Water, Salts, and Excretion at Work: Mammals of Deserts and Dry Savannas HWA Chapter 30
Temperature Regulation in the Heat: Keeping Cool many animals evaporate water at a rate equal to their metabolic heat production in a hot environment, much more evaporation (10 times as much) is needed to balance heat conduction and radiation into the animal one day in a hot desert without water will kill a human Rate of evaporation depends on body size & surface area
Figures 28.18 and 30.1 -The rate of evaporative water loss experienced by birds or mammals exposed to heat stress depends strongly on their body size Within a group, total rate of evaporative water loss is an allometric function of size Evaporation to maintain a constant body temperature increases rapidly with body size under hot desert conditions. anphys2e-fig-29-01-0.jpg
(percent of body weight per hour) Rate of evaporative water loss Figure 30.1 The rate of evaporative water loss experienced by mammals exposed to heat stress depends strongly on their body size If the red line is extrapolated, it predicts that mouse- and rat-sized animals would have extremely high rates of evaporative water loss while walking under the desert sun. 13% for 100-g animal 20% for 25-g animal 10 9 8 7 6 5 4 3 2 1 The red line and red points represent humans and animals that are walking under the sun in a hot desert. The line assumes that body-core temperature is maintained at 37°C by sweating or panting. The blue line and blue points represent humans and animals resting at high air temperatures. (percent of body weight per hour) Rate of evaporative water loss Lab mice Merriam’s kangaroo rats Dogs Sets of humans Lab rats Big burros Small burros Desert k-rats 100 200 300 400 500 Body weight (kg)
Keeping Cool: The Importance of Body Size Large body size is a physiological advantage in terms of water costs Smaller animals are at a disadvantage Due to higher surface area to volume, small animals heat up or cool down faster than large animals This is also true for organs with large surface area (elephant ears) A small animal in a hot environment will roast, a rat will die in about 1 hour
Small animal Large animal Effect of Body Size High water loss/unit body mass Cannot travel far Can use micro- climates Large animal Low water loss/unit body mass Travel long distance for water Cannot use large range of microclimates
Coexisting species are diverse in their relations to drinking water Drinking water independent Remain healthy for long periods without access to drinking water Drinking water dependent Must drink daily or every other day
Figure 30.2 Common wildebeests are drinking-water-dependent antelopes that seek shade anphys2e-fig-29-02-0.jpg
Figure30.3 Grant’s gazelles are drinking-water-independent antelopes noted for their indifference to sun or shade anphys2e-fig-29-03-0.jpg
Fig 30.4 The annual migration of drinking water dependent zebras and wildebeests in the Serengeti ecosystem anphys2e-fig-29-04-0.jpg
Fig 30.6 Experimental water budgets of wildebeests and oryxes All species of large herbivores require considerable amounts of preformed water Oryx are water independent and require less water (especially preformed). Less water loss means less water required anphys2e-fig-29-06-0.jpg
Figure 30.7 The moisture content of “dry” grass varies with time of day or night anphys2e-fig-29-07-0.jpg
Oryxes conserve water exceptionally well by several mechanisms Figure 30.10 Oryxes in deserts often exist on dead, dry grasses and the leaves of water-stressed bushes and trees Oryxes conserve water exceptionally well by several mechanisms anphys2e-fig-29-10-0.jpg
Figure 30.11 Average body-core temperatures of free-living Arabian oryxes over the 24-hour day anphys2e-fig-29-11-0.jpg
Figure 30.12 Dromedary camels—the “ships of the desert”—do not carry extra water in their bodies, as legend has often held AnPhys3e-Fig-30-12-0.jpg
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Advantages of being large in hot environment, e.g., camel. Camels do not carry water Camels evaporate little water per unit body mass due to size. When fully watered, camel can use water to evaporate off heat.
Changes in Core Body Temperature When unwatered, camel can tolerate increase in body temperature up to 41C. (SN Fig. 7.23) Their body temperature go up and down by about 7 C, which for a big animal is a lot of heat. Oryxes undergo similar changes (HWA Fig 29.11)
Figure 30.11 Average body-core temperatures of free-living Arabian oryxes over the 24-hour day anphys2e-fig-29-11-0.jpg
Advantages of being large in hot environment, e.g., camel. High body temperature allows: Water conservation (does not have to stay as cool). Reduces difference in external & internal temperatures Less heat absorbed from environment. slow conduction of heat into the body.
Advantages of being large in hot environment, Thick fur, e.g., camel Insulation reduces amount heat absorbed from environment. Knut and Bodil Schmidt-Nielsen et al. observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals’ skin remained more than 30°C cooler. The Schmidt-Nielsens reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating.
To test this hypothesis, they compared the water loss rates of unclipped and clipped camels. Removing the fur of a camel increased the rate of water loss through sweating by up to 50%. Control group (Unclipped fur) Experimental group (Clipped fur) 4 3 2 1 (L/100 kg body mass) Water lost per day
Advantages of being large in hot environment, e.g., camel. fur Fat stored in hump Camels can tolerate about twice the water depletion as a human. Human would die if lost ~12% of body weight. Camel can tolerate loss of ~25% without adverse effects. Water loss from interstitial fluid not plasma
Camel drinks more than third of body weight. camels have enormous capacity to rehydrate themselves can make up a 25% body weight loss in 10 minutes comparable to 200 lb man drinking 50 lbs water in 10 minutes (roughly 6 gal of water)