98 



ANALYSIS OF THE ENVIRONMENT 



and warm-blooded animals tends to break 

 down, also, when approached from the 

 homoiothermal side of the division line. 

 Monotremes afford an example of forms es- 

 sentially transitional between poikilothermy 

 and homoiothermy. The body temperature 

 of Echidna varies from 26.5° to 34° C. 

 without correlation with air temperature 

 (Semon, 1894). Nesthng birds may start as 

 poikilothermous animals and, later in ontog- 

 eny, develop the ability to regulate their 

 temperature. Hibernating mammals become 

 essentially poikilothermic during hiberna- 



tures near maximum toleration; the safet)^ 

 factor is much greater at the lower than at 

 the upper end of the tolerated temperature 

 range. This relationship is illustrated in a 

 generalized fashion in Figure 12; it holds 

 for the majority of aquatic as well as for 

 terrestrial forms. 



As with several other environmental fac- 

 tors, the temperature extremes that an ani- 

 mal can tolerate depend on a complex series 

 of relationships. Some of these are: (1) the 

 species or other taxonomic subdivision; (2) 

 the external temperature at which the spe- 



INCREASING TEMPERATURE 



HEAT COMA 



Fig. 12. The effect of temperature on activity of animals. (Redrawn after Verwom.) 



tion. The temperature of the extremities of 

 homoiothermal animals fluctuates somewhat 

 with the environment (see p. 120), and the 

 body temperatures of small birds and mam- 

 mals are independent of the environmental 

 temperature within a narrow range only 

 (Kendeigh and Baldwin, 1928; Rasmussen, 

 1916; Chevillard, 1935; Gerstell, 1939). 

 Thus, adult passerine birds have a nor- 

 mal body temperature between 38.9° and 

 44.6° C. The environmental temperature 

 can carry body temperatures of these birds 

 down to about 23.9° and up almost to 

 46.7° C. for short periods without being 

 necessarily fatal for the birds. The lower 

 margin of safety is rather large, some 15 

 degrees; this is not the extreme Limit of tol- 

 erance, for the tiny house wren {Troglo- 

 dytes aedai parkmani) has survived after 

 its body temperature was lowered to 

 16.7° C. for a short time. The upper mar- 

 gin of safety between the highest normal 

 and the lethal body temperature is much 

 less; it is only 2.1 degrees for adult passer- 

 ines. This condition seems to be general 

 (Kendeigh, 1934). Most animals, cold- 

 blooded as well as warm-blooded, operate 

 most efficiently in many ways at tempera- 



cies normally li\'es and that in which the 

 given individuals have lived recently; (3) 

 the length of exposure; (4) the internal 

 body temperature; (5) the rate of change 

 of internal temperature as extremes are 

 approached; (6) for low temperatures, the 

 presence or absence of internal ice; (7) the 

 general condition of the individuals as re- 

 gards items hke water content and thermal 

 insulation (cf. Luyet and Gehenio, 1938). 

 Roughly stated, most poikilothermous or- 

 ganisms are active at temperatures between 

 6"^ and 35° C. Numerous exceptions are 

 known, and even the more generous limits 

 from about —37° to -f64° C," such as 

 have been found on the surface of the Lake 

 Michigan dunes, do not reach the extremes 

 of endurance of active animals. Entire Ufe 

 histories are passed both above and below 

 the usual temperature limits. 



* Higher surface temperatures are known. 

 Geiger (1927) cites 71.5° at Tucson, Arizona, 

 and 69° C. at Agra, India. Johnson and Davies 

 ( 1927 ) estimate that it is unlikely that the sur- 

 face temperature of the soil will ever exceed 

 200° F. (93.3° C), and the highest soil 

 temperature that may be expected is about 

 180° F. (82.2° C). 



