diet of dry vegetation alone. Prolonged droughts bring it to the margin of total ex- 

 termination, except in certain favourable localities like the borders of swamps, irri- 

 gated fields, etc. In the typical desert rodents either the normal rate of ecological 

 destruction or the physiological resistance to dry food (largely replaced in the 

 desert by bulbs and succulent plants) must differ considerably from that of Microtus, 

 which is prevented from entering the desert at all. The seasonal and annual fluc- 

 tuations of the populations of the nocturnal desert mammals have not been studied. 

 Their enemies may be few, for there are not many nocturnal birds of prey, although 

 snakes are abundant. 



The seasonal reproductive cycle of desert birds usually differs from that of the 

 birds of neighbouring regions. Most of our desert birds breed rather early (mainly in 

 March) according to the rainfall, and breeding ends in late March. The desert offers 

 sufficient food for the rearing of young only during the short period of late winter 

 vegetation. The total number of eggs and of broods per female per year is reduced as 

 compared with that in more favourable biotopes. Thus Oenanthe lugens only has up 

 to 5, compared with the 42 eggs of the equal sized Erythropygia in Mediterranean • 

 territory. The great heat reduces the size of the hunting area, and still more impor- 

 tant, it enforces upon desert birds many hours of additional rest at noon. The energy 

 spent in finding the same amount of food is also much greater than in more favoured 

 biotopes. These observations easily explain the lack of attraction which the desert 

 even at its most favourable season, exerts upon hibernating, aestivating or resident 

 birds beyond its borders. On the other hand, Heim de Balzac observed that on the 

 southern borders of the Sahara, a number of resident birds extend their breeding 

 area into the savanna, far beyond the limits of the desert vegetation. We must as- 

 sume that in that season only are conditions favourable for them in the savanna. 

 Mobile birds are able to utilize this situation for extending their range during the 

 short but vital season of nidification and reproduction, when the food situation is 

 less favourable in their permanent habitat, the true desert. 



In reptiles a seasonal cycle of the gonads is also observed. Here the seasonal 

 occurrence of the main items of food as well as the historic origin of every species 

 are of importance. Whilst reptile- eating species may reproduce late in the season, 

 insectivorous reptiles breed early. The chamaeleon shows its historic affiliation 

 with the African savanna from which it originates, by its reproduction late in autumn, 

 when sycamore and charob trees are flowering. The reproduction of the few desert 

 amphibians depends entirely upon the incidence of heavy precipitation. 



Among small aquatic invertebrates as well as in the big Phyllopods (Estheria, 

 etc.) a similar relation to rainfall prevails. The insects however show many paral- 

 lels with the terrestrial vertebrates of the desert. Diapause in any stage from egg to 

 mature adult, is one of the mechanisms of adaptation. It is induced and regulated 

 by seasonal changes of the secretory glands in response to unfavourable stimuli. 

 More attention should be paid to the secretory seasonal changes of the reproductive 

 glands in desert animals, as well as to the analysis of the stimuli which induce 

 these changes in every species. 



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