38 



MAINTENANCE RATIONS OF FARM ANIMALS. 



body surface rather than to the weight, while the fact that the same 

 law holds true for cold-blooded animals, which assume the tempera- 

 ture of their surroundings and which, therefore, are subjected to no 

 demand for heat, points in the same direction. Apparently we have 

 here a general biological law of which the proportionality between 

 heat production and body surface is one expression. 



The internal work of the animal, however, as measured by the fast- 

 ing katabolism or the fasting heat production, constitutes, as we have 

 seen, its maintenance requirement. The maintenance requirements of 

 animals of different sizes, therefore, especially of those of the same 

 species, are proportional to their surfaces. 



COMPUTATION OF RELATIVE BODY SURFACE. 



Few actual determinations of the body surface of animals have 

 been made and almost none for farm animals, so that it is at present 

 impossible to express with accuracy the metabolism of the latter 

 animals per unit of surface. For purposes of comparison between 

 individuals of the same species, however, another method serves to 

 give at least approximate results. It is a familiar geometrical fact 

 that the surfaces of two solids of the same shape (i. e., similar figures 

 in the geometrical sense) are proportional to the two-thirds powers 

 of their volumes. By regarding all animals of the same species as 

 of the same shape and also as having the same specific gravit}^, so 

 that their weights are proportional to their volumes, it is a very 

 simple matter to compute their relative surfaces and the correspond- 

 ing maintenance requirements. For example, a steer weighing 583 

 kilograms was found to have a computed fasting katabolism (i. e., 

 maintenance requirement) of 8.671 therms. A steer weighing 500 

 kilograms, other things being equal, would have a maintenance re- 

 quirement in proportion to its smaller surface. The latter would be 

 to the surface of the larger animal, approximately, as (500) ^ is to 

 (583) § and the maintenance requirement would therefore be 8.671 



X^|^^ 8 = 7.878 therms. In this way it is a simple matter to com- 

 pute the relative maintenance requirements of different individuals 

 without the necessity of expressing them per unit of surface. 



Of course, such a comparison is only approximately correct. In 

 the first place, it may be presumed that there are differences in the 

 specific gravity of different individuals, although it may be doubted 

 whether these differences are sufficiently great to be of much sig- 

 nificance in this connection. Moreover, different animals are not of 

 the same shape. The young animal differs in conformation from the 

 older one, and the beef steer and the dairy cow, for example, are far 

 from being geometrically similar. It would be of much interest to 

 determine the relation of surface to weight in different species, types, 

 and ages of domestic animals, but lacking such determinations the 



