446 LAND ANIMALS 



Air currents have a free sweep in open lands. Winds and storms, 

 which are seasonally frequent, drive unhindered over the level or 

 rolling land, at times with terrific force; they carry along dust, sand, 

 or snow, and may carry off even full-grown sheep. The furious winds of 

 the African and Arabian deserts, the sand storms and sand hurricanes of 

 the Transcaspian, and the blizzards and tornadoes of the North Amer- 

 ican plains are notorious for their destructive effects. There is no 

 protection against them in open lands. 



And finally, protection against being seen, important to the plant 

 feeders as a protection against the predatory animals, and to the carni- 

 vores in stalking their prey, is for the most part wanting. 



The burrowing habit. — These extremes of temperature, wind, and 

 visibility explain the fact that burrowing animals are much more 

 common in open lands than elsewhere, since this habit provides ef- 

 fective shelter. Under ground, they are assured of protection against 

 heat and cold, against storms, and from many predators. The fluctua- 

 tion of temperature at the surface decreases progressively until a depth 

 of 0.5 m. the soil temperature approaches the average annual temper- 

 ature of the region. The soil, even at moderate depths, is considerably 

 coolor in summer and warmer in winter than the surface. Thus bur- 

 rowing animals find protection against the heat of the day and the 

 coldness of the night, against summer heat and winter frost. Many 

 animals of the steppes close their burrows during the day against the 

 penetration of heat, 7 just as the hibernating marmot {Marmota bobac) 

 does regularly to ward off the winter cold. 



The ants and termites of open lands belong primarily among the 

 burrowing animals. The ants, whose nests rise in high, loose mounds 

 on the ground in the forest or hang free from the boughs of the trees, 

 build their nests in open country under stones or burrow deep into the 

 soil. The surface mounds disappear, the more the region is exposed to 

 winds; in meadows and grassland, earth mounds are built up above 

 the subterranean part of the nest; in deserts, the entire nest is in the 

 soil, and the wind very quickly blows away the sand mound which 

 forms about the entrance during the building of the nest. In the forest, 

 the termites burrow out the trunks of trees or build carton or earth 

 nests on the branches in addition to their subterranean nests. In open 

 tropical savannas they burrow into the ground and overlay their bur- 

 rows with lasting, resistant structures built from the excavated mate- 

 rial with the help of a sticky secretion; these hillocks are able to 

 resist wind and weather because of the hardness of their walls and 

 are well insulated in the innermost parts by the numerous air cham- 

 bers. The form of the construction may be adjusted to insolation, as in 



