ENVIRONMENTAL RELATIONSHIPS 



of cold-blooded animals which become active and thrive only at temperatures 

 much too low to sustain metabolism in other forms. 



With rising temperatures, ectothermous animals become more active; their 

 metabolic rates increase, and they "live faster." At a certain point, how- 

 ever, again varying between different species, continued increase in tempera- 

 ture begins to affect the animal adversely. Eventually, a heat-induced 

 dormancy supervenes, followed very soon by the death of the animal. The 

 maximum temperatures that even the most heat-resistant species can with- 

 stand appear to lie between 48 and 52°C.; most animals are killed by 

 considerably lower temperatures. The primary effect of heat is a derange- 

 ment of the delicately balanced physical state of cellular constituents, prob- 

 ably through minute changes in proteins and lipids. 



For every species of ectothermous animal, there is thus a specific range of 

 temperature within which the organism remains active and capable of carry- 

 ing on its vital functions. Within this range, between the maximum and 

 minimum tolerable temperatures, there is a narrower zone at which the 

 animal operates with greatest efficiency; this is its optimum temperature. 

 Some difficulty is often experienced in precisely defining the optimum 

 temperature; it may vary between stages in the life cycle, and, as indicated in 

 Figure 19.1, it may differ slightly depending on the criteria used. From the 

 standpoint of the most rapid development, the optimum temperature for the 

 pupal stage of Drosophila ranges from 27.5 to 33°C.; however, this 

 temperature range is evidently slightly above the optimum judged by 

 the number of flies successfully completing the pupal stage. From this 

 latter standpoint, the optimum temperature range extends from 22.5 to 

 27.5°C., but here development proceeds somewhat more slowly. The 

 adverse effects of temperatures even slightly above the optimum, however 

 defined, are evident from the data presented. 



For a given species the temperature range may be extensive, or it mav be 

 restricted. Again, the range may cover only a few degrees near zero; it may 

 lie, for example, between 20 and 30°C.; or it may be much nearer, or even 

 beyond, the temperature at which most other animals are killed by heat. For 

 any species the temperature range and the optimum temperature can be 

 determined only by observation and experimentation. We may draw the 

 general conclusion, however, that animals with restricted temperature 

 ranges will be found in nature in the relatively few environments where 

 their temperature tolerances are never exceeded; but species with broader 

 temperature ranges are likely to be much more widely distributed. Thus, 

 temperature acts as a very significant factor in determining the survival 

 and distribution of ectothermous animals. 



By various special activities and characteristics, many kinds of ectothermous 

 animals are able to maintain fairly constant body temperatures through 

 a wide range of external temperature conditions. Notable among these are 

 such insects as wasps and honeybees and such reptiles as lizards. This 

 primitive temperature regulation is of limited significance, however, and 



597 



