Animals and Water

12.10 Animals and Water

The water budget for an animal can be written in the same form as the energy budget, namely: water in - water out = stored water. Unlike the energy budget, the water budget can seldom be analyzed as a collection of steady-state processes. Intake is from free-water sources, metabolism, and water in the animal's food. Water is taken in discrete events, not continuously. As we have shown, cutaneous and respiratory water loss are relatively continuous, and determined largely by animal activity and environment. Water is also lost in feces and urine, though this is often only a small fraction of the total water loss. Water loss reduces the amount of water stored in the blood, tissues, or digestive tract, and may decrease the osmotic potential (increase concentration) of body fluids.

osmotic potential of kangaroo rats (Dipodomys merriami) in the desert as an index of water balance. Their data (Fig. 12.7) show only slight variations in plasma osmotic potential through the season. Urine osmotic potential decreased in the summer and increased in the winter, indicating higher water deficits in the summer. Urine osmotic potentials of some other desert rodent species were less well correlated with seasonal temperature changes

and

(1975) had

plasma

Nov Mar Nov Mar Jul Nov Mar Nov Mar

12.7. Osmotic potentials of plasma and urine of desert dwelling Dipodomys merriami over a three-year period (data from

F IG URE

and Christopher, 1975).

References 205

because of changes in diet with season. For comparison, the osmotic potential of human plasma is -0.75

and human urine is generally between -2.1 and -3.3 also conducted laboratory studies on several desert rodent species to determine their ability to maintain water balance on a diet of dry birdseed. The kangaroo rat neither lost nor gained weight, but some other species fell far short of maintaining positive water balance. Others

actually gained weight. The pocket mouse (Perognathus longimembris) seemed to be able to maintain a particularly favorable water balance on this diet. One wonders first how these animals can get enough water from dry seeds to supply their needs, and second why the kangaroo rat would

have a less favorable water balance than the smaller mouse. Some light can be shed on these questions by a simple analysis.

When an animal oxidizes food to produce heat, water is also produced. One kilogram of glucose, when oxidized, produces 600 g of water. The ratio of latent heat from respiratory water to metabolic heat produced

0.1. We have already shown the respiratory latent heat loss for the kangaroo rat at 20°C to be

is M =

The skin latent heat loss (12.16)) is 5

= 40 and g,,

if we assume C,, -

be used to find M at =

2.8 (Table 7.2). Equation(l2.11)

0.14 mol for the kangaroo rat and 0.21 mol

20°C. We assume

for the pocket mouse (estimates from Figure 12.4 and Table 12.2). Also, assume that

0.8 mol The metabolic rates at

from these assumptions are 88 for the mouse and 65

for the rat. The ratio of water produced to water evaporated is

For the rat, the ratio is 1.03, and for the mouse, 1.3. These calculations are crude, but they show that the animals produce enough metabolic water to supply their water requirements without any additional water input. They also show that the more favorable water balance of the mouse is the result of the higher metabolic rate it requires to maintain constant body temperature. This metabolic requirement increases as temperature decreases and is apparently too high during the winter months for the mice to remain active because they hibernate during the winter.

References

G. S. (1981) A two-dimensional operative-temperature model for thermal energy management by animals. J.

Biol. 23-30. Bernstein, M.H. (1971) Cutaneous water loss in small birds. Condor

Calder, W. A. and J. R. King . (1974). Thermal and caloric relations of

birds. p. 259-413 S. Farner and J. R. King, eds. Avian Biology,

4. New York: Academic Press. Campbell, G. S., A. J.

and J. L. Monteith . (1980). Windspeed

dependence of heat and mass transfer through coats and clothing. Boundary Layer Meteorol.

Animals and their Environment

Cena, K. and J. L. Monteith (1975) Transfer processes in animal coats, Conduction and convection. Proc, R.

Lond. B. Kerslake, D.

(1972) The Stress of Hot Environments. London: Cambridge University Press. R. C., M. H. Bernstein, and R. D.

(1971) Cutaneous

water loss in the Roadrunner and Poor-will. Condor

The water relations of two populations ofnoncaptive desertrodents. Environmental Physiology of Desert Organisms.

R. E.

A. Christopher

Hadley, ed.) New York: John Wiley. S. A. and J. R. King (1977) The use of the equivalent blackbody temperature in the thermal energetics of small birds. J. Thermal Biology 2: 115-120.

Monteith, J. L., and M. H. (1990) Principles of Environmental Physics, 2nd ed. London, Edward Arnold. Porter, W. P. and D. M. Gates (1969) Thermodynamic equilibrium of animals with environment. Ecol. Monogr. Porter, W. P., J. C.

W. E. Stewart, S. Budaraju, and J. Jaeger (1994) Endotherm energetics: from a scalable individual-based model to ecological applications. Aust. J. Zool. 42: 125-162.

Robinson, D. E., G. S. Campbell, and J. R. King (1976) An evaluation of heat exchange in small birds. J. Comp. Physiol,

153- Schmidt-Nielsen, K. (1969) The neglected interface: the biology of water as a liquid-gas system. Quart. J. Biophys.

-304. Schmidt-Nielsen, K. (1972) How Animals Work. London: Cambridge University Press. Schmidt-Nielsen, K., J. Kanwisher, R. C.

J. E. and W. Le (1969) Temperature regulation and respiration in the Ostrich. Condor Scholander, P. F., V. Walters, R. Hock, and L. Irving (1950) Body in- sulation of some arctic and tropical mammals and birds. Bio. Bull.

Walsberg, G. E., G. S. Campbell, and J. R. King. (1978) Animal coat color and radiative heat gain: a re-evaluation. J. Comp. Physiol.

Webb,

D. R. and J. R. King . (1984). Effects of wetting on insulation of birds and mammals coats. J.

Biol.

Webster, M. D. (1985) Heat loss from avian integument: effects ofposture and the plumage. Indiana Academy of

1-686.

Problems

12.1. Compare the respiratory and skin water loss for humans and mice when air temperature is

C and air humidity is 0.4. Assume the expired air temperature for the mouse is the same as for the kangaroo rat in Fig. 12.3.

Problems 207

12.2. Estimate the upper and lower lethal limit environments for a spar- row. For this, assume that the maximum sustainable metabolic rate for thermoregulation is 3

and the maximum latent heat loss is

12.3. How much food does a 100 kg caribou need to survive an arctic winter with average

of

Assume

12.4. What is the operative temperature for a sunbather standing on a beach at noon on a clear day, with

2 and

a, =

Humans and their Environment

Human-environment interaction involves the same principles discussed in Ch. 12. However, we need to look at three additional factors. They are clothing, sweating, and comfort. These are examined by considering sur-

vival in cold environments, survival in hot environments, and the human thermoneutral energy budget. The variables that need to be considered

are metabolic rate, surface area, latent heat exchange, body temperature, and body (clothing and tissue) conductance.