194
Animals and their Environment
well matched, since any unnecessary weight would also limit the range
of the bird.
12.4 Conduction of Heat in Animal Coats and
Tissue
The conduction of heat from the animal core to the environment is first
through the vascularized tissues under the skin, then through the coat, and
finally through the boundary layer to the surrounding air. Heat transfer
from the body core to the skin surface of an animal depends on blood
flow and is subject to regulation, within limits, by vasoconstriction or
vasodilation. The regulation is important in control of body temperature. Table 12.2 gives maximum and minimum values of average tissue
conductance for several species. These conductances, and their range of
variation, would appear incapable of having much effect on overall heat
loss from animals with coats because they are so large in comparison with
coat conductances. Their important effect, however, is probably in controlling the surface temperature of poorly insulated appendages, which
also have small characteristic dimensions and therefore large boundary
layer conductances.
The conductance of animal coats is normally much lower than the
tissue conductance, and is therefore the limiting conductance controlling heat loss. Figure 12.4 shows conductance for pieces of fix under
laboratory conditions. In Ch. 7 conductances for layers of still air are computed. Since heat transport in animal coats can be by conduction through
the air, by longwave radiative transport, and possibly by free convection,
the conductance of air sets the lower limit for coat conductance. The coats
in Fig. 12.4 follow the air conductance line reasonably well and are well
below the line for radiative conductance in free space. It is interesting
that coat conductance appears to stay fairly constant at around 40 to 50
mmol m-2 s-' for coats thicker than 3 cm, no matter how thick the coat
is.
The radiative conductance of a coat depends on the average distance radiation can travel within the coat (Cena and Monteith, 1975). The shorter
the radiation path length the lower the radiative conductance. This difTABLE 12.2. Thermal conductance of peripheral tissue of animals (from
Monteith and Unsworth, 1990; and Kerslake, 1972)
Animal
Vasoconstriction conductance Vasodilation conductance
(mol m-2 s-I)
(mol m-2 s-')
steer
0.24
0.83
calf
0.38
0.83
pig (3 months) 0.42
0.69
down sheep
0.46
1.4
human
0.46
2.8
Animals and their Environment
well matched, since any unnecessary weight would also limit the range
of the bird.
12.4 Conduction of Heat in Animal Coats and
Tissue
The conduction of heat from the animal core to the environment is first
through the vascularized tissues under the skin, then through the coat, and
finally through the boundary layer to the surrounding air. Heat transfer
from the body core to the skin surface of an animal depends on blood
flow and is subject to regulation, within limits, by vasoconstriction or
vasodilation. The regulation is important in control of body temperature. Table 12.2 gives maximum and minimum values of average tissue
conductance for several species. These conductances, and their range of
variation, would appear incapable of having much effect on overall heat
loss from animals with coats because they are so large in comparison with
coat conductances. Their important effect, however, is probably in controlling the surface temperature of poorly insulated appendages, which
also have small characteristic dimensions and therefore large boundary
layer conductances.
The conductance of animal coats is normally much lower than the
tissue conductance, and is therefore the limiting conductance controlling heat loss. Figure 12.4 shows conductance for pieces of fix under
laboratory conditions. In Ch. 7 conductances for layers of still air are computed. Since heat transport in animal coats can be by conduction through
the air, by longwave radiative transport, and possibly by free convection,
the conductance of air sets the lower limit for coat conductance. The coats
in Fig. 12.4 follow the air conductance line reasonably well and are well
below the line for radiative conductance in free space. It is interesting
that coat conductance appears to stay fairly constant at around 40 to 50
mmol m-2 s-' for coats thicker than 3 cm, no matter how thick the coat
is.
The radiative conductance of a coat depends on the average distance radiation can travel within the coat (Cena and Monteith, 1975). The shorter
the radiation path length the lower the radiative conductance. This difTABLE 12.2. Thermal conductance of peripheral tissue of animals (from
Monteith and Unsworth, 1990; and Kerslake, 1972)
Animal
Vasoconstriction conductance Vasodilation conductance
(mol m-2 s-I)
(mol m-2 s-')
steer
0.24
0.83
calf
0.38
0.83
pig (3 months) 0.42
0.69
down sheep
0.46
1.4
human
0.46
2.8