GENERAL LIFE HISTORY OF THE TADPOLES 4I 



of conjecture in transferring these observations to another species, but 

 it seems hkely that something of the kind happens in the frog. 



On one of the visits to Upper Parkfield Pond, which contained 

 tadpoles of both species, I noticed that a large number of toad tadpoles 

 had gathered round a shcc of bread floating on the water. Wondering 

 whether they were feeding on the bread itself, or on organisms 

 attracted by the bread {B. bufo feeds mainly on plankton), I collected 

 some of these tadpoles. When these were weighed, they were found 

 to be significantly heavier than those collected at the same time from 

 the procession of swimming tadpoles that made up most of the 

 population. Next week, I took some bread and deliberately reproduced 

 the same conditions. In a short time, a significant size difference was 

 established. By watching the animals, it was possible to see what was 

 happening. The bread was surrounded by a crowd of large tadpoles 

 with vigorously waving tails. A small tadpole would approach, and 

 be swept aside by the currents of water. The interesting thing was that 

 the efforts of the small tadpoles did not persist. After one or two 

 attempts, they gave up, and turned away to swim in the open water of 

 the pond. There is surely notliing surprising in this. A weak tadpole 

 will do better for itself in a second-class feeding ground, if it can feed 

 there immolested, than in a first-class place where the activities of its 

 stronger competitors prevent it from feeding. This is just what happens 

 in our own species, for it does not pay a man to attempt work far beyond 

 his powers. He does better at an easier task that he can accomplish. If 

 frog tadpoles behave as toad tadpoles do, then perhaps this is what 

 happens : as soon as the tadpoles disperse, the largest of them find a 

 good place to feed, and remain there until they have more or less 

 exhausted the food. They then move off^in search of another grazing 

 ground. Very likely, tliis is occupied by smaller tadpoles, who now 

 have to move. If they move to the place just vacated by the large 

 tadpoles, we should have an explanation for the exchanges that take 

 place. When there are not many tadpoles in the pond, it often seems 

 that the biggest tadpoles are always found at the same place. Perhaps 

 there are not enough of them to exhaust the food, so that there is no 

 incentive to move. Lower Parkfield in 1948 and 1949 and Cat Hill 

 may be examples of this. 



I do not feel that the purely mechanical theory I have just outhned 

 will explain all cases. Sometimes, there hardly seem enough tadpoles for 

 it to apply. It may be that sometliing in tadpole psychology is involved, 

 4— (T.914) 



