Conduction of Heat in Animal Coats and Tissue

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

tissues under the skin, then through the coat, and 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 tempera- ture. Table 12.2 gives maximum and

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 con- trolling 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 control- ling heat loss. Figure 12.4 shows conductance for pieces of

under laboratory conditions. In Ch. 7 conductances for layers of still air are com- puted. Since heat transport in animal coats can be by conduction through the air, by

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

for coats thicker than 3 cm, no matter how thick the coat is. The radiative conductance of a coat depends on the average distance ra- diation can travel within the coat (Cena and Monteith, 1975). The shorter the radiation path length the lower the radiative conductance. This

T A BL E 12.2. Thermal conductance of peripheral tissue of animals (from Monteith and

1990; and Kerslake, 1972)

Animal Vasoconstriction conductance Vasodilation conductance (mol

0.38 0.83 pig (3 months) 0.42 0.69

down sheep 0.46 1.4 human

Conduction of Heat in Animal Coats and Tissue 195

- - radiative conductance

shrew squirrel lemming marten

o rabbit fox, dog, beaver reindeer, white fox

100 wolf, grizzly b e a r

polar b e a r 0 0 sheep

air conductance

Coat Depth

12.4. Coat conductance of animal fur compared to conductance of an equivalent thickness of still air and radiative conductance of open space.

F IGURE

ference in radiative path length is the main factor determining the quality of insulation in both homes and outdoor clothing. From Fig. 12.4 it ap- pears that these coats are surprisingly effective at minimizing radiative transport within them.

As with the tissue conductance, getting an overall picture of coat ef- fects on thermoregulation by looking only at conductances of pieces of is difficult. Coat depth varies from point to point, and an average thermal conductance for the entire body is needed. For this reason, con- ductances determined on live animals are likely to be more useful than those estimated on portions of animal coats. Calder and King (1974) give a relationship for the minimum conductance of birds, based on measurements of metabolic rate. It is

0.06 (12.17) where m is the body mass in kg. In the absence of other information,

or Fig. 12.4 could be used to estimate the minimum conductance for the animal as- suming blood flow is restricted to the best insulated part of the body. The

minimum conductance could be computed

Eq.

maximum conductance is achieved by shunting blood to poorly insulated appendages. Animals can often increase conductance by a factor of about three times the minimum by doing this. Figure 12.5 is a dramatic example of the range of conductance which can be achieved by an animal. Note that, for a resting white crown sparrow, minimum conductance occurs between about 10 and 25" C. By about 45°C the conductance has tripled.

Animals and their Environment

-40 -30 -20

0 10 20 30 40 50 60 Air Temperature ( C )

F IGURE 12.5. Variation of tissue and coat conductance of white crown sparrows with temperature (data from

and King, 1977).

Clearly this range of conductance is not achieved simply by vasodilation and vasoconstriction. Webster et al. (1985) showed that

doves posture, ptiloerection, and other responses to cold could substantially alter conductance.

It is interesting that conductance increases from its minimum with both increasing and decreasing temperature. The increase with increasing temperature is for thermoregulation, but the increase as temperatures drop below freezing is probably the result of shunting blood to appendages at these cold temperatures to avoid freezing them.

Wind has a major effect on the thermal resistance of clothing and animal coats. Campbell et al. (1980) analyzed much of the available data on windspeed dependence of coat conductance and obtained the equation:

where

is the conductance

is the conductance (for heat or vapor) in wind,

coat at zero windspeed, and is a constant that depends wind permeability

coat. Campbell et al. (1980) show variation in c between 0.03

and 0.23 with typical values for dense coats

3 to 4 cm thick being around 0.1 Therefore expect a 10 wind to approximately double the animal conductance.

Rain can also substantially alter the conductance of animal coats. Webb and King (1984) compared the conductance of wet and dry coats under

a range of conditions, and found that, on average, the conductance of wet coats is about double that of dry coats. Part of the decrease is due

Qualitative Analysis of Animal Thermal Response 197

to decreased coat thickness, but a more important part is the latent heat transport.