THE BREEDING BEHAVIOUR OF THE FROG I57 



Indeed, even when there is a pre-spaw^n period for the first arrivals, for 

 some frogs there is none, for they only reach the pond after breeding 

 has begun. 



The Choice of Site 



The most usual place for frogs to lay their eggs is in the shallow 

 water at the edge of the pond, often on a sub-aqueous plateau. As a 

 rule, the same place is chosen every year, so that if the pond is already 

 knovvoi, it is possible to walk up to the exact spot, look into it and see 

 the eggs just as in the previous year. There are, however, exceptions. 

 Perhaps the commonest is the use of mats of weed growing in deep 

 water, on which the frogs gather and lay their eggs, in much the same 

 depth of w^ater as when they lay directly on the bottom of the pond. 

 It will be remembered that in Chapter i I described a remarkable case 

 in which a frog laid on a twig just under water in a deep part of the 

 pond, in the shade of a bush. 



Running water is rarely used, but again there are exceptions, as I 

 mentioned in Chapter i. These are particularly interesting, and 

 probably important from the theoretical point of view. I have only 

 known five streams where frogs spawned. In one, they spawned at a 

 shallow ford, not in the main stream at all, so that perhaps this hardly 

 counts. In three of the others, spawning was associated with concrete 

 structures. In one, for example, the spavvoi was laid beside the concrete 

 support for a drain that crossed the stream. In another, a place beside 

 the concrete supports for a road bridge was chosen. In the third, the 

 only one where I saw spawning in progress, the frogs spawned mainly 

 on the concrete bottom where a footbridge crossed the stream. The 

 stream itself is about three feet across, and usually about two feet six 

 inches deep, but varying greatly with the rainfall, as is normal in 

 streams in the London Clay. It rises in Jack's Pond, an artificial lake 

 made by damming up the stream near its source. The water is typical 

 of London Clay. In most years, the frogs spawned in only one place 

 in the stream, within about half a mile of Jack's Pond, and chose the 

 south, downstream side of the footbridge. There they spavmed among 

 concrete blocks, on or near a concrete and brick floor forming part o^ 

 the structure of the bridge itself, and between concrete and brick walls 

 about six feet high, rising sheer from the water. When I visited this 

 place at night, I found that considerable modifications in behaviour 

 were forced on the frogs. It was an extraordinary experience. The 



