Ecology Physiological Ecology of Water Balance in Terrestrial Animals
by
Sue Nicolson
  • LAST REVIEWED: 06 June 2017
  • LAST MODIFIED: 29 May 2014
  • DOI: 10.1093/obo/9780199830060-0084

Introduction

Physiological ecology is concerned with the interactions between physiological systems and the environment. Water and temperature are the two most important abiotic factors affecting animals, and water plays a major role in their distribution and abundance patterns. The water relations of terrestrial animals have received much attention from physiological ecologists, with a bias, not surprisingly, toward assessing the performance of desert animals in extreme environments. There has been a general move from mainly comparative studies carried out under laboratory conditions to an increasing number of field studies of water balance physiology, with more emphasis on the ecological and evolutionary context, and behavior becoming more evident as the first level of response. The evaporating power of air depends on its water vapor density and temperature, and avoidance of desiccation when it cannot be tolerated physiologically depends on minimizing the vapor pressure gradient between the body and the environment. Small animals have a relatively large ratio of surface area to volume, yet despite this constraint, insects are highly successful in terrestrial environments. The focus in this article is on arthropods, reptiles, birds, and mammals as the main terrestrial groups. Water turnover may differ by orders of magnitude, with endothermic vertebrates having far-higher rates of metabolism and water use. Aquatic animals have a different set of environmental problems and will be dealt with in a separate article, except for occasional mention of some of the more terrestrial amphibians. There is huge variety in the ways that animals use water, and the avenues of water exchange with the environment vary greatly in both absolute and relative terms in different animals.

General Overviews

Textbooks on physiological ecology all have substantial sections dealing with the water relations of both terrestrial and aquatic animals. Schmidt-Nielsen 1997 (the last of several editions) is clear and approachable, while Withers 1992 is more comprehensive and strong on technical details. Willmer, et al. 2005 has a strong environmental slant. McNab 2002 is biased toward the author’s favorite topic of energetics. The most recent is Karasov and Martínez del Rio 2007, which focuses on nutritional ecology and treats water as a nutrient (not commonly done). Two smaller volumes, both characterized by a personal touch, are Louw 1993 and Bradshaw 2003.

  • Bradshaw, Don. 2003. Vertebrate ecophysiology: An introduction to its principles and applications. Cambridge, UK: Cambridge Univ. Press.

    DOI: 10.1017/CBO9780511840906Save Citation »Export Citation »E-mail Citation »

    Short, readable text with an emphasis on field studies related to stress and homeostasis (or the lack of it). Valuable for the Australian case studies, many from the author’s own research. Includes useful summaries of many techniques used in ecophysiological studies.

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    • Hill, Richard W., Gordon A. Wyse, and Margaret Anderson. 2012. Animal physiology. 3d ed. Sunderland, MA: Sinauer.

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      The final section consists of three chapters on water and salt physiology and excretion, with an “at work” chapter dealing with mammals of deserts and dry savannas. The companion website is also useful.

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      • Karasov, William H., and Carlos Martínez del Rio. 2007. Physiological ecology: How animals process energy, nutrients, and toxins. Princeton, NJ: Princeton Univ. Press.

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        This refreshingly different textbook is basically a broad account of how animals process and use food resources. Contains useful boxes on technical topics such as the doubly labeled water technique. Chapter 12 (pp. 608–644) deals with the water requirements and water fluxes of animals (mainly vertebrates).

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        • Louw, Gideon N. 1993. Physiological animal ecology. Harlow, UK: Longman Scientific & Technical.

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          Short and readable volume aimed at bridging the gap between physiology and ecology.

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          • McNab, Brian K. 2002. The physiological ecology of vertebrates: A view from energetics. Ithaca, NY: Cornell Univ. Press.

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            Uses energetics as the unifying theme in a synthesis of the literature on physiological ecology of vertebrates. Chapter 7 (pp. 174–218) deals with water and salt exchange in terrestrial vertebrates.

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            • Schmidt-Nielsen, Knut. 1997. Animal physiology: Adaptation and environment. 5th ed. Cambridge, UK: Cambridge Univ. Press.

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              The first edition of this hugely successful textbook appeared in 1975, and it has remained a favorite because of the clarity of its writing. Chapter 8 (pp. 301–354) deals with water and osmotic regulation; chapter 9 (pp. 355–391), with excretion.

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              • Willmer, Pat, Graham Stone, and Ian Johnston. 2005. Environmental physiology of animals. 2d ed. Oxford: Blackwell.

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                Comprehensive overview of the comparative physiology of animals, placed in a strong environmental context. Emphasizes behavioral as well as physiological responses and includes broad coverage of animal phyla.

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                • Withers, Philip C. 1992. Comparative animal physiology. Philadelphia: Saunders College.

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                  Chapter 16 (“Water and Solute Balance”; pp. 777–830) and chapter 17 (“Excretion”; pp. 831–891) are the most relevant. The author uses a quantitative approach emphasizing the physicochemical basis of physiology.

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                  Journals

                  The sampling of journals listed below includes four in comparative physiology (Comparative Biochemistry and Physiology, Journal of Comparative Physiology B, Journal of Experimental Biology, and Physiological and Biochemical Zoology), one with a focus on insects (Journal of Insect Physiology), and one with a broad ecological focus (Functional Ecology).

                  Taxon-Specific Literature

                  Insects are numerically dominant in terrestrial systems, and insect physiology is a strong field, represented by four books listed below. Edney 1977 and Hadley 1994 deal with all terrestrial arthropods, but the bulk of the material comes from studies on insects. Chown and Nicolson 2004 provides an overview of insect physiological ecology and attempts to integrate mechanisms at the individual level with large-scale patterns. Harrison, et al. 2012 emphasizes the timescale of physiological responses, the control systems involved, and the necessary trade-offs. On amphibians, Hillman, et al. 2009 is a valuable source and, like Harrison, et al. 2012, is from the Ecological and Environmental Physiology series of Oxford University Press. Small desert mammals, which are the most water stressed due to their high ratio of surface area to volume and high rates of respiratory water loss, are dealt with in Degen 1997; larger desert mammals, in Wilson 1989. Desert reptiles are covered in Bradshaw 1997 (in the same Springer series, Adaptations of Desert Organisms, as Degen 1997 and Wilson 1989).

                  • Bradshaw, S. Donald. 1997. Homeostasis in desert reptiles. Adaptations of Desert Organisms. Berlin: Springer-Verlag.

                    DOI: 10.1007/978-3-642-60355-6Save Citation »Export Citation »E-mail Citation »

                    Shows how desert reptiles relax their homeostasis during osmotically stressful periods.

