no 



ANALYSIS OF THE ENVIRONMENT 



leaves a large gap in fundamental knowl- 

 edge concerning temperature relations of 

 animals. 



It is not fair to compare the biological 

 eflFects produced by constant temperature 

 with those obtained under the variable con- 

 ditions found in nature, since, for many 

 animals, laboratory life at its best is highly 

 artificial. In the laboratory, organisms ex- 

 posed to variable temperatures frequently, 

 perhaps usually, show accelerated develop- 

 ment as compared with those held at a 

 constant temperature of the same mean 

 value, if other conditions remain equal. This 

 generalization might not hold true for ani- 

 mals that live in the steady temperatures 

 such as obtain in even the upper layers of 

 the soil of the tropical rain forests or at 

 some depth in lakes or oceans. Exceptions 

 have been recorded for terrestrial animals 

 from the middle latitudes (Headlee, 1914; 

 Ludwig, 1928). 



The amount of acceleration varies with 

 diflFerent stages in the life history, with 

 diflFerent species, and with the combinations 

 of temperatures that are used. When the 

 range is held between the minimum effec- 

 tive temperature and the maximum for the 

 given process, blowfly larvae and pupae 

 showed acceleration (Peairs, 1927). The 

 codling moth (Carpocapsa pomonella) was 

 accelerated between 7 and 8 per cent for 

 egg, larval, and pupal stages. Grasshopper 

 eggs held at 22° C. for sixteen hours and 5, 

 10, or 15 degrees higher for eight hours 

 daily showed an average acceleration for 

 Melanopltis mexicamis of 38.6 per cent and 

 for Camnula pellucida of 30.5 per cent, as 

 compared with the rate of development at 

 comparable constant temperatures. Grass- 

 hopper nymphs reared in such alternating 

 conditions were accelerated some 12 per 

 cent over expectation based on results from 

 constant temperatures (Parker, 1930). 



Certain complexities involved are illus- 

 trated by relations reported for the flour 

 beetle, Tribolium confusum. The eggs 

 develop more rapidly in constant tempera- 

 tures than in comparable variable ones 

 without reference to the position of mean 

 temperatures, except when the upper tem- 

 perature lies below the optimum and the 

 lower temperature is at the developmental 

 zero. In the pupae, if the mean of the com- 

 bination lies above or at the optimum with 

 symmetrical alternating temperature, devel- 

 opment is delayed; if the mean lies below 



the optimum, development is accelerated. 

 With Tribolium, the most rapid acceleration 

 comes with an alternating amplitude of 5 

 degrees. The thermal optimum in the action 

 of alternating temperatures may be different 

 from the optimum for constant tempera- 

 tures. 



Alternating temperatures may affect sur- 

 vival as well as the rate of development 

 Tribolium shows increased longevity with 

 alternating temperatures, especially in the 

 lower part of its favorable thermal range. 

 The range of constant temperatures, with a 

 survival greater than 50 per cent, is nar- 

 rower than the range of mean alternating 

 temperatures that produce the same result. 



Relations between effects produced by 

 variable as contrasted with constant tem- 

 perature of the same mean value depend, 

 among other things, upon the species, the 

 process measured, the location of the tem- 

 peratures used in relation to the effective 

 temperature range, and the length and 

 amplitude of the thermal cycle. It is unfor- 

 tunate that the majority of the experimen- 

 tal work has been done with terrestrial 

 insects and plants that are sensitive to 

 changes in humidity and under conditions 

 that at times leave doubt concerning the 

 exactness of the control of the latter factor 

 (Mikulski, 1936, 1936a). 



From an entirely different approach, 

 there is much evidence that man thrives 

 best and works most efficiently when ex- 

 posed to dailv or weekly changes of wea- 

 ther rather than in the same locality in 

 periods of relative constancy. Further 

 changeable temperate climates are more 

 stimulating to man than are relatively con- 

 stant tropical regions (Huntington, 1924; 

 Taylor, 1927). 



ECOLOGICAL TEMPERATURE ZERO 



The lowest temperature at which a given 

 physiological process, or development 

 through a given stage in the life history can 

 be carried through to completion is the ef- 

 fective temperature threshold for the 

 function under consideration. In the lower 

 temperature range, the highest externally 

 imposed temperature at which such a 

 functional unit cannot he successfully com- 

 pleted is its ecological temperature zero. 

 This is a new name for an old idea. The 

 concept of an ecological zero of develop- 

 ment is a general one; the ecological tem- 



