Intra-sexual selection in a North American annual killifish: does the color-polymorphism matter?

The decision to escalate aggression in a contest is based on the assessment of oneself and the opponent's capabilities, previous experiences and the asymmetries between contenders. Interestingly, there is increasing evidence that color polymorphism may also influence the result in a contest during agonistic encounters. Therefore, this research aimed to analyze, for the first time, the intra-sexual competition of adult males of the Mexican Rivulus Millerichthys robustus, testing the influence of color polymorphic variations during agonistic contests. The results of this study suggest that the aggressiveness between morphotypes is of orange and red over yellow. The orange morph was related to winning the greatest number of encounters, whereas yellow lost the most. The red morpho registered the greatest number of ties. Interestingly, in contests between more aggressive morphs (orange and red), fish tended to modulate and lower aggressiveness levels, possibly to avoid injuries. In contrast, the least aggressive morphotype (yellow) tended to raise its aggressiveness against the most successful morphotype and lower it in matches against its own color, changing to a scheme between contenders with symmetrical aggression levels. These results suggest that differential aggression between morphotypes can influence the evolution of color polymorphism in M. robustus, differentially influencing the fitness of the individuals.