Il6 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



occasions when by chance the dates will coincide. As I shall show in 

 the next chapter, we now know much about the nature of the spawn 

 stimulus itself. We do not know nearly as much about the migration 

 stimulus. 



It seems likely that a certain level of hormones is necessary for 

 migration, but there are always serious difficulties in accepting this kind 

 of explanation when synchronous behaviour is involved. It is quite 

 clear that the animals live during the autumn and winter under different 

 temperature conditions. For example, the frogs at Hibernating Pond 

 were not all in the water; some were in crevices in the bank. In 

 general, there is no doubt that the population of frogs in this area was 

 using a multitude of hibernacula. Now the generation of hormones is 

 a biochemical reaction, and it would be a most amazing thing if the 

 process had no temperature coefficient. Since frogs are poikilotherms, 

 the process must be proceeding at different rates in different frogs, 

 unless all are living at the same temperature. 



In the absence of any knowledge about it, let us suppose that the 

 beginning of the process is at the date when the frogs begin to hiber- 

 nate, say in October, and that hibernation begins simultaneously in 

 the whole population (a very unlikely assumption in itself). Then for 

 the fmal stage of stimulation to occur within three weeks, some four 

 months later, we would have to assume a rate of reaction correct to 

 about J;^ 8 per cent. This is quite good for a biochemical reaction in a 

 laboratory thermostat: it is quite incredible for one occurring in 

 hundreds of frogs at liberty in the field. In this, as in other similar 

 cases, it must surely be that the hormones merely provide a part of the 

 mechanism that transmits some effect in the environment to the organs 

 of the frogs. Hormones bear to the environment and the frog the same 

 relation as a telephone system does to the telephone conversation. The 

 physiologist is interested in telephone systems, the ecologist in the con- 

 versations. The whole phenomenon includes both, but no useful pur- 

 pose is served by stretching ideas about hormones to cover ecological 

 concepts. Sometimes, however, the synchronization of migration is 

 not at all accurate. In 1932, for example, I found some migration 

 taking place two months before spawning, and, for a rough adjustment 

 of this kind, perhaps nothing more than a physiological explanation 

 is necessary, but it would be rash even then to suppose that the full 

 explanation is as simple as a reaction rate. 



Although t have suggested that the guide for migration is different 



