104 



ANALYSIS OF THE ENVIRONMENT 



that in the lower ranges of lethal tempera- 

 tures, the effect of heat is a function of the 

 duration of exposure as well as the absolute 

 temperature. No satisfactory generaUzed 

 formulation of these time-temperature rela- 

 tions has been worked out. 



There is a large, though somewhat con- 

 fused, hterature dealing with heat death; 

 much of it has been summarized by Heil- 

 brunn (1943, 1946). Death from heat does 

 not necessarily occur during or immediately 

 after exposure and, if deferred, is not evenly 

 distributed through the passing horns. 

 Rather, as the hfe history develops, death 

 may be restricted mainly to times of 

 greatest physiological activity when there 

 is need for the closest interaction between 

 crucial processes (Larsen, 1943). 



Attention has been paid to heat hardiness 

 as distinct from heat death. Adaptive proc- 

 esses that make for heat hardiness include, 

 among others, evaporation of water from 

 skin or lungs (p. 183), evaporation of 

 water from the nests of social bees or 

 wasps (p. 215), and aestivation in some 

 more or less completely quiescent stage (p. 

 185). Many animals emigrate, burrow, or 

 become nocturnal, and so escape more ex- 

 treme heat. Others apparently acchmate 

 themselves by synthesizing Hpoids with a 

 higher melting point. As with cold hardi- 

 ness, for many organisms, heat hardiness is 

 favored by a decrease in the amount of the 

 water content of the organism (Heilbrunn, 

 1943). 



Ecology, like genetics and evolution, can 

 gain much sound knowledge from the study 

 of domestic animals. Not only do the breeds 

 of cattle wdth a smaller body size tend to 

 have greater heat hardiness (Davidson, 

 1927), as called for by Bergmann's rule (p. 

 119), but a tentative scale of heat hardiness 

 has been worked out by Rhoad (1941) as 

 follows : 



100- lO(Tb-lOl) zzHt 



The formula is based on 101.0° F. as the 

 normal body temperature for cattle: 100 

 represents perfect efficiency in maintaining 

 body temperature at 101° F.; 10 is a factor 

 to convert degrees of deviation in body 

 temperature from the temperature scale to 

 a convenient unit basis; Tb is the observed 

 body temperature under conditions of a 

 "severe" test such as would be furnished by 

 exposure under field conditions to a tem- 

 perature of 95° F.; r. h. of 72 per cent with 



a wind velocity of only 4.5 miles per hour; 

 Ht is the heat tolerance index. A group ol 

 cattle with a mean body temperature of 

 104.3° F. would have an indicated heat 

 tolerance of 



100 - 10( 104.3 - 101 ) = 100 - 33 = 67 



Using tliis scale, Rhoad reports that cattle 

 tested under comparable conditions on a 

 Texas farm showed the heat tolerance as 

 follows: Braliman cows, 93; Jersey cows, 

 86; Hereford steers, 73; and Aberdeen 

 Angus cows, 56. These indexes of heat tol- 

 erance are approximate and tentative. The 

 method is promising and can be applied 

 widely. 



HIBERNATION, AESTIVATION, AND DOR- 

 MANCY OF VERTEBRATES 



Considered in a broad sense, hiberna- 

 tion and aestivation are related phenomena 

 and are united under the concept of dor- 

 mancy. A period of dormancy under con- 

 ditions of heat and drought is much more 

 familiar in invertebrates than in mam- 

 mals (p. 185). The passage of a dry season 

 in summer (and by extension, of any dry 

 season) is commonly referred to as aestiva- 

 tion, which bears directly on the water 

 relations of animals (pp. 183-189). Over 

 wintering in a dormant state is commonly 

 referred to as hibernation. 



Poikilotherms 



The overwintering of cold-blooded ver- 

 tebrates in temperate and northern latitudes 

 is much like that of insects and other inver- 

 tebrates. Most fishes, amphibians, and 

 reptiles are killed by complete freezing, but 

 not by freezing of the extremities. Perma- 

 nently frozen subsoil accordingly Hmits their 

 northward spread. The Alaska blackfish 

 (Dallia pectoralis) is said to survive being 

 frozen solid. In some fresh-water fishes ol 

 the temperate zone there appears to be a 

 tendency to suspend feeding and to form 

 loose or even compact aggregations, some 

 times in the water, sometimes in the bottom 

 mud (Norman, 1931, p. 243; Anonymous, 

 1943, p. 129). Many fresh-water fishes are, 

 of course, active throughout the winter. 

 Marine fishes are not known to exhibit any 

 suspension of activity in the cold season. 



An aestivation period becomes a fixed 

 part of the life cycle of many tropical fresh- 

 water fishes in regions with a sharply de- 



