1. Water, Salts, and Excretion at Work: Mammals of Deserts and Dry Savannas HWA Chapter 30

2. 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

3. Figures and 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.
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4. (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)

5. 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

6. 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

7. 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

8. Figure 30.2 Common wildebeests are drinking-water-dependent antelopes that seek shade
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9. Figure30.3 Grant’s gazelles are drinking-water-independent antelopes noted for their indifference to sun or shade
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10. Fig The annual migration of drinking water dependent zebras and wildebeests in the Serengeti ecosystem
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11. 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
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12. Figure 30.7 The moisture content of “dry” grass varies with time of day or night
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13. Oryxes conserve water exceptionally well by several mechanisms
Figure 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
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14. Figure 30.11 Average body-core temperatures of free-living Arabian oryxes over the 24-hour day
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15. Figure Dromedary camels—the “ships of the desert”—do not carry extra water in their bodies, as legend has often held
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16. anphys2e-table jpg

17. 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.

18. Changes in Core Body Temperature
When unwatered, camel can tolerate increase in body temperature up to 41C. (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)

19. Figure 30.11 Average body-core temperatures of free-living Arabian oryxes over the 24-hour day
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20. 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.

21. 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.

22. 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

23. 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

24. 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)