Complexities of Animal Energetics

12.9 Complexities of Animal Energetics

The models we have presented for interaction can

be very useful for analyzing organism response to environment and un- derstanding the most important factors in the animal environment. There

Complexities of Animal Energetics

are many cases, however, where our simplifying assumptions are too re- strictive, and can lead to incorrect conclusions. The limitations we have imposed on latent heat loss exclude any analysis of sweating. A more complete analysis, however, will be given in Ch. 13. We also failed to consider heat loss by conduction to the ground or other substrate (even though the equations for that are given in Ch. 8). Perhaps the most serious omissions are a failure to consider the possibility that radiation can pen- etrate the animal coat, and the failure to consider the three-dimensional nature of the animal. To add these complexities goes beyond the objec- tives of this book, but excellent work has been done in both areas, and we briefly refer to the results of that work.

Our energy balance equations are essentially for a one-dimensional an- imal. We assume that the heat is well enough mixed internally to maintain an essentially constant internal temperature. We also chose a single char- acteristic dimension and a single

value for the animal in spite of the fact that we know both of these values vary widely over the surface of the animal. Coat conductance also varies substantially fromplace to place de- pending on the thickness of the coat and exposure to wind.

1) addressed these issues with what he calls a two-dimensional operative temperature model. This new model just divides the animal up into many

zones (head, legs, body in sun, body in shade, etc.), each of which can be adequately analyzed by an equation similar to Eq. (12.11). An operative temperature for each zone is also computed. The overall energy budget is

the area-weighted average of all zones. From this

of analysis

he concludes that in strong wind or sun the one-dimensional model can give substantially different results than the two-dimensional model. In one example, the operative temperature from the two-dimensional model

was 6°C lower than for the one-dimensional model. If radiation penetrates the coat of an animal, the location of energy absorption ceases to be the outer boundary of the coat. Dissipation of heat, however, still occurs at the outer boundary, so the effective radiation heat load on the animal is higher. This is a kind of miniature greenhouse

effect. Walsberg et al. (1978) determined that, for small animals with high boundary layer conductances, radiation penetration is important in determining the optimum coat color for animals in desert environments. Solar radiation penetrates to deeper depths in white coats than black. Even though the total energy absorbed by a black coat is much greater than that absorbed by a white one, the additional heat load from radiation penetration of the white makes the black coat more suitable for desert environments. Observations of coat color in desert dwelling animals seem to confirm this result.

In sparse animal coats, both long and shortwave radiation penetrate the coat, and it becomes impossible to treat the

interface as a definite boundary as we have in this chapter. To deal with it properly as a continuum, computer models must be used. Porter et al. (1994) have developed such models and have shown them to work well in ecological applications. The model has the advantage that it properly treats all of the

204 Animals and their Environment

complexities of the interaction. The disadvantage is that it provides little opportunity for understanding the physical principles involved in the exchange processes except to the person who creates the model.