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                    • Chown, Steven L., and Sue W. Nicolson. 2004. Insect physiological ecology: Mechanisms and patterns. Oxford Biology. Oxford: Oxford Univ. Press.

                      DOI: 10.1093/acprof:oso/9780198515494.001.0001Save Citation »Export Citation »E-mail Citation »

                      Chapter 4 (pp. 87–114) provides an overview of the water balance physiology of insects.

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                      • Degen, A. Allan. 1997. Ecophysiology of small desert mammals. Adaptations of Desert Organisms. Berlin: Springer-Verlag.

                        DOI: 10.1007/978-3-642-60351-8Save Citation »Export Citation »E-mail Citation »

                        Chapter 6 (pp. 93–162) of this monograph is on water requirements and water balance of small mammals (defined here as those up to 5 kilograms).

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                        • Edney, Eric B. 1977. Water balance in land arthropods. Zoophysiology and Ecology 9. Berlin: Springer-Verlag.

                          DOI: 10.1007/978-3-642-81105-0Save Citation »Export Citation »E-mail Citation »

                          Systematic examination of all avenues of water loss and gain in terrestrial arthropods. This classic monograph was an inspiration to many insect physiologists.

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                          • Hadley, Neil F. 1994. Water relations of terrestrial arthropods. San Diego, CA: Academic Press.

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                            New synthesis at the time, and much more than an update of Edney 1977. Includes a strong focus on the non-insect arthropods.

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                            • Harrison, Jon F., H. Arthur Woods, and Stephen P. Roberts. 2012. Ecological and environmental physiology of insects. Ecological and Environmental Physiology 3. Oxford: Oxford Univ. Press.

                              DOI: 10.1093/acprof:oso/9780199225941.001.0001Save Citation »Export Citation »E-mail Citation »

                              The authors have not tried to be comprehensive but rather to focus on fundamental questions related to temperature, water, food, and oxygen. Contains a useful chapter on techniques.

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                              • Hillman, Stanley S., Philip C. Withers, Robert C. Drewes, and Stanley D. Hillyard. 2009. Ecological and environmental physiology of amphibians. Ecological and Environmental Physiology 1. Oxford: Oxford Univ. Press.

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                                A thorough synthesis of research on the comparative physiology of amphibians as a vertebrate model system for studying the transition from an aquatic to a terrestrial environment. Strong on the biophysical aspects and with the expected emphasis on water balance.

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                                • Wilson, Richard T. 1989. Ecophysiology of the Camelidae and desert ruminants. Adaptations of Desert Organisms. Berlin: Springer-Verlag.

                                  DOI: 10.1007/978-3-642-74483-9Save Citation »Export Citation »E-mail Citation »

                                  The chapter on water balance and kidney function in camels and desert ruminants (both domesticated and wild) takes up half of this slim volume and includes quantities of useful data summarized in tables.

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                                  Water Budgets and Water Turnover

                                  Water balance depends on maintaining the internal body water pool at a steady state. Mass-specific water flux (total volume passing through the body) is greater in small animals because their higher ratio of surface area to volume affects rates of water exchange with the environment. In considering the various ways in which arthropods achieve water balance, Hadley 1994 suggests five different “strategies”: (1) behavioral avoidance (see Microclimates and Behavioral Avoidance), (2) enhanced water conservation, (3) dehydration tolerance, (4) high fluid turnover, and (5) dependence on water vapor absorption. The last strategy applies to certain arthropods only (see O’Donnell and Machin 1988, cited under Replenishing the Body Water Pool), but the others are broadly applicable to other taxa. Similarly, Wolcott 1992 examines the impact of hydration on performance of land crabs, emphasizing that adequate hydration levels can be achieved with vastly different turnover rates in different species.

                                  • Hadley, Neil F. 1994. Water relations of terrestrial arthropods. San Diego, CA: Academic Press.

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                                    At the whole-animal level, Hadley classifies arthropods into different adaptive types that manage their body water and osmotic balance in very different ways.

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                                    • Wolcott, Thomas G. 1992. Water and solute balance in the transition to land. American Zoologist 32.3: 428–437.

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                                      Terrestrial crabs are physiologically similar to their marine relatives but have adopted varied solutions to the challenges of life on land. Wolcott points out that balancing water and ion budgets is more important than absolute turnover rates, the latter varying widely according to microhabitat, activity, and water availability.

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                                      Water Budgets

                                      Desiccation resistance may be accomplished in three ways at the organismal level: by increasing the water content, decreasing the rate of water loss, or becoming more tolerant of dehydration. Gibbs 2002 assesses these aspects of water balance in Drosophila melanogaster subjected to laboratory selection and in Drosophila species from desert and mesic environments. Woods and Harrison 2001 describes water budgets of caterpillars, in a test of the beneficial-acclimation hypothesis. Benoit and Denlinger 2010 contrasts the water balance challenges facing blood-feeding arthropods after a meal and during the off-host stage. Nicolson 2009 discusses water balance in bees, which frequently have to deal with excess water at both individual and colony levels. Water requirements and water fluxes, with a strong vertebrate focus, are dealt with in Karasov and Martínez del Rio 2007; Shoemaker and Nagy 1977, although outdated, covers both amphibians and reptiles. Plasma volume maintenance during dehydration is essential in mammals and birds, but camels can tolerate dehydration equivalent to 30–40 percent of their body mass (Grenot 1992). Finally, anhydrobiosis, an extreme version of dehydration tolerance in some invertebrates, involves the reversible loss of virtually all body water; the mechanisms involved are reviewed in Crowe, et al. 2002.

                                      • Benoit, Joshua B., and David L. Denlinger. 2010. Meeting the challenges of on-host and off-host water balance in blood-feeding arthropods. In Insect molecular physiology—Basic science to applications: A special issue in honour of Dr. Judith H. Willis. Journal of Insect Physiology 56.10: 1366–1376.

                                        DOI: 10.1016/j.jinsphys.2010.02.014Save Citation »Export Citation »E-mail Citation »

                                        Review of the challenges of the blood-feeding lifestyle for water balance—these arthropods must cope with excess water after blood meals, followed by long periods of dehydration.

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                                        • Crowe, John H., Ann E. Oliver, and Fern Tablin. 2002. Is there a single biochemical adaptation to anhydrobiosis? Integrative and Comparative Biology 42.3: 497–503.

                                          DOI: 10.1093/icb/42.3.497Save Citation »Export Citation »E-mail Citation »

                                          The most extreme dehydration tolerance is seen in some small animals such as tardigrades and nematodes that exhibit anhydrobiosis; this review discusses the mechanisms involved, especially the role of stabilizing disaccharides such as trehalose.

