Behavioural ecology: transient sexual mimicry leads to fertilization.

Sexual mimicry among animals is widespread1,2, but does it impart a fertilization advantage in the widely accepted ‘sneak–guard’ model3 of sperm competition? Here we describe field results in which a dramatic facultative switch in sexual phenotype by sneaker-male cuttlefish leads to immediate fertilization success, even in the presence of the consort male. These results are surprising, given the high rate at which females reject copulation attempts by males, the strong mate-guarding behaviour of consort males, and the high level of sperm competition in this complex mating system4,5.