BREEDING HABITS OF RAINBOW DARTER. 47 



6. Sex Recognition. 



Sometimes when a female slips out of sight of the male that 

 has been guarding her, he, seemingly unaware of her absence, 

 approaches a small male as if he mistook him for the female. 

 As he approaches the young male the latter sometimes raises his 

 lightly colored first dorsal as if to show his identity and then 

 usually flees. In this case the large fish apparently fails to dis- 

 criminate between the light-colored male and the female. 



This failure was especially apparent in one instance when sev- 

 eral males rushed from various distances and directions toward a 

 young male that happened to burrow into the sand while feeding 

 and thus took nearly the position of a female preparing to spawn. 

 None of the males followed the young male as they would have 

 followed a female, nor did they feed when they came near the 

 spot where he had been, as they would have done if their ap- 

 proach had been due to the suggestion of food by the attitude of 

 the young male. It thus seems probable that at a little distance 

 the male fails to distinguish between dull males and females. 

 The more nearly the behavior of a dull male simulates that of a 

 female, as in the case of the male burrowing for food, the more 

 is he likely to be mistaken for a female. Upon the near approach 

 of the brilliant male the young male erects the first dorsal and 

 rapidly escapes, modes of behavior not observed in the female. 

 It appears then that the brilliant fish distinguishes between the two 

 by their behavior ; a mode of sex recognition pointed out by 

 Holmes (1903) in the case of amphipods. In the case of very 

 young males the sex recognition must be wholly of this character, 

 while males which already show some little sexual coloration are 

 probably distinguished upon near approach by means of it as 

 well as by behavior. Holt (1898) believes that a similar method 

 of recognition occurs in the dragonets. 



V. Observations Bearing on Sexual Selection. 

 The preceding description of the breeding activities of the rain- 

 bow darters at once suggests that a field study of their behavior 

 may be made to yield evidence as to the occurrence of sexual 

 selection among them. The fish are crowded together in the 

 breeding areas ; the sexes are easily distinguishable ; they may 



