Colour cues play an important role in sexual selection and conspecific recognition. Literature shows that conspecifics might enjoy their everyday chat, without ever worrying about occasional eavesdroppers (e.g., predators), when information interchange evolves into a private communication channel. Yet, when signalling is converted into foraging cues by predators, their prey must pay the due cost for sustaining conversation. For that matter, fiddler crabs draw attention for having flashy enlarged claws that could potentially attract the attention of many predators. Surprisingly, the costs associated with claw colouration in fiddler crabs are still poorly understood and have never been studied in American species. Here, we initially examine whether hypertrophied claws of American thin-fingered fiddler crabs (Leptuca leptodactyla) reflect UV-light and how conspecific females react to these cues. Then we test two alternative hypotheses concerning the role of claw colouration in fiddler crabs' mate choice: a) that claw colouration evolved into a private communication channel, which could have significantly lowered signalling costs for males; b) that claw colouration is conspicuous to potential reproductive partners, as well as to predators, making colour signalling by males very costly (i.e., a handicap). Thereafter, we measured the reflectance spectra from several enlarged claws and modelled their chromatic contrast against the background spectrum, considering the visual systems of conspecific fiddler crabs and two kinds of predators (foxes and plovers). We also tested female conspecifics' preference towards enlarged claws that reflected UV-light or other colour cues, by artificially altering claw colouration. Our results show a clear female preference for UV reflecting males. We also found that natural enlarged claws should be highly detectable by avian predators, refuting the private communication channel hypothesis. Furthermore, since female fiddler crabs select the most flamboyant claws from the sandy background, claw colouration in fiddler crabs can be understood as an honest signal.