34 VERTEBRATE REPRODUCTIVE CYCLES 



8. THE QUESTION OF THE TROPICS 



All the experiments, observations, and conclusions so 

 far set out in this chapter refer to the temperate regions. 

 It is obvious, however, that some parts of the tropics 

 offer an intriguing field for research since they experience 

 almost uniform climatic conditions throughout the whole 

 year, and in such countries it has often been observed 

 that some animals can always be found breeding. 



The most important work done in these regions was 

 that of the Oxford University Expedition to the New 

 Hebrides in 1933-4, the results of which have been 

 published in a series of papers. ^^-lo At Hog Harbour on 

 Espirito Santo where the expedition was based the mean 

 temperature varies by only 2° C. between the hottest 

 and coldest months, the longest day is only if hours 

 longer than the shortest, and there is no dry season, the 

 least wet month having on the average about twice as 

 much rain as the wettest month in England. The land is 

 covered by typical rain forest, and within this it is 

 probably true to say that environmental conditions vary 

 less from month to month than they do anywhere else on 

 land. The two questions which the expedition attempted 

 to answer were whether in such a uniform climate the 

 animals reproduce continually or seasonally, and 

 whether, if breeding seasons are evident, these can be 

 correlated with environmental changes of any kind. 



To the first question a clear answer was given. Most of 

 the terrestrial organisms, plants as well as animals, 

 showed marked seasonal changes in their reproductive 

 habits. Only one of the vertebrates studied, the lizard 

 Emoia cyanura, reproduced continuously, and even in 

 this species egg-laying almost ceased during two months. 

 At the other extreme the most clearly defined breeding 

 season was that of the insectivorous bat, Miniopterus 

 australis. The females became pregnant only during the 

 first few days of September, and afterwards the epididy- 

 mides of the males were found to be empty of sperm- 



