NATURAL AREAS AND REGIONS 



553 



larger mammals, apparently because 

 of the relative scarcity of places of 

 retreat. 



2. Ability to burrow. Examples are 

 the pocket-gopher, ground-squirrel, 

 badger, prairie-dog, mice, vole. About 

 70% of the species rear their young in 

 burrows and nearly 50% of the species 

 spend much of their time underground. 

 Burrows are retreats from heat, cold, 

 wind, and some enemies, and for the 

 prairie-dog, at least in some cases, 

 furnish access to the underground water 

 supply. 



3. Many have acute long-range vision; 

 the fleet runners all have. This appears 

 to have been developed by the same 

 factors mentioned in the discussion of 

 the vision of birds. 



4. A gray or tawny type of coloration 

 which harmonizes well with dead leaves 

 is possessed by nearly all. The skunk, 

 an exception, is less in need of protective 

 coloration than are the other mammals 

 of the plains. 



5. Ability to do without much drink- 

 ing water. Water for physiological 

 activity and for cooling by perspiration 

 is secured mainly from the food eaten. 

 Footprints of most steppe animals are 

 seldom seen in mud about water holes. 

 This is especially true in regard to the 

 rodents, including the jackrabbit. 



6. The daily period of activity is 

 chiefly in the early morning, in the 

 evening, and to a lesser degree at night. 



Voluntary activity of almost all 

 abundant mammals of the grassland is 

 very limited during the heat of the 

 summer day, when as many as may be 

 are in the shade. Activity generates 

 heat which must be eliminated by 

 perspiration, which in turn requires 

 water, an article which often is precious. 



7. Ability to hibernate. The long- 

 distance runners and the carnivorous 

 do not hibernate, but, with the exception 

 of the rabbits, the rodents, the most 

 numerous mammals on the plains, do, 

 and for longer periods than related 

 species in other formations. Hiberna- 

 tion is a response to the unfavorable 

 conditions which prevail during the 

 winter months. 



8. The larger herbivores, the bison 

 and antelope, migrated chiefly in re- 

 sponse to irregularity of rain and snow. 

 Wolves, which preyed upon them, 

 accompanied them in their wanderings. 



9. A few of the mammals of the grass- 

 land are gregarious; the bison and ante- 

 lope congregated probably for protection 

 from wolves and bears, and in response 

 to the lack of numerous places of escape; 

 but also in winter for the heat accumu- 

 lated in a closely packed herd. Prairie- 

 dogs are grouped in towns for protection 

 against coyotes and certain hawks, 

 and possibly by the somewhat restricted 

 soil and ground water conditions which 

 they appear to require. 



Reptiles. The reptiles hibernate for 

 nearly half the year, or even longer; can 

 get along without drinking water; and 

 are of small size as compared with 

 related forms in more humid areas. 

 Although the vast majority of reptiles 

 lay eggs, three of the five abundant 

 steppe reptiles (the rattler, plains 

 garter-snake, and horned-lizard) bring 

 forth their young alive. This may be 

 in response to the "hard" conditions 

 imposed by the climate. 



Insects. Concerning the insects of the 

 grassland, the following statements may 

 be made : 



1. The diurnal period of greatest 

 activity is in the forenoon from 7 to 11 

 o'clock, after the chill of the early 

 morning is gone but before the heat 

 becomes oppressive. Coition is carried 

 on chiefly between 11 o'clock and one. 

 During the rest of the twenty-four hours 

 most of the insects are quiescent except 

 when disturbed. 



2. Seasonal activity is limited nearly 

 to the warmer and more moist three or 

 four months of the year, chiefly June, 

 July and August. There is almost 

 complete inactivity during the colder 

 and drier months of the year. Most 

 species are dormant during ten or 

 eleven months of the year, when many 

 are represented chiefly by eggs. 



3. Many forms burrow or occupy 

 mammalian burrows, in so doing having 

 retreats from wind, heat, cold and some 

 enemies. Grassland species in many 



