PROBLEMS OF PHYSIOLOGY AND ECOLOGY OF DESERT ANIMALS 



Professor F. S. Bodenheimer 

 (Jerusalem) 



The endocrine cycle of the reproductive glands in desert animals. 



In all climates manifesting distinct seasonal contrasts the majority of 

 terrestrial vertebrates undergo a conspicuous annual cycle with regard to the sea- 

 sonal activity and the histological structure of the endocrine glands, especially of 

 the gonads. These seasonal changes occur in response to changes of external 

 stimuli, such as temperature, humidity, duration of day, etc., and their effect always 

 results in the birth of the young at the season of the most luscious vegetation. Ac- 

 cordingly, the rutting season is usually in the climatic autumn, and that of birth in 

 the climatic spring. This synchronization of the reproductive cycles into annual 

 cycles of climate and vegetation is doubtless of the greatest ecological importance. 

 It is such a conspicuous phenomenon that it can scarcely be overlooked. In many 

 cases this synchronization is fixed by heredity and is more or less rigid at least so 

 long as no counteracting external stimuli change the normal cycle. Thus we know 

 that sheep or deer transported from a moderate climate in the northern hemisphere to 

 a corresponding one in the southern hemisphere, where the winter corresponds to the 

 northern summer, adapt themselves within one or two seasons to the climatic cycle 

 of the new environment. Yet for the camel a transfer into the summer- rain regions 

 of the Sudan means an experiment which is rarely survived and still more rarely 

 leads to reproduction within the Sudanese cycle of precipitation. 



The habitual seasonal cycle of reproduction is however often maintained for a 

 long time in a not entirely different climate where only the normal releasing external 

 stimuli are missing. Major Flower has published birth data for a number of species 

 of gazelles normally living in rather varied climatic conditions in N. E. Africa, which 

 with their offspring were kept for a long time in the Cairo Zoo. In Cairo rain is 

 practically absent. Fresh berseem - clover or lucerne (produced by irrigation) is fed 

 to the animals throughout the year, and the trend of temperature and of day -length 

 is more or less identical with that in their home countries. He showed that the 

 monthly birth incidence of gazelles in the Cairo Zoo remained for many years, in full 

 agreement with the seasonal rain cycles of their native regions, the peak of the 

 births usually following that of the rains by one month. The only typical domestic 

 animal of our deserts is the camel, whose reproductive endocrine cycle has recently 

 been studied in the Negeb, S. Israel, by R. Volcani. The camel, in contrast to many 

 other domestic animals, has preserved a pronounced rutting season from January to 

 March. Its pregnancy lasts 12 months, suckling 3 to 4 months, and the interval bet- 

 ween births is two years. Both birth and rutting seasons coincide with the season 

 of luxuriant vegetation on the margins of the desert. This is an extreme and most 

 remarkable adaptation to the desert environment with its short period of green vege- 

 tation. In consequence reproduction occurs only once in two years. 



During the rutting season the female is 'on heat' for periods of 7 days with 20- 

 day intervals, until fertilization has taken place. The seasonal changes of the 



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