4O2 SAMUEL F. HILDEBRAND. 



A total of 41,073 adult fish, consisting of 7,337 males and 33,736 

 females, were used in the experiments described in the preceding 

 paragraph. The loss among the males was 29% per cent, and 

 among the females it was 22^/3 per cent. This mortality, as a 

 whole, was very high, that is, much higher than usually occurs in 

 Gambusia that are transported from one place to another, because 

 many of the fish, for the sake of experimentation, were held under 

 very adverse conditions. For example, in working with the ef- 

 fects of high temperatures the containers were set in the sun for 

 the purpose of determining at how high a temperature the fish 

 could survive. The males and females, however, were subjected 

 to the same tests and, therefore, the data should be nearly as rep- 

 resentative of one sex as of the other. Two discrepancies, never- 

 theless, may be mentioned. First, in some lots all the males died 

 before the experiments were concluded. In such cases the mor- 

 tality of the females (for additional ones usually died) is .greater 

 than it would 'have been, if the experiments had been terminated 

 when the last male had died. Second, it is probable that some 

 immature males that exceeded a length of 20 mm. died and, if so, 

 they were classed as females. Therefore, the percentage of deaths 

 given for the females is subject to a small error, as it is somewhat 

 higher than it should be for direct comparison with the percentage 

 of deaths among the males. It may be concluded, then, that the 

 differential death rate among the fish used in the experiments was 

 greater than indicated by the data. 



The data show that under artificial conditions the death rate 

 almost constantly was higher among males than among females, 

 and it seems probable, that the males are generally less resistant 

 to harmful environmental factors and this probably is true in na- 

 ture as Geiser (1924) has contended, as well as under artificial 

 conditions. 



The present writer has no other pertinent data to offer bearing 

 upon the cause or causes for the greater decrease in males than 

 females, occurring annually. Geiser (1924) argues that the 

 smaller males are more liable to be devoured by small predaceous 

 fish than the much larger females and he also points out that the 

 female, herself, is an enemy of the male. Barney and Anson 

 (1921), on the other hand, argue (see quotation, p. 393) that the 



