158 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



males were unable to float about as they usually do, for the current 

 would have carried them down. They crawled about on the bed of 

 the stream, with httle or no croaking. Apart from this, everything 

 went on normally. Frogs of both sexes were to be seen in the stream 

 heading towards the site, and spent females crouched down in the 

 shallows near by. Large numbers of clumps of spawn were laid, 

 so that the site was not the aberrant choice of a few mistaken frogs. In 

 one year, they spawned more to the south of their usual position, at 

 the shelving natural edge of the stream, just south of the end of the 

 wall. In another year, a few clumps were found higher up stream, 

 where there is a widening of the brook, and in yet another year some 

 spawn was found at another concrete dam lower down. Once a 

 few clumps were found in a pool at the foot of a small concrete spillway 

 leading water out of Jack's Pond, and only a few yards from the pond 

 itself. One or two isolated clumps have been seen, not in every year, 

 at other parts of the stream, but never in Jack's Pond itself. There is 

 another bridge near the pond with a concrete bottom, but spawn has 

 never been seen there. The main spillway is also of concrete, but the 

 current is so swift at this point that it cannot be counted as a possible 

 place. The length of stream that I surveyed is about half a mile, and 

 there are numerous httle bays and shallows where the frogs could 

 spawn with less apparent difficulty than in the places they chose. Out 

 of four patches of concrete in this stream, one is the principal site, and 

 spawn has been seen at two of the others. Out of the five streams in 

 which I have seen spawn, it has been on or near concrete in three. 

 A rigid statistical test of an association with concrete is rather difficult, 

 because it is uncertain what length of stream must be counted as a unit 

 for the negative classes, but there is little doubt that this association is 

 not due to chance, and that there is something about concrete in 

 streams that favours the spawning of the frogs. Perhaps it provides a 

 firm substratum for sessile algae. 



The other place where large numbers spawned in a stream was in the 

 drainage channel of a gravel quarry. In this area, mechanical excavators 

 are used, and produce numerous ponds after they have taken the sand 

 and gravel away. All are temporary, in the sense that, after excavation, 

 the over-burden (clay that lies over the gravel) is returned and house- 

 hold refuse dumped to fill in the pits. Through the area, a channel is 

 dug to drain the working faces of the pit, and a large electric pump lifts 

 the water at the lower end and produces a fairly rapid flow in the 



