150 Temperature 



type of migration serves to avoid the severity of winter. Some ani- 

 mals work their way into rotten logs, others dig deep in the soil, and 

 many species squirm into the mud of swamps or pools. This move- 

 ment of animals into winter quarters is often loosely referred to as 

 hibernation, and should not be confused with the structural and 

 physiological changes often occurring in winter dormancy and de- 

 scribed by the same term. 



Somewhat longer journeys that result either primarily or secondarily 

 in the avoidance of extreme temperatures are made by larger land ani- 

 mals. Bear, deer, and other game animals descend from the moun- 

 tains into the sheltered valleys when the weather gets cold. In the 

 spring they return to higher elevations and to more exposed situations. 

 The same type of thermal migration of moderate length is seen in the 

 aquatic environment. Many fishes and other active aquatic animals 

 leave the shore in the summer when the water has become too warm. 

 Conversely, other species migrate into deeper water during the 

 winter to avoid what are for them excessively low temperatures. 

 Such migrations at contrasting seasons are illustrated very neatly by 

 two species of flounder that inhabit the coastal areas of New England. 

 The so-called summer flounder (Parolichtliys dentatiis) enters the 

 shallow water of the bays in June and departs in October. The winter 

 flounder (Pseudopleuronectes americaniis), on the other hand, is not 

 able to tolerate the inshore temperatures characteristic of July and 

 August. Fish of the latter species are abundant in the upper reaches 

 of Great South Bay, Long Island, during the winter. As spring comes 

 on the population moves toward the mouth of the bay, and, when the 

 summer sun warms the water beyond about 20°C, the majority of this 

 species migrates to the cooler, deeper water outside the inlet (Fig. 

 5.9). During the autumn months the winter flounder returns once 

 more into the bay. 



Migrations of still greater length are carried out by certain mam- 

 mals, insects, and birds. Although change of temperature is not the 

 primary cause of some of these mass movements, escape from climatic 

 extremes is certainly an important consequence in many instances. 

 The caribou migrate long distances, and in former times gigantic herds 

 of bison traveled hundreds of miles across the American plains from 

 their winter grounds to their summer feeding areas. The extensive 

 north-south migrations exhibited by some insect species should also 

 be considered as thermal migrations in part at least. Most spectacular 

 of all are the trips made by migrating birds. Considerable evidence 

 has now accumulated to indicate that change in day length is the 

 factor that initiates the southward migration of birds at the end of the 



