May I now turn to cattle, which though not primarily desert animals, are widely 

 grazed nomadically in semi -arid areas. I should perhaps first refer to the Yak, 

 which while not classed as true cattle, are close relatives and are typical of the cold 

 Asiatic deserts of Mongolia and of the Tibetan plateau. This animal again shows all 

 the characteristics of cold resistant types, with relatively heavy and compact body, 

 small extremities and a thick insulating coat. The same may be said of our own 

 temperate cattle as illustrated in the Highland and Galloway breeds. It is instruc- 

 tive to compare these with their counterpart in the tropical desert areas. Here, how- 

 ever, contrary to Davidson's findings for North America, Bergmann's rule regarding 

 larger body size in cold areas no longer holds; cattle in desert and semi- arid areas 

 are among the largest in the world. The explanation may well lie in the fact that, in 

 such animals, the skin surface is very greatly increased by the abnormal develop- 

 ment of the dewlap and, in the male, of the sheath. Both developments are parall- 

 eled by a marked increase in the fineness of the coat in comparison with temperate 

 breeds. It would perhaps not be out of place to note at this point that the large size 

 and exaggerated skin areas of the cattle of the hot deserts is not found in the hot 

 but humid areas. Here the animals, while having normal sized extremities, are quite 

 definitely dwarfed. 



Although I have no personal experience of the colder Asiatic deserts, it appears 

 that Bergmann's and Allen's rules are equally applicable to camels, — perhaps the 

 best known desert animal. Thus the two- humped Bactrian camel, common to the 

 cold deserts of Turkestan, is described as 'distinguished from its Arabian relative 

 not only by the presence of two humps, but by the facts that it is heavier, more com- 

 pact, and shorter in the leg, and that it has a heavier coat of longer hair'. 



Camels form the obvious introduction to the second aspect of climate in rela- 

 tion to domesticated animals inhabiting desert areas, i.e. to the indirect effect of 

 climate via the vegetation and water supplies. One minor but none the less impor- 

 tant adaptation is that associated with the nature of the vegetation. Xerophyllous 

 vegetation is notably hard and frequently spiked or thorny ; yet the camel is able to 

 derive its nutrients from such material which, indeed, forms an important part of its 

 natural grazing. But the most striking feature of the camel, which makes it of spec- 

 ial value as a true desert animal, is its ability to store a reserve of fat in the hump 

 to provide energy (not water) to tide over rainless and therefore vegetationless 

 periods. 



This feature is not confined to camels; it is less widely recognized but prob- 

 ably more important from the aspect of desert utilization as a feature of fat- tailed 

 and fat-rumped sheep. For some six months of the year deserts are (as I pointed 

 out earlier) normally devoid of rain. During much of this period the surface of the 

 desert is practically free from any vegetative growth. With the onset of the rains 

 the whole picture is transformed'by the growth of a thick carpet of annual herbage 

 plants. The duration of this vegetation is, however, relatively short, and within a 

 few weeks of the cessation of the rains it dries up and fragments, leaving only the 

 sparsest supply of grazing nutrients for stock. Nevertheless, owing to the wide 

 variations in the locality of rainfall, a flock may be able to secure more or less con-' 

 tinuous grazing by constant movement across the desert — the basis of nomadism. 



171 



