ECOLOGICAL FACTORS 409 



the Transcaspian steppe. 116 Amphibians withdraw into crevices and 

 holes in the ground and lie there in a death-like sleep. Of the reptiles, 

 the hydric crocodiles, turtles, and snakes aestivate; even many lizards 

 in the Sudan crawl away and hide during the driest period. Crocodiles 

 bury themselves in the mud through the dry period. Aestivation is 

 also known among the mammals. The aardvark of the African steppes 

 aestivates, 117 and so, in Madagascar, do the peculiar insectivores 

 Centetes and Ericulus, and the lemurs Chirogale milii and Micro- 

 cebus. 118 Birds are not known to aestivate or to hibernate. 



Hibernation in the winter corresponds to aestivation in the dry 

 season. Earthworms crawl deeper into the ground and discontinue their 

 activities. Many of the insects and spiders overwinter in the egg stage, 

 and thus are less susceptible to the injurious effect of the cold; many 

 other insects survive the unfavorable period as pupae. Many over- 

 winter as larvae and imagoes hidden away in cracks and holes in the 

 earth, under pieces of bark, and in hollow trees. Snails bury themselves 

 under the covering material on the ground or bore into available 

 fissures and crevices. Some amphibians hibernate at the bottom of 

 bodies of waters; others bury themselves in the mud or in dry earth. 

 Similarly, the reptiles creep into suitable places, sometimes in groups, 

 as for example, the rattlesnakes. Many mammals also sink into a state 

 of sleep in some frost-free hiding place, in holes in the ground, or in 

 hollow trees, during which their rate of metabolism is much lowered; 

 the frequency of the heartbeat and breathing is greatly decreased, the 

 temperature of the body sinks to 10°. Thus bats, marmots, and bears 

 survive the winter without ingestion of food, since they live entirely 

 on the stored-up supplies in the body. At Kodiak Island, according 

 to guides, the bears gorge themselves with wild cranberries just before 

 hibernation and purge out their intestines thoroughly until they are 

 as clean as though washed with soap and water. They then eat the 

 root of some plants that make a tough fibrous plug at the anus. This 

 is evacuated soon after hibernation ends. The young cubs are born 

 while the mother is still in hibernation, and new-born cubs may be 

 removed without the mother's paying attention. This capacity for 

 suspension of activity with cold is not common to all mammals. 



In general, only homoiothermal land animals can be active during 

 winter. Even these require a sufficient food supply and a means or 

 protection against excessive radiation of heat. Most of the north tem- 

 perate mammals, even those that do not hibernate, have hiding places 

 for the winter into which they can withdraw when they are not occu- 

 pied in the search for food. These recesses are mostly holes in the 

 ground (moles, mice, hamsters, rabbits, foxes), at times the hollows of 



