556 NUTRITION OF FARM ANIMALS 



Species. The difference just noted between the efficiency 

 of the human body and that of the horse is evidently due largely 

 to differences in the kind of work performed, since the work of 

 ascent is done with about equal efficiency in both cases. Klein l 

 finds that the work of ascent is performed by the ox with about 

 the same net efficiency as by the horse but that the former 

 animal expends much more energy in horizontal locomotion per 

 unit of distance traveled than does the horse, viz., about 0.53 to 

 0.55 gram calories, per meter distance and kilogram live weight. 



660. Individuality. Zuntz and Hagemann's experiments 

 upon the horse show interesting individual differences between 

 animals, presumably due to differences in conformation. For 

 example, Horse No. XIII carried a given load on his back with 

 less expenditure of energy than did Horse No. III. Horse No. 

 II expended more energy than Horse No. Ill in horizontal 

 locomotion at a walk but less in trotting. No. II likewise 

 utilized energy to a slightly less extent than No. Ill in ascend- 

 ing a grade and to a considerably less extent in horizontal draft 

 but, on the other hand, like No. XIII, carried a load on his 

 back more economically than No. III. 



The possible bearing of these facts upon questions of heredity 

 and breeding opens up an interesting field of speculation. 



661. Training and fatigue. It is a familiar experience that 

 any unaccustomed form of work is much more fatiguing at first 

 than it is later. This is due in part to the fact that in making 

 unfamiliar motions more accessory groups of muscles are called 

 into activity than are necessary later when more skill has been 

 acquired. The experience of a learner on the bicycle is an ex- 

 cellent example of this. In the second place, however, simple 

 exercise of a group of muscles in a particular way seems to in- 

 crease their average mechanical efficiency. 



This effect may be illustrated by the results of two series 

 of experiments by Gruber upon himself in which he de- 

 termined the carbon dioxid excreted during work. Thus in 

 hill climbing the amounts excreted in twenty minutes were : 



Series I : 



Hill climbing without practice 40.98 grams 



Hill climbing after 12 days' practice .... 32.22 grams 



^entbl. Physiol., 26 (1912), 722. 



