References
205
because of changes in diet with season. For comparison, the osmotic
potential of human plasma is -0.75 Wkg and human urine is generally
between -2.1 and -3.3 kJkg.
MacMillian 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
is h Epr / M = 0.1. We have already shown the respiratory latent heat
loss for the kangaroo rat at 20°C to be 0.02M. The skin latent heat loss
(Eq. (12.16)) is 5 w/m2, if we assume C,, - C,, = 40 mmol/mol and
g,, = 2.8 mmol m-2 s-' (Table 7.2). Equation(l2.11) cannow be used to
find M at Te = T, = 20°C. We assume g ~ b
= 0.14 mol m-2 s-' for the
kangaroo rat and 0.21 mol m-2 s-' for the pocket mouse (estimates from
Figure 12.4 and Table 12.2). Also, assume that g~~ = 0.8 mol m-2 s-'.
The metabolic rates at 20°C, from these assumptions are 88 wlm2 for
the mouse and 65 w/m2 for the rat. The ratio of water produced to water
evaporated is Ep,/Ee, = O.lMl(5 + 0.02M). 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
Bakken, G. S. (1981) A two-dimensional operative-temperature model
for thermal energy management by animals. J. Them. Biol. 23-30.
Bernstein, M.H. (1971) Cutaneous water loss in small birds. Condor
73:468-469.
Calder, W. A. and J. R. King . (1974). Thermal and caloric relations of
birds. p. 259-413 inD. S. Farner and J. R. King, eds. Avian Biology,
V. 4. New York: Academic Press.
Campbell, G. S., A. J. McArthur, and J. L. Monteith . (1980). Windspeed
dependence of heat and mass transfer through coats and clothing.
Boundary Layer Meteorol. 18:485-493.
205
because of changes in diet with season. For comparison, the osmotic
potential of human plasma is -0.75 Wkg and human urine is generally
between -2.1 and -3.3 kJkg.
MacMillian 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
is h Epr / M = 0.1. We have already shown the respiratory latent heat
loss for the kangaroo rat at 20°C to be 0.02M. The skin latent heat loss
(Eq. (12.16)) is 5 w/m2, if we assume C,, - C,, = 40 mmol/mol and
g,, = 2.8 mmol m-2 s-' (Table 7.2). Equation(l2.11) cannow be used to
find M at Te = T, = 20°C. We assume g ~ b
= 0.14 mol m-2 s-' for the
kangaroo rat and 0.21 mol m-2 s-' for the pocket mouse (estimates from
Figure 12.4 and Table 12.2). Also, assume that g~~ = 0.8 mol m-2 s-'.
The metabolic rates at 20°C, from these assumptions are 88 wlm2 for
the mouse and 65 w/m2 for the rat. The ratio of water produced to water
evaporated is Ep,/Ee, = O.lMl(5 + 0.02M). 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
Bakken, G. S. (1981) A two-dimensional operative-temperature model
for thermal energy management by animals. J. Them. Biol. 23-30.
Bernstein, M.H. (1971) Cutaneous water loss in small birds. Condor
73:468-469.
Calder, W. A. and J. R. King . (1974). Thermal and caloric relations of
birds. p. 259-413 inD. S. Farner and J. R. King, eds. Avian Biology,
V. 4. New York: Academic Press.
Campbell, G. S., A. J. McArthur, and J. L. Monteith . (1980). Windspeed
dependence of heat and mass transfer through coats and clothing.
Boundary Layer Meteorol. 18:485-493.