The social implications of winner and loser effects.

Winner and loser effects have now been documented in a number of species. To our knowledge, experimental work, however, has focused exclusively on pairwise interactions, and not the extent to which winner and loser effects impact hierarchy formation. We report the results of experimentally manipulated winner and loser effects on hierarchy formation in a socially living species, the green swordtail, Xiphophorus helleri. Our results demonstrate that randomly chosen winners in pairwise aggressive contests were more likely to emerge as top-ranked individuals in a hierarchy, whereas randomly chosen losers were more likely to emerge as the lowest-ranking individuals, and that 'winner-neutral-loser' hierarchies were significantly overrepresented.