GENERAL FACTORS CONDITIONING AGGREGATIONS 67 



water caused by the early melting of ice in a pond with proper ex- 

 posure. Banta (1914), Yerkes (1903, 1905), and Noble (1923) find 

 evidence that frogs may respond to frog calls and splashings, par- 

 ticularly during the spring breeding season. Studies on the breeding 

 migration of toads indicate that with them the voice serves as a sex 

 call (Courtis, 1907; Miller, 1909; Wellman, 191 7). Boulenger (191 2) 

 concluded that the voices of frogs and toads do not control migra- 

 tions toward breeding grounds or movements of individuals at the 

 grounds. Cummins later came to the same conclusion as a result of 

 his observations on a partially fenced pond, since he found that 

 heavy migrations followed periods in which there was no croaking in 

 or near the pond, and that, on the other hand, great vocal activity 

 was not accompanied by increased migration. Certainly, vocal ac- 

 tivity cannot account for the similar spring migration of the voice- 

 less Arnhystoma. 



The immediate inception of the migratory impulse must be in- 

 trinsic and is probably associated with the conditions of the sexual 

 glands. In frogs it is secondarily conditioned by weather, since waves 

 of migration are coincident with high relative humidity and with a 

 temperature of from 41° to 52° F. The migration is independent of 

 daylight. All of Cummins' illuminating observations still give no 

 information as to why the frogs congregate in a given pond or how 

 they learn of its existence. He does record that migration routes are 

 not direct, so that we may assume that we are dealing, at least in 

 part, with random movements, probably controlled largely by tem- 

 perature. Blan chard (1930) concludes that the external control for 

 the breeding migration is to be found in rainfall rather than in tem- 

 perature relations. 



During the breeding season a gregariousness appears among frogs 

 which does not exist under usual circumstances. This is not entirely 

 accounted for by the tendency which the animals exhibit to seek a 

 similar habitat for breeding, for if there are only a few pairs of frogs 

 in a given place, they force themselves together as closely as possible 

 and the eggs form a continuous mass. 



At the height of the breeding season several males will struggle for 

 the possession of a single female (Banta, 1914) ; the struggles attract 



