8 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



in England on that day, spawn was in danger from the heat. England 

 is a long way north of the southern hmits of distribution, and it is 

 even more hkely that somewhere in Europe, some spawn was killed. 

 A spring like that may not happen again in England for a long time 

 (three weeks later, the hot weather continued, and the tadpoles in this 

 pond were in water at 29°C), and the best chance of verifying the 

 hypothesis of Moore and Douglas would be at the southern limits of 

 the species. Even a few cases of heat death in eggs laid naturally in a 

 pond would be of much interest. 



There is, however, another more comphcated effect of high tempera- 

 tures at spawning time. If spawning is delayed for any reason, the eggs 

 within the frog may become over-ripe and develop abnormally, or 

 even die altogether. Nussbaum (1897) considered that under these 

 conditions R. esculenta absorbs the eggs without laying them, but that 

 R. temporaria under the same circumstances lays infertile eggs. Witschi 

 (1952) has pointed out that spoiled eggs of this species are rarely found 

 in the field, and I agree with him. He made a detailed study of the 

 abnormalities that can be produced by delayed spawning. He found 

 that over-ripe eggs are much more sensitive to the deleterious effects 

 of high temperatures than those that have only been in the ovisacs for 

 a few days. If they remain there for a week at 20°C, they die. Now, 

 delayed spawning can be produced by several conditions, but hot 

 weather at spawning time is one of them. It is therefore possible that 

 at the southern limits of the species, spawning will sometimes be 

 delayed by the hot weather, and that many of the eggs will as a result 

 be infertile. Moreover, the delay itself makes it more likely that, as 

 the season advances, lethally hot weather may kill those eggs that are 

 alive when laid, so that the effects of Moore and Douglas become 

 reinforced. I have seen events in the field that support this suggestion. 

 In 1958, spawning in my area was delayed. The cause of the delay was 

 not in this case high temperature at spawning time, so that only the 

 effect of the delay could be observed. In one pond, there were thirteen 

 clumps of eggs. Of these, six contained spoiled eggs in an abnormally 

 high proportion, and there was in addition a clump of jelly with no 

 vitelli at all in it. It seems almost certain that here was a field instance 

 of the effects that Nussbaum and Witschi have described, and that 

 these frogs were breeding so late that they only just succeeded in 

 doing so at all. Very few ponds contained spawn and the breeding 

 population was much reduced. In i960 this pond again had spawn later 



