Competition for mating between ordinary and supermale Japanese medaka fish

Abstract Fourteen pairs of white fish of the species, Oryzias latipes , competed once daily to mate with one female. Each pair consisted of a super-male (YY) and an ordinary male (XY). The YY males were responsible for 88 per cent of 155 spawnings, the XY males for 12 per cent ( P = 0·01). The YY males, compared with the XY males, initiated more actions directed toward the female and toward the other male. Quantitative studies showed significantly higher mean values in YY than in XY males for 7 component actions in the behaviour under observation, all of which reflected dominance. In view of the effects of a double Y chromosomal complex in XYY men (despite simultaneous presence of an X) it is postulated that the presence of two Y chromosomes, rather than the absence of an X chromosome, may produce differences between YY and XY male fish.