June, 1941] Animal Bkfkding and Nutrition 17 



(as in a race) or in slow plodding all day effort of applying its 

 weight in draft work. 



Obviously the effect of selective breeding has been to redesign in- 

 ternal motor pattern to meet the requirements that exaggeration of 

 function entails. 



These experiments have shown that the genetic factor represents 

 clearl}- an adaptation in metabolic activity to the particular function. 

 Even with a similar caloric intake the energy conserving- type be- 

 comes fleshy and the energy secreting type remains thin. 



This is illustrated by the production record of two outstanding 

 bovine examples of the result of selective breeding. This result pre- 

 sents an extreme hormone contrast in functional adaptation. One. 

 the energy secreting type, with a high hormone stimulus produced 

 an average of fifty quarts of milk for 365 consecutive days, the other, 

 the energy conserving type, with a lower hormone stimulus, develop- 

 ed into a ton of l^eef. These of course were not experimental sub-, 

 jects so their basal metabolism was not measured. However, the 

 experimental result of measuring the energy balance of two average 

 purebred cows of the same breeds at moderate production and feed- 

 ing indicated the same urge to fulfill the function to which the or- 

 ganism is adapted. In this instance the basal metabolism of the 

 dairy cow or energy expending type was about 25 per cent greater 

 than that of the beef cow or energy conserving type. 



Muscular effort increases the energy expenditure enormously and 

 as a result the Avorking type is provided with a motor corresponding 

 to the needs. Thus our experiments show that the horse has a high- 

 er basal metabolism than any other species so far measured, but even 

 in this species there exists a difference according to the character of 

 work performed. The race horse for example requires an internal 

 adaptation for a greater momentary fuel consumption during a burst 

 of speed than does a draft horse for slow j^lodding work, and this 

 internal adaptation is reflected by a difference in basal metabolism. 



Thus of two unusual type representatives, a thoroughbred race 

 horse, and a purebred Percheron draft horse. Avhose basal metabol- 

 ism we measured, the former had a basal heat production of 12000, 

 the latter of 8800 calories. 



The cost of work in terms of ener_gy rises rapidly in proportion to 

 the effort involved. The measurements -which Ave have carried out 

 with the horse along this line supply surprising results. From an 

 energy expenditure of 750 calories (per hour) when standing quietly 

 the output increased to three times that amount at a slow -walk (no 

 load), to four times at a fast walk and to seven tiiues at a ten mile 

 per hour trot. The same amount of energy ('5100 calories) was spent 

 to pull a 160 pound load at slow walk, but when the load Avas in- 

 creased to 250 pounds the energy output jumped to 9000 calories. 



The relative efficiency of horse and tractors has been frequently 

 discussed but no such fundamental data of caloric cost had CA-er been 

 ascertained for this si^tecies. 



Xo attempt has so far been made to measure the energ\- expended 

 during a race, or of the speed Avith which the energy output regains 

 normality after exertion. The practical result of such study aahII 



