6Water and Temperature Relations

6 Water and Temperature Relations nimals are mostly water. Water makes up be- creases, the lipid membranes of cells and cell organelles tween 65% and 75% of the body mass of rep- become more permeable, the rate at which electrical im- A tiles and some 75% to 85% of the body mass of pulses travel along nerve axons increases, and the speed amphibians (Shoemaker and Nagy 1977; Hillman et al. and force of muscular contractions increase. How could an 2009). Cells swell or shrink as the volume of water in them animal function in the face of such chaos? The solution is increases or decreases, and those changes alter the con- for an animal to regulate its body temperature and water centrations of dissolved substances within the cells as content so as to minimize the disruptive effects of variation. well as the confi guration of intracellular structures. Blood The study of how animals exchange heat and water becomes more viscous as its water content drops, fl ow- with their environments is known as biophysical ecology, ing more sluggishly and requiring more effort from the and reptiles and amphibians have been especially impor- heart to move it. Thus, any departure from an animal’s tant in the development of this fi eld because interaction normal water and salt content upsets the balance of its with the physical environment is such a conspicuous part biochemical and physiological processes and may limit of their lives. Early studies of temperature regulation fo- its ability to engage in normal behaviors, such as loco- cused on lizards and were carried out in the Soviet Union motion or capturing prey. and North America (Sergeyev 1939; Cowles and Bogert The effect of body temperature is equally profound. 1944), and our knowledge has increased greatly since The rates of many biochemical reactions approximately then (reviewed by Angilleta 2009; Hillman et al. 2009). double when the temperature increases by 10°C and fall Although water balance and temperature regulation are to half the original rate when the temperature decreases closely intertwined, we will discuss them separately for by 10°C. This phenomenon is known as the Q10 effect. If the sake of clarity. the rate of a reaction doubles with a temperature change of 10°C, the Q10 value is 2, and if the rate decreases by half, the Q value is 0.5. Not all reactions have the same 10 6.1 Water Uptake and Loss temperature sensitivity, and Q10 values of biochemical reactions extend from 1 (no change in reaction rate as Regulating the amount of water in the body requires bal- temperature changes) to 3 (tripling of reaction rate) or ancing gain and loss. In a steady state (i.e., no change in occasionally even more. body water content), the total intake of water must equal the Viewed from a biochemical perspective, an organism is total water loss, and each side of the water-balance equation a series of linked chemical reactions, and the product of has several components: each reaction is the substrate for the next reaction in the Gain (liquid water + preformed water + metabolic water) = series. If each reaction has a different Q value, you can 10 Loss (evaporation + urine + feces + salt glands) imagine what havoc a variable body temperature causes with the integration of cellular processes. To complicate This is a general equation that fits terrestrial or aquatic am- the situation still further, these reactions take place in a phibians or reptiles, but some of the details of water move- cellular environment that also changes with temperature. ment apply only to particular situations or certain kinds of As temperature increases, the viscosity of cytoplasm de- animals. © 2015 SINAUER ASSOCIATES, INC. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. 4 Chapter 6 Water and Temperature Relations schel and Seely 2008). The lizard drinks droplets of water Routes of water gain that collect on vegetation and can consume nearly 15% of All of the water an organism needs for its metabolic pro- its body mass in 3 minutes. The snake uses its own body cesses and to replenish water lost by evaporation or in the surface to collect water, flattening to present a large surface feces, urine, and salt glands enters through the mouth or area. The skin of the viper, like that of most snakes, is hy- across the skin, and some amphibians and reptiles have drophobic (i.e., it repels water, much as a freshly waxed car structures and behaviors that enhance collection of water. does), and small droplets of water on the skin flow together Three sources of water are available to animals: liquid wa- to form large drops. The snake moves its head along the ter, preformed water, and metabolic water. length of its body, swallowing the water it has collected. M. anchietae and B. peringueyi occur only in the fog belt of LIQUID WATEr Liquid