ECOLOGY OF TADPOLES 45 



To get these times into proper perspective, it is necessary to compare 

 them with those found in other animals. The digestive enzymes of 

 homoiothermous animals such as the mammals do not seem to have 

 temperature optima different from those of the poikilotherms, so that 

 we should expect that animals such as cows, wliich also cat vegetable 

 food, but digest at about 37°C, should be able to complete the process 

 in much less time. But digestion in the cow takes i8 hours. Using the 

 common assumption that a rise of io°C doubles the rate of reaction, 

 five hours in the tadpole corresponds to less than two in the cow. 

 Digestion in the mammal has by then scarcely begun. In a review, 

 Barrington (1942) has collected many observations on digestion in the 

 lower vertebrates. For example, the dogfish, Scyllium, sometimes 

 takes 18 days to digest a meal, even though it lives on animal food, 

 which is more digestible than vegetable. A carp takes 18 hours at io°C 

 or 4^ at 26°C. Sokolov and Chvaliova (1936) reported that the stomach 

 of the fish Gambusia is completely emptied of a meal o£ Anopheles or of 

 Daphnia in 3 to 4 hours at 30°C, and Barrington considered this a 

 remarkably short time. This comment emphasizes the exceptional 

 nature of digestion in the tadpole, for this animal not merely empties 

 one part of its alimentary canal in this time at a much lower tempera- 

 ture, but empties the whole of it, and yet lives on relatively indigestible 

 food. Direct evidence for cellulose digestion was sought by the 

 methods of Baker and Nasr (1948), who employed direct micro- 

 scopic observation of areas of attack by enzymes. This method, 

 although devised for studying farm animals, was adapted for use in 

 tadpoles. There was no evidence that cellulose was being digested and, 

 indeed, having regard to the known properties of cellulases, which take 

 a long time to act, it would be astonishing if any activity could be seen 

 after such a short time. We must therefore accept the idea that tadpoles 

 cannot digest cellulose. But they can live on algae, enclosed witliin 

 cellulose walls. The field observations showed that in some ponds, 

 such as Dagger Lane, algae predominated in the diet, and there seemed 

 to be a relation between the growth of the algae and that of the 

 tadpoles. These observations were supported by experiments in wliich 

 artificial and rapidly growing cultures of single species of various algae 

 were used as food for tadpoles put to live in the culture vessels. The tad- 

 poles grew well at first, but in one year died before metamorphosis, 

 and in the next, were clearly about to do so, and were fixed at tliis 

 time. How can we reconcile these somewhat conflicting observations? 



