392 LAND ANIMALS 



dies of hunger; on the other hand, at a temperature of 30°-35°, it 

 thrives and is active. If the feeding time is increased about two or 

 three hours by artificial illumination, it can endure a temperature of 

 14° and even of 13°. A certain minimum size is essential for homoio- 

 thermal animals in order that they may subsist in cold regions; warm- 

 blooded animals of the size of the tiny hummingbirds can leave the 

 tropics only during the warm summer. 



Increasing metabolism, however, inflicts increased demands upon 

 the heart, for an increased circulation is correlated with increased 

 food and oxygen consumption and with increased excretion of meta- 

 bolic products. Through size adjustment, the heart can adapt itself 

 within certain limits to these greater demands. Thus, weight of the 

 heart of the sparrow, Passer montanus, in Leningrad is 15.74% of the 

 body weight, in northern Germany 14.0% , in southern Germany 

 13.1%o. The same is true of the squirrels Sciurus, whose relative heart 

 weight increases from 5%o in the middle Neckar region to 5.9%o in the 

 Black Forest, 6.2% in East Prussia, and 6.5%o in the Brocken region. 

 The tundra birds that migrate to Germany in the winter all have a 

 greater relative heart weight than their nearest relatives of similar 

 sue living in these regions; these values for the rough-legged buzzard 

 {Buteo lagoptis) in comparison with the common buzzard (Buteo 

 buteo) are as 8.35% to 7.1%e; and many similar relationships could 

 be cited. 52 



Other temperature relations. — The surface temperature of the 

 desert soil may reach 60° in parts of Palestine; 53 readings of 78°-84° 

 have been obtained in other desert regions and of 65° on the exposed 

 sandy surface of the Indiana dunes near Chicago. Nymphs of two 

 species of Mantis and adult grasshoppers have been observed moving 

 about where the surface soil temperature was 50.8° (Buxton), and 

 thermopyle tests showed the internal temperature of the insects was 

 approximately that of the surrounding air. Such high temperatures 

 are found only during the day, and even then only on the surface 

 exposed to the full rays of the sun. 



Many tropical animals quickly perish when they are exposed for 

 some time to the unabated effect of the tropical sun. A simple method 

 used by collectors for killing large crocodiles quickly without injury 

 consists in exposing them for a time to the full rays of the tropic sun ; 

 in the Zoological Garden of Antwerp a number of ostriches died because 

 they could not obtain shade. 54 Such effects have usually been ascribed 

 to increased temperatures, but more recent work indicates that light 

 may have lethal effects for other reasons. At any rate, tropical and 

 desert animals in general are forced to seek shade. For homoiothermal 



