Strategic concealment of sexual identity in an estrildid finch

One explanation for the evolution of sexual monomorphism is the sexual indi stinguishability hypothesis, which argues that in group-living species indi viduals might benefit by concealing their sex to reduce sexual competition. We tested this hypothesis in long-tailed finches Poephila acuticauda. Male s and females could not be reliably distinguished morphologically or by ana lysis of the reflectance spectra (300-700 nm) from the plumage and bill. Ma les seemed unable to distinguish the sex of an unfamiliar individual in the absence of behavioural cues; they were equally likely to court and copulat e with unfamiliar males and females but rarely courted familiar males. Here we report the first experimental evidence that sexual monomorphism enables strategic concealment of sex. Males were more likely to reveal their sex w hen faced with a solitary unfamiliar individual than a group of unfamiliar individuals. When encountering an unfamiliar male that revealed his sex, su bordinate males were more likely to conceal their sex than dominant males.