HEAT 



97 



important in deserts and other hot, dry 

 lands, where the animals by burrowing can 

 escape midday heat and the great fluctua- 

 tions that characterize desert conditions (p. 

 219). Temperature stratification is found 

 also in the air, especially in regions covered 

 by relatively dense vegetation; such strati- 

 fication is well developed in forests. There 

 the daytime temperature gradient is largely 

 a result of dijfferential insolation and tends 

 to disappear on heavily overcast days, es- 

 pecially if there is little air movement. 



Vertical gradients of other environmental 

 factors also exist under the forest canopy as 

 well as in lake or soil. For the forest, the 

 more notable ones include differential dis- 

 tribution of intensity of sunhght, of wind 

 velocity and of evaporating power of the 

 air. These matters are considered in some 

 detail in dealing with biotic factors of the 

 environment (p. 228) and especially in 

 stratification in communities, as discussed in 

 Chapter 28. 



EFFECTS OF HEAT ON ORGANISMS 



The relation of animals to temperature 

 supplies another basic ecological division, 

 that between the animals whose body tem- 

 perature approximates the temperature of 

 their environment, the poikilothermous 

 animals, as contrasted with the so-called 

 warm-blooded or homoiothermous birds and 

 mammals. The body temperature of the 

 homoiotherms may be independent of that 

 of the environment within rather wide 

 hmits. Of the million or so known species 

 of animals, all but about 20,000 are 

 poikilotheiTnal. Homoiothermal mechanisms 

 are not required or fully acquired before 

 hatching or birth. The ability to maintain 

 a given temperature normally improves to 

 a steady state with early development. In 

 seasonally variable chmates, many species 

 of birds and mammals hatch or are born 

 near a time of optimal temperature, but 

 there are so many exceptions that it is 

 questionable whether this involves a definite 

 adaptation or is merely a tendency. The 

 degree of approximation between the body 

 temperature of cold-blooded animals and 

 their immediate environment may be 

 close; the earthworm Lumbriciis agricola, 

 for example, when immersed in water, be- 

 comes adjusted to a change of 10 degrees 

 within two minutes with an accuracy of 

 0.05 degrees (Rogers and Lewis, 1914, 

 1916). 



Small aquatic animals, especially if tlieir 

 muscular activity is low, have a body tem- 

 perature that closely approximates that of 

 the surrounding water. Active fishes, how- 

 ever, may show temperatures some 10 de- 

 grees above their environment, and with 

 passivity the temperature of the surround- 

 ing water is approached slowly. Under 

 many conditions the body temperature of 

 fishes is about that of the surrounding water 

 (Clausen, 1934). 



Terrestrial poikilotherms, especially in- 

 sects and other similarly active forms, may 

 have their body temperatures raised above 

 that of the surrounding air as a result either 

 of their own activity or of insolation. With- 

 in a period of ten minutes, the air tempera- 

 ture remaining constant at 28° C, the 

 internal temperature rose from 27.9° in 

 the shade to 42.7° C. when a third instar 

 grasshopper nymph {Locusta migratoria) 

 was directly exposed to sunrays that had 

 an intensity of 1.07 gram-calories (Strel'- 

 nikov, 1936). As might be expected, the 

 body temperature of black-brown locusts 

 exposed to the sun is higher than ttiat ot 

 green ones. The amount of air movement 

 in the micro-habitat is an important agent 

 in lowering the temperature of insects and 

 other animals. This becomes more eflFective 

 when combined with the evaporation of 

 water from the exposed body surface. Thus, 

 land amphibians usually maintain a body 

 temperature below that of the surrounding 

 air as a result of constant water loss. 



Aggregations of insects may have tem- 

 peratures within the aggregations decid- 

 edly higher than the surrounding air even 

 when there is Httle integration between the 

 members of the aggregation. Social insects, 

 notably the honeybees in their winter clus- 

 ters, are much more independent of en- 

 vironmental temperature. As a result of the 

 heat produced by muscles in vibrating the 

 wings and as a further result of the insula- 

 tion furnished by the covering shell of bees, 

 such a cluster may maintain a tempera- 

 ture decidedly higher than that outside the 

 cluster. At high temperatures honeybees are 

 able to lower their temperature slightly, 

 probably by increased evaporation. Such 

 social insects show partial control over their 

 immediate microclimate and have become 

 facultative homoiotherms as a result of so- 

 cial activities (Pirsch, 1923; Phillips and 

 Demuth, 1914). 



The distinction between cold-blooded 