water is what you normally think of the Namib, suggesting that the collection of water from fog as water—that is, enough water molecules collected in one plays a critical role in their overall water balance. place to form a pool, puddle, drop, or at least a film of water Foggy deserts are limited to coastal locations, but some on the surface of a rock or leaf. Amphibians and reptiles reptiles and amphibians from inland habitats collect rain make use of all those sources of water. (Comanns et al. 2011). The Australian thorny devil (Moloch Many reptiles drink from pools or puddles, just as birds horridus) and several other species of agamid lizards, as well and mammals do. More intriguing are the ways some as some species of North American horned lizards (Phry- reptiles have of obtaining liquid water in habitats where nosoma), and several species of vipers harvest rain (Sher- puddles rarely form. In deserts, water can disappear into brooke 1990, 1993, 2002, 2004; Vesley and Modry 2002; the soil as quickly as it falls, and some reptiles catch water Glaudas 2009). The rain-harvesting behavior of the Mo- droplets before they reach the ground. have rattlesnake (Crotalus scutulatus) has been described by In the Namib Desert of southern Africa, the small lac- Michael Cardwell (2006). This species normally rests in an ertid lizard Meroles anchietae and the viper Bitis peringueyi ambush posture with the anterior portion of the body lying collect droplets of fog that form when moist air from the sea on the posterior part and the head and neck held in an S- blows across the cold Benguella Current (Louw 1972; Hen- shaped striking position (Figure 6.1A). During rainstorms, the snake repositions its body like a coiled garden hose, maximizing the area exposed to rain and al- (A) Ambush posture lowing water to collect between the coils; the head is then inserted between the coils and the snake imbibes the collected water (Figure 6.1B). Like the fog-collecting mechanisms of the Namib reptiles Head in described above, this water-collection mechanism striking is based on the hydrophobic properties of the position scales, which cause water droplets to bead up on the body surface and flow into channels formed between coils of the body. In contrast, the water-collection mechanisms Body rounded of several agamid and iguanid lizards are based on scale surfaces to which water adheres. Many of these lizards adopt a distinctive humpbacked water-collecting posture when they are sprayed with water. Hinges of the scales of Moloch hor- (B) Rain-collecting posture ridus and the Texas horned lizard (Phrynosoma cornutum) form semienclosed channels that con- Head between coils (drinking position) Figure 6.1 Rain-harvesting behavior of the Mohave rattlesnake (Crotalus scutulatus) (A) Resting snake in ambush posture. The body is rounded and the head rests over the posterior in a Body attened, striking position. (B) When it rains, the snake coils channeling and flattens its body, maximizing the area exposed to water into valleys rainfall and drinking the water that collects between the coils. (From Cardwell 2006; photographs by Michael D. Cardwell.) © 2015 SINAUER AssOCIATES, INC. This material cannot be copied, reproduced, manufactured or disseminated in any form without express written permission from the publisher. Pough 4e Sinauer Associates In house Pough4e_06.01.ai 03-11-15 6.1 Water Uptake and Loss 5 (A) (B) Direction of water ow Figure 6.2 Water transport by lizard skin. (A,B) Both the Australian thorny devil (Moloch horridus; A) and the Texas horned lizard (Phrynosoma cornutum; B) harvest rain, although Moloch lacks a stereotyped rain-harvesting stance. (C) Capillary flow carries the harvested water through a network of channels formed by hinge joints between the scales. Rainwater flows down between the scales into a network of interconnected (C) Top view hinge-joint channels. Capillary flow moves water through Channel for rain the channels. Each jaw movement pulls a little water from the Scale water below scales channels into the lizard’s mouth and moves water further away Direction of water ow in the channel system closer to the mouth. (After Sherbrooke et al. 2007 and Wade C. Sherbrooke, pers. comm.) To head duct water to specialized scales at the corners of the mouth (Figure 6.2). Soon after a lizard’s dorsal Plane of surface has been wetted, it begins to open and close cross section its jaws rhythmically with swallowing movements that draw water into the mouth, thereby maintain- (D) Cross section ing the flow of water through the scale channels Direction of (Sherbrooke 2004; Sherbrooke et al. 2007). water ow Condensation of water vapor on the body surface Scales (i.e., the formation of dew) is another potential source of water (Comanns et al.

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