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                                          • Gibbs, Allen G. 2002. Water balance in desert Drosophila: Lessons from non-charismatic microfauna. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 133.3: 781–789.

                                            DOI: 10.1016/S1095-6433(02)00208-8Save Citation »Export Citation »E-mail Citation »

                                            Desiccation is a powerful agent of selection in small insects. This useful review compares the results of laboratory selection (desiccation-resistant populations of D. melanogaster compared with control populations) and natural selection (desert species of Drosophila compared with mesic species). Not all possible adaptive mechanisms evolve in conditions of desiccation stress.

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                                            • Grenot, Claude J. 1992. Ecophysiological characteristics of large herbivorous mammals in arid Africa and the Middle East. Journal of Arid Environments 23.2: 125–155.

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                                              A rather fragmented account of water and energy budgets in domesticated and wild ungulates of arid regions. Stresses the great variation in water flux rates of mammals—for example, during lactation and evaporative cooling—and in their capacity for dehydration.

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                                              • Karasov, William H., and Carlos Martínez del Rio. 2007. Physiological ecology: How animals process energy, nutrients, and toxins. Princeton, NJ: Princeton Univ. Press.

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                                                In a book that emphasizes budgets throughout, chapter 12 (pp. 608–644) deals with water budgets. A large American desert hare, the jackrabbit, is selected as a focal animal because there is sufficient information for an integrative view of the significance of water in its ecology.

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                                                • Nicolson, Susan W. 2009. Water homeostasis in bees, with the emphasis on sociality. Journal of Experimental Biology 212.3: 429–434.

                                                  DOI: 10.1242/jeb.022343Save Citation »Export Citation »E-mail Citation »

                                                  Review of water homeostasis in bees at two levels of organization (the individual and the colony), both subject to high water fluxes.

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                                                  • Shoemaker, Vaughan H., and Kenneth A. Nagy. 1977. Osmoregulation in amphibians and reptiles. Annual Review of Physiology 39:449–471.

                                                    DOI: 10.1146/annurev.ph.39.030177.002313Save Citation »Export Citation »E-mail Citation »

                                                    Review of the avenues of water gain and loss in these two groups, and their ability to tolerate temporary osmotic imbalances.

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                                                    • Woods, H. Arthur, and Jon F. Harrison. 2001. The beneficial acclimation hypothesis versus acclimation of specific traits: Physiological change in water-stressed Manduca sexta caterpillars. Physiological and Biochemical Zoology 74.1: 32–44.

                                                      DOI: 10.1086/319302Save Citation »Export Citation »E-mail Citation »

                                                      Water budgets of the tobacco hornworm, Manduca sexta, were compared on artificial diets of low and high water content. Most important are water gain in the food and water loss in the feces, caterpillars in general having high water fluxes through the alimentary canal.

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                                                      Water Turnover Rates

                                                      Tritiated water (deuterium may be substituted for tritium) has long been used to measure the water turnover of free-living animals (and total body water, obtained from the dilution space). Doubly labeled water enables simultaneous measurement of water turnover and energy expenditure. Nagy and Peterson 1988 presents extensive allometric analyses of isotopically measured water flux rates for the major taxonomic groups, showing high rates of water turnover in small animals, and higher rates in free-living than in captive animals. The decline in specific activity is due to elimination of the isotope through excretion and evaporation, and its dilution by input of unlabeled water (food, drink, and metabolic water). The method does not provide information about these individual avenues of water gain or loss, and separate experiments are required to quantify these. This is illustrated here by the studies of Ken Nagy and colleagues on desert tortoises (Nagy and Medica 1986) and kangaroo rats (Nagy and Gruchacz 1994). Munn, et al. 2013 compares energy and water turnover in kangaroos and sheep. The text in Bradshaw 2003 has a strong emphasis on the use of radioisotopes in turnover measurements in various vertebrate taxa. In contrast, the application of this technique to insects has been limited. The metabolic rates of tethered flying bumblebees were measured with both doubly labeled water and respirometry in Wolf, et al. 1996, which documents good agreement between the two methods.

                                                      • Bradshaw, Don. 2003. Vertebrate ecophysiology: An introduction to its principles and applications. Cambridge, UK: Cambridge Univ. Press.

                                                        DOI: 10.1017/CBO9780511840906Save Citation »Export Citation »E-mail Citation »

                                                        This volume contains many case studies involving the use of doubly labeled water, and it also includes a useful chapter on turnover methodology.

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                                                        • Munn, Adam J., Terence J. Dawson, Steven R. McLeod, Todd Dennis, and Shane K. Maloney. 2013. Energy, water and space use by free-living red kangaroos Macropus rufus and domestic sheep Ovis aries in an Australian rangeland. Journal of Comparative Physiology B 183.6: 843–858.

                                                          DOI: 10.1007/s00360-013-0741-8Save Citation »Export Citation »E-mail Citation »

                                                          In this study of resource competition in Australian rangelands, the authors measured water turnover rates and urine concentrations of free-living red kangaroos and merino sheep, finding large differences in water use (kangaroos being more efficient). There was also good agreement with earlier studies carried out on animals in an enclosure.

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                                                          • Nagy, Kenneth A., and Mark J. Gruchacz. 1994. Seasonal water and energy metabolism of the desert-dwelling kangaroo rat (Dipodomys merriami). Physiological Zoology 67.6: 1461–1478.

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                                                            Used tritiated water and stomach content analysis to look at the water balance of kangaroo rats Dipodomys merriami in the Mojave Desert. Animals did not drink but increased their water intake in food by consuming green vegetation in season and by storing seeds in burrows.

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                                                            • Nagy, Kenneth A., and Philip A. Medica. 1986. Physiological ecology of desert tortoises in southern Nevada. Herpetologica 42.1: 73–92.

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                                                              Classic study of osmoregulation and energetics of the large desert tortoise Gopherus agassizii, studied in an enclosure containing natural vegetation. They tolerated large seasonal imbalances in water, energy, and salt budgets. Fluctuating water and electrolyte content of the vegetation do not permit year-round homeostasis.

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                                                              • Nagy, Kenneth A., and Charles C. Peterson. 1988. Scaling of water flux rate in animals. University of California Publications in Zoology 120. Berkeley and Los Angeles: Univ. of California Press.

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                                                                Presents allometric relationships between water flux and body mass for many species, both captive and free living. The water economy index is used to relate water use to energy metabolism: low values in desert species mean that their low water requirements are due to water-conserving mechanisms, not merely low energy requirements.

