TOLERANCE OF HIGH TEMPERATURES BY FISHES, ETC. 



171 



Several considerations led to the choice of these animals. In the first place, it 

 was decided that this introductory work should be done on the members of a well- 

 defined group, within which the relation of ecology to modifiability of tolerance 

 could be studied effectively. An attempt to compare animals belonging to different 

 phyla involves such profound differences in structure and physiology that several 

 members of each great group must be studied in detail in order to obtain results that 

 are of ecological significance, so it was thought that each of the larger groups should 

 be considered in a separate series of experiments. The present study, therefore, is 

 limited to gill-breathing vertebrates. 



The fishes named were chosen for the reasons that they are abundant and easily 

 obtained in Lake Mendota, and that they represent a fairly wide range of habitat 

 preferences (Pearse, 1918 and 1921; Pearse and Achtenberg, 1920), Micropterus 

 salmoides, Eupomotus gibbosus, and Lepomis incisor being essentially shallow-water 

 fishes, while Perca flavescens is, by preference, an inhabitant of lower levels. Toad 

 tadpoles were studied as another type of gill-breathing vertebrate because, commonly 

 living in small bodies of water, they would be expected to withstand rather wide and 

 rapid fluctuations in temperature. 



In most of the experiments on fishes, individuals between 1 and 2 years of age 

 were used, those less than 1 year old being very sensitive and hard to handle, while 

 older specimens could be used only in very small numbers because of limited space 

 iti the constant-temperature baths. The young fishes possessed the further advan- 

 tage that they were not passing through the physiological changes that accompany 

 the different phases of the annual reproductive cycle. All of the tadpoles used had 

 easily visible hind legs. They developed rapidly during the experiments, but, unless 

 otherwise stated, none of them had visible front legs at the times of the tests. 



All of the fishes used were seined from Lake Mendota. The tadpoles were 

 obtained in some large, shallow pools in South Madison, several collections being 

 made in the course of the work. All specimens were brought to the laboratory as 

 soon as possible after being collected and were kept in large aquaria and battery jars 

 with a constant flow or frequent changes of water. No animals were used in the 

 tests until they had been in the laboratory at least 48 hours, as it was found that in 

 the first day or two after being brought in their resistance to heat was extremely 

 variable. 



METHOD OF MEASURING TOLERANCE 



In this quantitative study it was necessary (1) to decide what should be con- 

 sidered as constituting tolerance and (2) to find a fairly accurate, numerical means 

 of expressing the degrees of tolerance possessed by different individuals. After a 

 number of preliminary experiments it was decided to express tolerance in terms of 

 the maximum temperatures that could be survived for periods of 1 minute, 4 minutes, 

 15 minutes, 1 hour, 4 hours, and 24 hours. The 24-hour tests were considered the 

 most significant and the others served as checks upon them. The maximum tem- 

 peratures that could be endured for the various lengths of time are referred to as 

 the "24-hour tolerance limit," the "4-hour tolerance limit," etc. The general 

 method of procedure was as follows: 



1. Tests were made (usually simultaneously) at two or three constant tempera- 

 tures, 2° C. apart, the lowest of which was believed to be near the upper limit of 

 tolerance of the individuals tested. 



