SEX RATIO IN GAMBUSIA. 403 



larger and more conspicuous female is the one suffering most from 

 predatory fish. The writer believes that the former view may be 

 correct but that in addition to fish, other natural enemies, such as 

 water snakes, birds and insects should be considered as they all 

 take part in destroying Gambusia. 



CONCLUSION. 



It has been shown quite conclusively that in immature Gainbns\i 

 the sexes are evenly represented, as the 2,593 fish (all 20 mm. and 

 under in length) that were examined, consisted of 1,298 males 

 and 1,295 females. It was shown also, that a large " seasonal " 

 variation in the abundance of males among adult Gambusia takes 

 place, the data presented showing that the fluctuation in sex ratio 

 may vary from i male to 2.5 females in June to i male to 11.3 

 females in August. Some evidence was produced tending to show 

 that this great " thinning out " of males may be due to their lack 

 of resistance to adverse environmental conditions, and the possi- 

 bility that they are more extensively preyed upon by natural 

 enemies, also, was advanced. What the greater mortality of the 

 males may be due to, however, very largely, remains for further 



investigation. 



Although male Gambusia are constantly fewer in number than 

 females, it seems certain, nevertheless, that enough males for 

 breeding purposes always are present. It has been shown that 

 copulation between broods is not necessary, as females separated 

 from males early in the spring produced young throughout the 

 breeding season. In the spring when apparently the majority of 

 the females are fertilized for the duration of the breeding season, 

 the males are proportionately much more numerous. The great- 

 est scarcity of males occurs in midsummer (July and August) 

 when the females born the previous season have nearly all ma- 

 tured and quite certainly have been fertilized. At that time few 

 males are needed. It, at least, is unquestioned that when condi- 

 tions are favorable, Gambusia are quite capable of perpetuating 

 their kind. Certainly few fish multiply more rapidly or become 

 more numerous. The thinning out of males, therefore, appears 

 to be nature's process of eliminating " surplus " animals. 