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                                                                • Wolf, Thomas J., Charles P. Ellington, Simon Davis, and Mark J. Feltham. 1996. Validation of the doubly labelled water technique for bumblebees Bombus terrestris (L.). Journal of Experimental Biology 199.4: 959–972.

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                                                                  Validation study that showed excellent agreement between metabolic rates of tethered flying bumblebees measured with doubly labeled water and flow-through respirometry.

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                                                                  Replenishing the Body Water Pool

                                                                  Water gain is through food and drink (collectively termed preformed water) and metabolism. In a general account, Karasov and Martínez del Rio 2007 compares the relative importance of these avenues for herbivorous and granivorous vertebrates. Gains from food depend on its water content and the amount ingested. An extreme example of high preformed water gain is seen in nectar-feeding birds, which are able to cope with huge variation in dietary water intake (Nicolson and Fleming 2003). Fruit, fresh leaves, blood, and other animals have high water contents, although Wright, et al. 2013 shows that carnivorous Gila monsters do not get enough water in their diet and must also drink. Fog and dew may be important sources of drinking water in arid environments (Hamilton and Seely 1976). Metabolic water production is calculated from metabolic rate and food composition: high metabolic rates during flight make it easier to achieve positive water balance, even in animals as small as Drosophila (Lehmann, et al. 2000). An additional avenue of water gain, absorption of water vapor from the atmosphere, is unique to certain arthropods such as tenebrionid beetle larvae and ticks. This has been an active research area among physiologists intrigued by the diverse mechanisms involved in moving water against sometimes-steep gradients (reviewed in O’Donnell and Machin 1988; see also Edney 1977, cited under Taxon-Specific Literature).

                                                                  • Hamilton, William J., III, and Mary K. Seely. 1976. Fog basking by the Namib Desert beetle, Onymacris unguicularis. Nature 262.5566: 284–285.

                                                                    DOI: 10.1038/262284a0Save Citation »Export Citation »E-mail Citation »

                                                                    Many desert arthropods depend on advective fog for moisture; this tenebrionid beetle positions its body as a fog-collecting device.

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                                                                    • Karasov, William H., and Carlos Martínez del Rio. 2007. Physiological ecology: How animals process energy, nutrients, and toxins. Princeton, NJ: Princeton Univ. Press.

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                                                                      Chapter 12 (pp. 608–644) deals with water requirements.

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                                                                      • Lehmann, Fritz-Olaf, Michael H. Dickinson, and Jocelyn Staunton. 2000. The scaling of carbon dioxide release and respiratory water loss in flying fruit flies (Drosophila spp.). Journal of Experimental Biology 203.10: 1613–1624.

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                                                                        During tethered flight in four species of Drosophila of different sizes, metabolic water was found to replace, on average, 42 percent of evaporative losses. This offset was much smaller in resting flies.

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                                                                        • Nicolson, Susan W., and Patricia A. Fleming. 2003. Energy balance in the whitebellied sunbird Nectarinia talatala: Constraints on compensatory feeding, and consumption of supplementary water. Functional Ecology 17.1: 3–9.

                                                                          DOI: 10.1046/j.1365-2435.2003.00692.xSave Citation »Export Citation »E-mail Citation »

                                                                          The challenge of drinking dilute nectar is that birds must process large volumes of preformed water. Compensatory feeding by white-bellied sunbirds involves adjustment of drinking rates to maintain constant energy intake over a ten-fold range of sucrose concentrations.

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                                                                          • O’Donnell, Michael J., and John Machin. 1988. Water vapor absorption by terrestrial organisms. Advances in Comparative and Environmental Physiology 2:47–90.

                                                                            DOI: 10.1007/978-3-642-73375-8_2Save Citation »Export Citation »E-mail Citation »

                                                                            Comprehensive review of the mechanisms involved in extracting water from unsaturated air. Different oral and rectal structures are used for this purpose in groups of unrelated arthropods.

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                                                                            • Wright, Christian D., Marin L. Jackson, and Dale F. DeNardo. 2013. Meal consumption is ineffective at maintaining or correcting water balance in a desert lizard, Heloderma suspectum. Journal of Experimental Biology 216.8: 1439–1447.

                                                                              DOI: 10.1242/jeb.080895Save Citation »Export Citation »E-mail Citation »

                                                                              The diets of carnivores contain about 70 percent water, so drinking is assumed to be unnecessary. However, Gila monsters showed no evidence of rehydration after meals, and their blood osmolality increased. It is likely that these animals rely on water stored in their urinary bladders until the wet season.

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                                                                              Evaporative Water Loss

                                                                              Relative humidity and temperature of the air together determine its water vapor density and hence the gradient for evaporative water loss from animals. This occurs via cutaneous and respiratory routes, which are not always easily partitioned and are dealt with in Cutaneous Water Loss and Respiratory Water Loss. Because birds are small and diurnal, don’t burrow, and have high metabolic rates, they are particularly prone to high rates of evaporative water loss: see the allometric analysis in Williams 1996. Dehydration may limit the duration of sustained flight, sometimes studied under controlled conditions in wind tunnels, but the authors of Giladi and Pinshow 1999 used trained pigeons to measure evaporation and excretion in flight.

                                                                              • Giladi, Itamar, and Berry Pinshow. 1999. Evaporative and excretory water loss during free flight in pigeons. Journal of Comparative Physiology B 169.4–5: 311–318.

                                                                                DOI: 10.1007/s003600050226Save Citation »Export Citation »E-mail Citation »

                                                                                Prolonged flapping flight in birds requires the maintenance of plasma volume, in spite of high metabolic demands and heat dissipation requirements. Water loss in free flight was measured as a function of different air temperatures and water vapor densities, and excretory loss was 10 percent of the total.

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                                                                                • Williams, Joseph B. 1996. A phylogenetic perspective of evaporative water loss in birds. Auk 113.2: 457–472.

                                                                                  DOI: 10.2307/4088912Save Citation »Export Citation »E-mail Citation »

                                                                                  Analysis of a large data set (102 species, ranging in body mass from hummingbirds to ostriches) for total evaporative water loss. The finding of lower values in birds from arid environments was confirmed after controlling for the effects of phylogeny.

