168 Temperature 



of sufficient warmth prevents the estabHshment of a native breeding 

 population despite the fact that the animal's range extends much 

 farther north in situations where the heat requirements are met. 



The northward distribution of turtles in inland waters is similarly 

 determined not by the extremes of temperature but by the need for 

 the minimum amount of heat for the completion of the life cycle. 

 Since turtles burrow in the mud during the winter, the extreme mini- 

 mum air temperature at that season is of no consequence. But these 

 reptiles require the heating action of the summer sun to incubate their 

 eggs. The snapping turtle, for example, lays its eggs in the sand dur- 

 ing the month of May, and sufficient heat must reach them during the 

 summer in order for the young turtles to hatch out successfully in 

 September. The northerly distribution of turtles is correspondingly 

 limited by this long heat requirement to the upper or central part of 

 the United States, but snakes can extend their range well over the 

 Canadian border because heat for the incubation of their eggs is 

 needed for only a short period. 



Differential control of geographical range is also illustrated by the 

 distribution of frogs, whose temperature requirements are widely di- 

 vergent. Some species of frogs seem to need little more heat than 

 that necessary to thaw out their ponds just long enough for reproduc- 

 tion. The eggs of the wood frog, Rona stjlvotica, develop at a tem- 

 perature as low as 2.5°C, and the larval stage requires only 60 days. 

 This species extends northward in Canada to the mouth of the Mc- 

 Kenzie River. The pickerel frog, Rona palustris, on the other hand, 

 must have a temperature of 7.5°C and a period of at least 90 days for 

 its development. The range of this species is correspondingly limited 

 to the latitude of James Bay. In contrast, a third species, Rana clam- 

 itans, will not develop until the temperature exceeds 11°C, and its 

 range extends only slightly above the southern boundary of Canada. 



Control by Need for Chilling. Some organisms require a certain 

 period of cold weather for their life cycles to be completed. No one 

 would plant tulips or crocuses in June and expect them to bloom in 

 August. These plants come into flower only after the bulbs have 

 passed through a winter period of low temperature. The dormant 

 buds of certain fruit trees and berry plants similarly require a period 

 of chilling before they will flower successfully. Experiments have 

 shown, for example, that some types of blueberries require an ex- 

 posure of 800 hours to temperature below 7°C before the dormant 

 buds will develop. In other species the initial formation of flower 

 buds will take place only at low temperatures. 



