PHYSIOLOGY 61 



There are, however, limits to the adjustment which an animal can make. When environ- 

 mental conditions become more severe than it can cope with, death results. 



But even though survival is not endangered, changes in the conditions to which an animal 

 is subjected affect its vigor and behavior. The varying cumulative effect of such forces at 

 different places undoubtedly is a major factor in determining the extent of the distribution of a 

 species. Similarly, at different points within its range these relationships govern to a consid- 

 erable degree seasonal need for various types of food and shelter. Again, severe conditions 

 during one year may so lower an animal's vitality that it is unable to recuperate in time to 

 survive the next. 



Therefore, a knowledge of the principal environmental influences involved and of how they 

 affect the species in which one is interested will aid both layman and technician in recog- 

 nizing habitat deficiencies. Likewise the game manager will be better able to devise effec- 

 tive development measures. 



Only recently, however, have detailed studies of these relationships been undertaken deal- 

 ing with game species. The data presented herewith are the first pertaining to the ruffed 

 grouse*. 



With respect to this species the basic environmental forces affecting it physiologically are 

 air temperature, air movement, radiation of heat, evaporation, humidity and light. Factors 

 such as shelter and food merely modify a bird's response to the other niore fundamental 

 influences. 



A GROUSE WITH ITS FEATHERS FLUFFED OUT TO PROVIDE INSULATION AGAINST THE COLD 



In measuring the effect on the bird of such conditions or combinations, the jirimary indices, 

 in addition to survival itself, are body temperature, respiration rate and heart rate. But 

 before significant variations can be recognized one must know what the normal is. Initial 

 experiments were jilanned accordingly. Then, following these, others were conducted to 

 determine the effect of various environmental changes. In connection with the data secured, 

 however, one must bear in mind that the birds used were from hand-raised stock and may 

 not be entirely representative of wild grouse. They were also all adult. 



* A more detailed accuiinl of these experiments is included in the Appendix, p. 749. 