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                                                                                  Cutaneous Water Loss

                                                                                  Correlations between water loss rates and cuticular lipids have been demonstrated for many insects: Gibbs 1998 reviews the waterproofing function of cuticular lipids. Cutaneous water loss is high in most amphibians, whose thin, vascular skin offers no resistance to water loss, but a few arboreal frogs are “waterproof,” with water loss rates like those of reptiles. Shoemaker, et al. 1989 shows that arboreal waterproof frogs from Africa and South America are also capable of evaporative cooling during heat stress. At high ambient temperatures, the water requirements for evaporative cooling may be high: sweating contributes to cutaneous water loss (for an insect example, see Toolson 1987) while panting adds to respiratory water loss. Cutaneous and respiratory routes of evaporation are not easy to separate experimentally, as shown by studies on bats (Thomas and Cloutier 1992; see also Ben-Hamo, et al. 2013, cited under Microclimates and Behavioral Avoidance) and birds (Webster and King 1987; see also McKechnie and Wolf 2004, cited under Plasticity in Water Balance Physiology). The main determinant of cutaneous water loss in birds and mammals is the lipid composition of the outer layer of the epidermis, the stratum corneum (Haugen, et al. 2003). The final paper highlighted in this section is a large-mammal example: elephants lack sweat glands but have high integumental permeability compared to other mammals (Dunkin, et al. 2013).

                                                                                  • Dunkin, Robin C., Dinah Wilson, Nicolas Way, Kari Johnson, and Terrie M. Williams. 2013. Climate influences thermal balance and water use in African and Asian elephants: Physiology can predict drivers of elephant distribution. Journal of Experimental Biology 216.15: 2939–2952.

                                                                                    DOI: 10.1242/jeb.080218Save Citation »Export Citation »E-mail Citation »

                                                                                    Captive elephants showed high rates of cutaneous water loss (measured with a ventilated capsule technique). Elephants lack sweat glands, but their skin is very permeable, especially on the ears and especially in summer, indicating seasonal acclimation. The implications for elephant water use in relation to climate are discussed.

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                                                                                    • Gibbs, Allen G. 1998. Water-proofing properties of cuticular lipids. American Zoologist 38.3: 471–482.

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                                                                                      The water permeability of insect cuticle depends on the amount and composition of epicuticular lipids. Long-chain saturated hydrocarbons are the most abundant, and changes in chemical composition affect physical properties of the lipids, such as melting temperature.

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                                                                                      • Haugen, Michael J., B. Irene Tieleman, and Joseph B. Williams. 2003. Phenotypic flexibility in cutaneous water loss and lipids of the stratum corneum. Journal of Experimental Biology 206.20: 3581–3588.

                                                                                        DOI: 10.1242/jeb.00596Save Citation »Export Citation »E-mail Citation »

                                                                                        Hoopoe larks acclimated to 35oC had lower rates of cutaneous water loss and altered lipid composition in the stratum corneum.

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                                                                                        • Shoemaker, Vaughan H., Mary Ann Baker, and John P. Loveridge. 1989. Effect of water balance on thermoregulation in waterproof frogs (Chiromantis and Phyllomedusa). Physiological Zoology 62.1: 133–146.

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                                                                                          These unusual frogs from Africa and South America are arboreal and relatively waterproof, but they can increase evaporative water loss markedly during heat stress. Evaporative cooling depends on hydration state, occurring only when frogs have water reserves in their bladders. These two unusual amphibians also excrete uric acid.

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                                                                                          • Thomas, Donald W., and Danielle Cloutier. 1992. Evaporative water loss by hibernating little brown bats, Myotis lucifugus. Physiological Zoology 65.2: 443–456.

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                                                                                            Total evaporative water loss in hibernating little brown bats is overwhelmingly by the cutaneous route because of their low metabolic rate.

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                                                                                            • Toolson, Eric C. 1987. Water profligacy as an adaptation to hot deserts: Water loss rates and evaporative cooling in the Sonoran Desert cicada, Diceroprocta apache (Homoptera: Cicadidae). Physiological Zoology 60.4: 379–385.

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                                                                                              Evaporative cooling is assumed to be insignificant in insects because of their small water reserves, but desert cicadas, which feed on dilute xylem sap, are an interesting exception.

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                                                                                              • Webster, Marcus D., and James R. King. 1987. Temperature and humidity dynamics of cutaneous and respiratory evaporation in pigeons, Columba livia. Journal of Comparative Physiology B 157.2: 253–260.

                                                                                                DOI: 10.1007/BF00692370Save Citation »Export Citation »E-mail Citation »

                                                                                                Separating cutaneous and respiratory water losses requires that birds wear a mask or are restrained with their heads physically isolated from the rest of the body. Measurements at various air temperatures and humidities showed that cutaneous water loss plays a greater role in avian water balance than previously thought.

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                                                                                                Respiratory Water Loss

                                                                                                Trade-offs between water loss and gas exchange are necessary in all terrestrial animals (Woods and Smith 2010). Chown 2002 points out that the relative importance of respiratory water loss in insects is contentious. Many insects (and some other arthropods) exhibit discontinuous gas exchange when at rest, and the evolutionary origins of this interrupted pattern of respiration and its adaptive function(s) have been hotly debated. While early work pointed to a function in reducing respiratory water loss, early-21st-century rigorous studies such as Terblanche, et al. 2008 have also tested other hypotheses involving respiration in subterranean environments and reduction of oxidative damage. Chown 2011 provides a concise update and suggests a nonadaptive neural hypothesis. High metabolic rates of endothermic vertebrates mean high respiratory water losses. Nasal countercurrent heat and water exchange is hypothesized to reduce the dehydration resulting from high body temperatures in birds and mammals (Schmidt-Nielsen, et al. 1970); however, see Tieleman, et al. 1999 for a more recent test in heat-stressed larks.

                                                                                                • Chown, Steven L. 2002. Respiratory water loss in insects. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 133.3: 791–804.

                                                                                                  DOI: 10.1016/S1095-6433(02)00200-3Save Citation »Export Citation »E-mail Citation »

                                                                                                  Discusses problems associated with quantifying the relative importance of respiratory losses (usually less than 20 percent of total evaporative water loss).

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                                                                                                  • Chown, Steven L. 2011. Discontinuous gas exchange: New perspectives on evolutionary origins and ecological implications. Functional Ecology 25.6: 1163–1168.

                                                                                                    DOI: 10.1111/j.1365-2435.2011.01879.xSave Citation »Export Citation »E-mail Citation »

                                                                                                    The evolutionary origins and adaptive significance of discontinuous gas exchange in insects have been the subject of prolonged controversy and intensive research. Chown provides an update on the controversy and a new hypothesis, also drawing attention to the model in Woods and Smith 2010 and implications for the water-saving hypothesis.

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                                                                                                    • Schmidt-Nielsen, Knut, F. Reed Hainsworth, and David E. Murrish. 1970. Counter-current heat exchange in the respiratory passages: Effect on water and heat balance. Respiration Physiology 9.2: 263–276.

