SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 



TABLE III. 



67 



From the above tables it is evident that the sex-ratios have suf- 

 fered reversal from immature to mature conditions ; also that the 

 change is very marked. One can perceive that such change could 

 be brought about in two ways : differential viability and sex in- 

 version. 



Oxygen consumption experiments have shown (Bellamy, 1922) 

 that the males consume approximately twice as much oxygen as 

 the females. The males are also by far the more active. From 

 these data one might expect to find greater mortality among the 

 males than among the females. This is borne out experimentally. 

 If both sexes of equal chronological age are subject to unfavorable 

 conditions such as weak solutions of potassium cyanide, alcohol, 

 or excesses of temperature, the male invariably succumbs first. 

 With some of these conditions, such as cyanide or alcohol, the 

 females live twice as long on the average as the males and some- 

 times even longer. That the reversal of the sex-ratio is not due 

 to mortality of the females can also be ascertained by keeping 

 strict records of broods of fishes from birth to sexual maturity. 

 The mortality of Xip/iopJwnts hcllcri under proper conditions is 

 slight, indeed, and whenever death occurs the sex can be estab- 

 lished cytologically. The results of such observations are decidedly 

 in consonance with the experimental results and it is perfectly safe 

 to conclude that the female is at least as sturdy as the male. 



If the reversal of sex-ratios from immature to mature condition 

 is not due to differential viability, there is only one other possibility, 

 and that is sex inversion. This occurs most cofnmonly in fishes 

 from 16-27 mm. in total length, but may occur at any size from 

 16 mm. upward. 



