Ecology of Animals 649 



animals whose body temperature is not constant (the poikilotherms) are 

 affected more than those maintaining an approximately constant one 

 (the homoiotherms). For the former group, the rate of development 

 and growth of the young is dependent upon the external temperature, 

 and they must devise means of surviving the colder and hotter portions 

 of the year. Many forms go into a period of dormancy during the win- 

 ter which is known as hibernation, and others become dormant during 

 periods of drought. The latter type of dormancy is known as aestivation. 

 Further, poikilothermic animals, in contrast to homoiothermic ones, are 

 often limited in their northern distribution, for they have no means of 

 withstanding the extremely low temperatures of northern areas. 



Some few homoiothermic animals do have a dormant period during 

 the winter. This also is known as hibernation. 



Temperature also may affect an animal's structure; this has led to 

 the formation of certain generalizations. One of these states that homoio- 

 thermic animals from colder areas tend to be larger than those from warm 

 ones ; this is correlated with the rate of heat loss. Another generaliza- 

 tion points out that animals from warmer areas have longer extremities. 

 This latter fact has been verified experimentally, that is, experimental 

 animals (mice) raised at higher temperatures have longer tails. Further, 

 the jack rabbit of Arizona has much longer ears than does a closely 

 related form from the Arctic. 



Wind. — While the effects of winds are not so pronounced with ani- 

 mals as with plants, they are nevertheless important. Their chief im- 

 portance is in the manner in which they may distribute smaller forms 

 from one area to another. Winged insects rely upon winds for much 

 of their spread ; ballooning spiders soar to great heights and are trans- 

 ported long distances. Insects living in areas where the winds are ex- 

 treme often are flightless. Birds in mountainous areas usually build their 

 nests behind sheltering rocks ; one bird dwelling in the windy deserts 

 of high Tibet actually constructs a windbreak of stones to protect its 

 nest. Sudden violent windstorms such as tornadoes or hurricanes may 

 have very pronounced local effects on terrestrial forms. During July 

 of 1940 a tornado in Iowa killed about 1,000 birds in a limited area of 

 100 acres. 



Water Currents. — For aquatic forms, water currents are of im- 

 portance in distribution and physiological and structural adjustments. 

 Sessile forms have their pattern of growth affected. 



