Multiple benefits of gregariousness cover detectability costs in aposematic aggregations

Understanding the early evolution of aposematic (warning) coloration has be en a challenge for scientists, as a new conspicuous morph in a population o f cryptic insects would have a high predation risk and would probably die o ut before local predators learnt to avoid it(1-4). Fisher(5) presented the idea of aggregation benefit through the survival of related individuals; ho wever, his theory has been strongly debated(6-8) as the mechanisms that fav our grouping have never been explored experimentally with the incorporation of detectability costs. Here we create a comprehensive 'novel world' exper iment with the great tit (Parus major) as a predator to explore simultaneou sly the predation-related benefits and costs for aposematic aggregated prey , manipulating both group size and signal strength. Our results show that g rouping would have been highly beneficial for the first aposematic prey ind ividuals surrounded by naive predators, because (1) detectability risk incr eased only asymptotically with group size; (2) additional detectability cos ts due to conspicuous signals were marginal in groups; (3) even naive preda tors deserted the group after detecting unpalatability (dilution effect); a nd (4) avoidance learning of signal was faster in groups. None of these mec hanisms require kin selection.