4 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



use is intended. In this book the word **egg" is kept for the whole 

 object, and the black body is referred to as a "vitellus.") At least when 

 the spawn is fresh, and in clear water for much of the time that it 

 contains the embryos, the jelly is transparent, so that there is little 

 hindrance to the heat and light from the sun which strike the vitellus 

 and are there converted almost entirely into heat. Surrounded as it is 

 by the relatively enormous mass of solidified water, this heat is not 

 lost, but is absorbed with nearly the minimum possible rise of tempera- 

 ture. When the sun goes down, the process is not exactly reversed. 

 The temperature of the mass has been raised, it is true, but it is still 

 quite low, and so radiation is small. There are no convection currents, 

 so that this cause of heat loss does not exist. Only conduction remains, 

 but water is not a good conductor. There is therefore a tendency for 

 the mass of eggs to remain during the night at a higher temperature 

 than a similar mass of liquid water. This has been shown experiment- 

 ally (Savage, 1950) for both eggs and for an artificial jelly, exposed to 

 the light from an electric lamp and compared with a similar quantity 

 of water in a similar vessel. The light was switched on and off at 

 various intervals, and, for example, after a period of illumination in 

 the evening, the eggs were o-8°C warmer than the water next morning, 

 although the light was left off all night. In the field, 73 measurements 

 of the temperatures of egg clusters were made. In 63 cases, the eggs 

 were warmer than the surrounding water, and in 10 they were colder, 

 but in 3 of them, there were exceptional conditions, such as shade or 

 very freshly laid eggs. The mean temperature difference between the 

 eggs and the water was 0-63 °C. This difference may not seem large, 

 but in the following paragraphs, it will be shown that it may well be 

 of considerable importance. 



The frog population of the British Isles probably entered the 

 country at a time when the islands were still joined to the Continent. 

 If at any time since then, weather at the time of spawning had been of 

 such exceptional severity that the thermal death hmits had everywhere 

 been exceeded, there would probably be no frogs in the islands today. 

 If, on the other hand, even one pond existed where the temperature 

 was just a little warmer, the whole country would in course of time 

 become re-populated. Very small differences of temperature become 

 important when the temperatures are near the critical hmits. 



So far, I have been considering the temperature of ponds, but of 

 course just the same apphes to masses of eggs ; if the eggs at the warmest 



