138 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



Spawning time. I cannot so easily credit that observers' errors are at 

 work in the phenomenon now under consideration. James (1944) 

 found that the calling of Hyla nigropes, which breeds in Costa Rica 

 in the dry season, occurred regularly at sundown, except when the 

 afternoon had been raining. Evening rain had no effect of this kind. 



ISOPHENES IN YEAR-DAYS 



100 200 30O 



RAINFALL IN MILLIMETRES 



Fig. 37. Joint Functional Regression Diagram, Showing the 



Relation between Rainfall and Temperature, and Spawn Date 



for the Month Mo 



The diagram at rainfalls higher than about 80 mm resembles that for Mi, 

 but is distinguished by a well-marked ridge of lateness almost vertically 

 through the diagram at about this value for rainfall. There is something that 

 determines that frogs should be later when Mo is fairly wet than when its is 

 drier. At high rainfalls the usual relation is observed. 



This could, of course, be a coincidence, and nothing to do with the 

 phenomenon we are discussing. Forty millimetres for a whole month 

 is light rainfall. That it should fall so often at the exact time when the 

 observers were about to inspect their ponds is rather diificult to 

 believe. I feel fairly certain that this is a real event, and perhaps we 

 should seek for a possible common cause for both phenomena. When 

 we come to consider light it may be possible to suggest a rational 

 explanation. 