                                                                                                      DOI: 10.1016/0034-5687(70)90075-7Save Citation »Export Citation »E-mail Citation »

                                                                                                      Classic paper about temporal countercurrent heat exchange in the nasal passages, with the emphasis on kangaroo rats and cactus wrens. Cooling of the exhaled air as it passes over previously cooled nasal surfaces leads to condensation and recovery of water.

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                                                                                                      • Terblanche, John S., Elrike Marais, Stefan K. Hetz, and Steven L. Chown. 2008. Control of discontinuous gas exchange in Samia cynthia: Effects of atmospheric oxygen, carbon dioxide and moisture. Journal of Experimental Biology 211.20: 3272–3280.

                                                                                                        DOI: 10.1242/jeb.022467Save Citation »Export Citation »E-mail Citation »

                                                                                                        Tested the predictions of competing hypotheses for discontinuous gas exchange in moth pupae (Samia cynthia) by exposing them to atmospheres with different concentrations of oxygen, carbon dioxide, and water vapor. The experiments provided support for the oxidative-damage hypothesis, as well as partial support for the water-saving hypothesis.

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                                                                                                        • Tieleman, B. Irene, Joseph B. Williams, Gilead Michaeli, and Berry Pinshow. 1999. The role of the nasal passages in the water economy of crested larks and desert larks. Physiological and Biochemical Zoology 72.2: 219–226.

                                                                                                          DOI: 10.1086/316658Save Citation »Export Citation »E-mail Citation »

                                                                                                          Examines the role of the nasal passages in the water economy of larks. When total evaporative water loss was quantified in larks, occlusion of the nares (so that the nasal membranes could not function as a heat exchanger) had little effect.

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                                                                                                          • Woods, H. Arthur, and Jennifer N. Smith. 2010. Universal model for water costs of gas exchange by animals and plants. Proceedings of the National Academy of Sciences of the United States of America 107.18: 8469–8474.

                                                                                                            DOI: 10.1073/pnas.0905185107Save Citation »Export Citation »E-mail Citation »

                                                                                                            Presents a universal model for the water costs of gas exchange in terrestrial species (plants too). The model shows that adult insects lose more water than other organisms for the same levels of gas exchange; this may be related to discontinuous gas exchange.

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                                                                                                            Excretory Water Loss

                                                                                                            High fecal water losses in herbivores compared to seed eaters are discussed in Karasov and Martínez del Rio 2007 (cited under General Overviews). Fecal and urinary water losses are combined in insects, birds, and reptiles, and Woods and Bernays 2000 shows how the water balance of wild caterpillars depends on modulation of fecal water loss. Phillips 1981 explains how the rectal pads or rectal complex modify the isosmotic fluid secreted by Malpighian tubules; the mechanisms of this fluid secretion, with emphasis on the interaction between physiology and molecular biology, are reviewed in Beyenbach, et al. 2010. Among vertebrates, only birds and mammals possess loops of Henle in their kidneys and are capable of producing hyperosmotic urine, via a countercurrent multiplier system and reabsorption of water by the collecting duct (controlled by antidiuretic hormone). Beuchat 1996 is a detailed analysis of scaling relationships in mammalian kidneys. Bradshaw 2003 provides examples of renal function and its endocrine control in a diversity of animals. In terms of nitrogenous excretion, insoluble uric acid has no osmotic effect and is the best excretory product from a water-saving perspective; Wright 1995 compares the physiological functions of different nitrogen end products. Uric-acid excretion and salt gland secretions compensate for the lesser renal-concentrating ability of birds, as do the absorbing epithelia of the lower gut (reviewed in Goldstein and Skadhauge 2000).

                                                                                                            • Beuchat, Carol A. 1996. Structure and concentrating ability of the mammalian kidney: Correlations with habitat. American Journal of Physiology 271.1: R157–R179.

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                                                                                                              Examines the relationship between kidney morphology and maximum urine concentration. Appendix B is an exhaustive compilation of data from over 330 mammal species from arid, mesic, and freshwater environments. The analysis focuses on allometric relationships and the effect of habitat.

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                                                                                                              • Beyenbach, Klaus W., Helen Skaer, and Julian A. T. Dow. 2010. The developmental, molecular, and transport biology of Malpighian tubules. Annual Review of Entomology 55:351–374.

                                                                                                                DOI: 10.1146/annurev-ento-112408-085512Save Citation »Export Citation »E-mail Citation »

                                                                                                                Mechanisms and control of fluid secretion by Malpighian tubules have been studied in considerable detail. This review focuses on transport mechanisms in mosquito tubules, and development and functional genomics of the Drosophila tubule, a model transporting epithelium.

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                                                                                                                • Bradshaw, Don. 2003. Vertebrate ecophysiology: An introduction to its principles and applications. Cambridge, UK: Cambridge Univ. Press.

                                                                                                                  DOI: 10.1017/CBO9780511840906Save Citation »Export Citation »E-mail Citation »

                                                                                                                  Discusses kidney function in a variety of animals, including various desert rodents. One of Bradshaw’s conclusions is that because granivores have a high protein intake, their extraordinary concentrating abilities may be related more to excreting high urea loads than to water conservation.

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                                                                                                                  • Goldstein, David L., and Erik Skadhauge. 2000. Renal and extrarenal regulation of body fluid composition. In Sturkie’s avian physiology. 5th ed. Edited by G. Causey Whittow, 265–297. New York: Academic Press.

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                                                                                                                    Broad review of osmoregulation in birds, including kidney function and extrarenal contributions from the salt glands (mainly in marine birds) and lower intestine.

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                                                                                                                    • Phillips, John. 1981. Comparative physiology of insect renal function. American Journal of Physiology 241.5: R241–R257.

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                                                                                                                      Classic overview of insect excretory systems, with equal emphasis on fluid secretion by the Malpighian tubules and reabsorptive processes in the hindgut.

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                                                                                                                      • Woods, H. Arthur, and Elizabeth A. Bernays. 2000. Water homeostasis by wild larvae of Manduca sexta. Physiological Entomology 25.1: 82–87.

                                                                                                                        DOI: 10.1046/j.1365-3032.2000.00167.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                        Simple observational study demonstrating the short-term control of fecal water loss by caterpillars in the field. Fecal water content was modulated in response to time of day, leaf water content, and attacks by tachinid flies that prompted water loss through regurgitation.

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                                                                                                                        • Wright, Patricia A. 1995. Nitrogen excretion: Three end products, many physiological roles. Journal of Experimental Biology 198.2: 273–281.

