Recent theory predicts that males should choose the social context that maximizes their relative attractiveness to females while minimizing sperm competition risk. By preferentially associating with less attractive and less competitive sexual rivals, a male may increase his reproductive success. Using the Trinidadian guppy (Poecilia reticulata), I tested for non-random social associations among males in mixed-sex groups based on two phenotypic traits (body length, body colouration) that predict relative sexual attractiveness to females. In a dichotomous-choice test, focal males exhibited a
significant preference for mixed-sex groups that included a less colourful and smaller male rival, thereby potentially increasing their relative attractiveness, as predicted. However, this preference was not expressed in nature. Males in mixed-sex shoals in a natural stream population in Trinidad were not assorted by either body length or colour, perhaps owing to constraints placed on preferred social associations by sexual conflict and the fission-fusion nature of guppy shoals.