrates must be made cautiously, and the actual mechanisms used to regulate body temperature must be studied carefully.

Regional Heterothermy Regulation of body tempera-ture is not an all-or-nothing phenomenon for verte-brates. Regional heterothermy is a general term used to refer to diff erent temperatures in diff erent parts of an animal’s body. Dramatic examples of regional heterothermy are found in several fi shes that main-tain some parts of their bodies at temperatures 15°C warmer than the water in which they are swimming.

Th at’s a remarkable accomplishment for a fi sh because each time the blood passes through the gills it comes into temperature equilibrium with the water. Th us, to raise its body temperature by using endothermal heat production, a fi sh must limit the loss of heat to the water via the gills.

Warm Muscles Th e mechanism used to retain heat is a countercurrent system of blood fl ow in retia mirabilia.

As cold arterial blood from the gills enters the warm part of the body, it fl ows through a rete and is warmed by heat from the warm venous blood that is leaving the tissue. Th is arrangement is found in some sharks, es-pecially species in the family Lamnidae (including the mako, great white shark, and porbeagle), which have retia mirabilia in the trunk. These retia retain the heat produced by activity of the swimming muscles, with the result that those muscles are kept 5°C to 10°C warmer than water temperature.

Scombroid fi shes, a group of teleosts that includes the mackerels, tunas, and billfi shes (i.e., the swordfi sh and spearfi sh as well as the sailfi shes and marlins), have also evolved endothermal heat production. Tunas have an arrangement of retia that retains the heat produced

For terrestrial mammals, the furry body covering (the pelage ) traps metabolic heat in the dead air spaces be-tween hairs and reduces the movement of heat out of the body, just as the air trapped between strands of fi berglass insulations reduces the movement of heat through the wall of a building. A furry body covering their bodies warm, but other parts are at water

tem-perature. Air-breathing aquatic tetrapods avoid that problem because they have lungs instead of gills.

Mammals are endotherms, and their high metabolic rates combined with the muscular metabolism that accompanies activity release heat that warms the body.

30

20

20 15

10

5 25 30

35

25

Muscle temperature (°C)

Water temperature (°C) T_m= 25.3 + 0.24 T_w

Tmuscle

=T^water

(c)

(a) (b)

Rete region

21.3° 23.3° 25.3° 27.3°

21.3° 23.3° 25.3° 27.3°

29.3° 31.4°

Water 19.3°

Figure 4–14 Details of body temperature regulation by the bluefin tuna. _{(a) Th e red} muscle and retia are located adjacent to the vertebral column. (b) Cross-sectional views showing the temperature gradient between the core (at 31.4°C) and water temperature (19.3°C). (c) Core muscle temperatures of bluefi ns compared to water temperature.

total surface area is 6 cm ² . The volume of that cube is one cubic centimeter (1 cm . 1 cm . 1 cm), and the ratio of surface to volume is 6 cm ² /1 cm ³ ( Figure 4–15 a ).

If we increase the linear dimensions of the cube so that a side becomes 5 centimeters long, each face has a surface area of 25 cm ² (that is, 5 cm . 5 cm), and the total surface area is 150 cm ² (6 . 25 cm ² ). The volume of this larger cube is 125 cm ³ (that is, 5 cm . 5 cm . 5 cm), and the ratio of surface to volume is 150 cm ² /125 cm ³ , which reduces to 1.2 cm ² /1 cm ³ . Th us the larger cube has only one-fi fth as much surface area per unit vol-ume as the smaller cube. A cube 10 centimeters on a side has a surface/volume ratio of 600 cm ² /1000 cm ³ , which is 0.6 cm ² /1 cm ³ —only one-tenth the surface/