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                                                                                                                          Concise review of patterns of nitrogen excretion in animals, comparing the metabolism, synthesis, and transport of ammonia, urea, and uric acid.

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                                                                                                                          Plasticity in Water Balance Physiology

                                                                                                                          Physiological diversity in water balance occurs at levels of the individual, population, and species, and understanding this variation is necessary to predict responses to climate change. Tracy and Walsberg 2001 reports a significant study on kangaroo rats, in which the authors investigated the genetic, developmental, and environmental determinants of total evaporative water loss. Also at the individual level, McKechnie and Wolf 2004 shows that heat-acclimated doves increase their cutaneous water loss (see also Haugen, et al. 2003, cited under Cutaneous Water Loss), and Woods and Harrison 2001 uses water budgets in caterpillars to test whether acclimation confers fitness benefits on animal performance. At the population level, clinal variation in a species is the variation in phenotype along an environmental gradient, illustrated by the work in Rourke 2000 on cuticle permeability in a grasshopper. Lastly, Addo-Bediako, et al. 2001 is an example of broad-scale, multispecies work on physiological variation in water loss.

                                                                                                                          • Addo-Bediako, Abraham, Steven L. Chown, and Kevin J. Gaston. 2001. Revisiting water loss in insects: A large scale view. Journal of Insect Physiology 47.12: 1377–1388.

                                                                                                                            DOI: 10.1016/S0022-1910(01)00128-7Save Citation »Export Citation »E-mail Citation »

                                                                                                                            Using a macroecology approach and analyzing data across broad geographic scales, the authors show that water loss rates of insects are related to rainfall and that respiratory water loss is a greater proportion of total water loss in xeric than in mesic species.

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                                                                                                                            • McKechnie, Andrew E., and Blair O. Wolf. 2004. Partitioning of evaporative water loss in white-winged doves: Plasticity in response to short-term thermal acclimation. Journal of Experimental Biology 207.2: 203–210.

                                                                                                                              DOI: 10.1242/jeb.00757Save Citation »Export Citation »E-mail Citation »

                                                                                                                              Measured the partitioning of evaporative water loss in doves in response to acclimation, showing an increase in cutaneous water loss in heat-acclimated doves, both in absolute terms and as a proportion of total evaporative water loss.

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                                                                                                                              • Rourke, Bryan C. 2000. Geographic and altitudinal variation in water balance and metabolic rate in a California grasshopper, Melanoplus sanguinipes. Journal of Experimental Biology 203.17: 2699–2712.

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                                                                                                                                Six populations of this grasshopper, collected from different altitudes, showed large differences in total water loss rates. These were attributed to clinal variation in permeability of the cuticle (increased amounts and higher melting points of cuticular lipids) rather than to reductions in respiratory water loss.

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                                                                                                                                • Tracy, Randall L., and Glenn E. Walsberg. 2001. Developmental and acclimatory contributions to water loss in a desert rodent: Investigating the time course of adaptive change. Journal of Comparative Physiology B 171.8: 669–679.

                                                                                                                                  DOI: 10.1007/s003600100218Save Citation »Export Citation »E-mail Citation »

                                                                                                                                  Young kangaroo rats (Dipodomys merriami) were raised either under desiccating or hydrating conditions, so that the authors could determine the effects first of development and then of acclimation on evaporative water loss (the main avenue of water loss). They found considerable phenotypic plasticity, with acclimation predominating over genetic and developmental effects.

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                                                                                                                                  • Woods, H. Arthur, and Jon F. Harrison. 2001. The beneficial acclimation hypothesis versus acclimation of specific traits: Physiological change in water-stressed Manduca sexta caterpillars. Physiological and Biochemical Zoology 74.1: 32–44.

                                                                                                                                    DOI: 10.1086/319302Save Citation »Export Citation »E-mail Citation »

                                                                                                                                    A test of the beneficial-acclimation hypothesis, which proposes that acclimation has a fitness benefit in the environment that caused it, by using caterpillars reared on artificial diets of low and high water content. Larval water budgets showed that short-term changes in fecal loss and long-term changes in evaporative loss were beneficial.

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                                                                                                                                    Microclimates and Behavioral Avoidance

                                                                                                                                    In terms of microclimatic conditions, air temperature and relative humidity are the most-important parameters, although these are continuously modified by wind and solar radiation. Small size enables animals to take advantage of favorable microclimates in vegetation or underground, often while quiescent (reviewed for insects in Willmer 1982). The undersurfaces of leaves are relatively still, cool, and humid and are exploited by many herbivores that are small enough to stay within the boundary layer. Kingsolver, et al. 2011 considers the effect of leaf biophysics on insect-plant associations. Aggregations of conspecific insects reduce surface area and create and maintain local microclimates; a nice caterpillar example is described in Klok and Chown 1999. Dehydration is one of the challenges of overwintering, evident in both dormant insects (Danks 2000) and hibernating bats, in which periodic arousals may be due to the need to drink (Ben-Hamo, et al. 2013; see also Thomas and Cloutier 1992, cited under Cutaneous Water Loss). The burrows of nocturnal desert rodents are considered refuges from extreme conditions on the surface. However, the authors of Tracy and Walsberg 2002 measured the microclimate in burrows of the model desert mammal, the kangaroo rat, and found conditions to be warmer and less humid than expected. Many arid-zone amphibians are fossorial and inactive during the dry season, burrowing deep to access soil moisture or forming protective cocoons from layers of shed skin, thus reducing their water loss dramatically (Withers 1998).

                                                                                                                                    • Ben-Hamo, Miriam, Agustí Muñoz-Garcia, Joseph B. Williams, Carmi Korine, and Berry Pinshow. 2013. Waking to drink: Rates of evaporative water loss determine arousal frequency in hibernating bats. Journal of Experimental Biology 216.4: 573–577.

                                                                                                                                      DOI: 10.1242/jeb.078790Save Citation »Export Citation »E-mail Citation »

                                                                                                                                      Bats hibernate in caves with high relative humidities. Periodic arousals, which are energetically expensive, may depend on hydration state. The authors found an inverse relationship between torpor bout duration and total evaporative water loss, which supports the hypothesis that bats arouse in order to drink.

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                                                                                                                                      • Danks, Hugh V. 2000. Dehydration in dormant insects. Journal of Insect Physiology 46.6: 837–852.

                                                                                                                                        DOI: 10.1016/S0022-1910(99)00204-8Save Citation »Export Citation »E-mail Citation »

                                                                                                                                        For dormant or diapausing insects, water balance is crucial because dehydration is prolonged and access to water is limited. Overwintering insects benefit from microclimates, and reductions in water content also contribute to cold hardiness. See also Benoit, et al. 2010, cited under Water Balance in a Changing Climate.

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                                                                                                                                        • Kingsolver, Joel G., H. Arthur Woods, Lauren B. Buckley, Kristen A. Potter, Heidi J. MacLean, and Jessica K. Higgins. 2011. Complex life cycles and the responses of insects to climate change. Integrative and Comparative Biology 51.5: 719–732.

                                                                                                                                          DOI: 10.1093/icb/icr015Save Citation »Export Citation »E-mail Citation »

                                                                                                                                          Emphasizes that different stages of the life cycle may be exposed to very different microclimates and may vary in their sensitivity to water and heat stress. For example, larvae of Manduca sexta develop successfully at high temperatures that are lethal to eggs.

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                                                                                                                                          • Klok, C. Jaco, and Steven L. Chown. 1999. Assessing the benefits of aggregation: Thermal biology and water relations of anomalous emperor moth caterpillars. Functional Ecology 13.3: 417–427.

                                                                                                                                            DOI: 10.1046/j.1365-2435.1999.00324.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                            Caterpillars of a large emperor moth form large aggregations in the first three instars, and this results in reduced rates of water loss and increased body temperatures. Aggregation has physiological benefits because it increases the effective body size of the caterpillars.

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                                                                                                                                            • Tracy, Randall L., and Glenn E. Walsberg. 2002. Kangaroo rats revisited: Re-evaluating a classic case of desert survival. Oecologia 133.4: 449–457.

                                                                                                                                              DOI: 10.1007/s00442-002-1059-5Save Citation »Export Citation »E-mail Citation »

                                                                                                                                              Kangaroo rats, common nocturnal granivorous rodents of North American deserts, are a classic case study in the physiological ecology of water conservation. This field study contradicts some earlier assumptions about their diet, activity, and burrow microclimate (not so mild after all).

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                                                                                                                                              • Willmer, Patricia G. 1982. Microclimate and the environmental physiology of insects. Advances in Insect Physiology 16:1–57.

                                                                                                                                                DOI: 10.1016/S0065-2806(08)60151-4Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                A comprehensive review of microclimates and their physiological advantages for insects, with case histories from the major orders.

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                                                                                                                                                • Withers, Philip C. 1998. Evaporative water loss and the role of cocoon formation in Australian frogs. Australian Journal of Zoology 46.5: 405–418.

                                                                                                                                                  DOI: 10.1071/ZO98013Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                  Reports measurements of evaporative water loss and resistance to water loss for a variety of resting and cocooned Australian frogs.

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                                                                                                                                                  Water Balance in a Changing Climate

                                                                                                                                                  Forecasting species responses to climatic warming requires knowledge of how temperature impacts may be exacerbated by other environmental stressors, especially water shortage. For insects, Chown, et al. 2011 examines the buffering effect of behavior, regulation of the components of water loss, plasticity in the responses, and the implications of environmental change for insect populations. Birds may be particularly vulnerable to climate change, due to their high rates of evaporative cooling at high air temperatures. McKechnie and Wolf 2010 models the effects of hot weather on avian water balance and concludes that catastrophic mortality events will become more frequent. The authors of Henen, et al. 1998 carried out a long-term study of climate effects on three populations of desert tortoises in the Mojave Desert. In the ecophysiological forecasting approach in Kearney, et al. 2013, biophysical models of heat and water exchange are combined with nutrition and applied to hatchling and adult lizards. For an invertebrate example of nutrient availability interacting with water shortage, see Benoit, et al. 2010. Physiological ecology will play an important role in predicting future shifts in geographic range (e.g., of insect disease vectors) and in devising conservation strategies for threatened species.

                                                                                                                                                  • Benoit, Joshua B., Kevin R. Patrick, Karina Desai, Jeffrey J. Hardesty, Tyler B. Krause, and David L. Denlinger. 2010. Repeated bouts of dehydration deplete nutrient reserves and reduce egg production in the mosquito Culex pipiens. Journal of Experimental Biology 213.16: 2763–2769.

                                                                                                                                                    DOI: 10.1242/jeb.044883Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                    The ability of the mosquito Culex pipiens to survive repeated bouts of dehydration improved when sugar was available during rehydration. Also important as a study of the effects of repeated desiccation stress.

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                                                                                                                                                    • Chown, Steven L., Jesper G. Sørensen, and John S. Terblanche. 2011. Water loss in insects: An environmental change perspective. In Special issue: “Cold and desiccation tolerance” honoring Karl Erik Zachariassen. Journal of Insect Physiology 57.8: 1070–1084.

                                                                                                                                                      DOI: 10.1016/j.jinsphys.2011.05.004Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                      Critical review of key questions concerning the responses of insects to changing water availability, and the interactions with temperature. For example, increases in metabolic rate due to warming lead to increases in respiratory water loss in ectotherms.

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                                                                                                                                                      • Henen, Brian T., Charles C. Peterson, Ian R. Wallis, Kristin H. Berry, and Kenneth A. Nagy. 1998. Effects of climatic variation on field metabolism and water relations of desert tortoises. Oecologia 117.3: 365–373.

                                                                                                                                                        DOI: 10.1007/s004420050669Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                        This study of the effect of climate on water turnover and field metabolism in Mojave Desert tortoises found extreme variability in these parameters on a seasonal and annual basis, as well as between sites due to rainfall patterns.

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                                                                                                                                                        • Kearney, Michael R., Stephen J. Simpson, David Raubenheimer, and Sebastiaan A. L. M. Kooijman. 2013. Balancing heat, water and nutrients under environmental change: A thermodynamic niche framework. In Special issue: Mechanisms of plant competition. Functional Ecology 27.4: 950–965.

                                                                                                                                                          DOI: 10.1111/1365-2435.12020Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                          The authors use a microclimate-modeling approach to consider the consequences of foraging behavior for heat, water, and nutrient balance in a herbivorous, rock-dwelling skink from Australia. Highlights the much-better knowledge of thermal than of hygric traits. Use of hatchlings and adults demonstrates the physiological consequences of surface area to volume ratios.

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                                                                                                                                                          • McKechnie, Andrew E., and Blair O. Wolf. 2010. Climate change increases the likelihood of catastrophic avian mortality events during extreme heat waves. Biology Letters 6.2: 253–256.

                                                                                                                                                            DOI: 10.1098/rsbl.2009.0702Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                            Global warming will increase the conflict that desert birds face between conserving water and maintaining body temperature below lethal levels. A model of evaporative water requirements and survival times during the hottest part of the day reveals that small birds will be particularly vulnerable to acute dehydration.

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