Private signalling, where conspecifics use mating signals that are difficult to detect by predators, can reduce the compromise between opposing natural and sexual selection. We investigated whether guppies, Poecilia reticulata, use an hypothesized private signal. In some areas, guppies, who can detect ultraviolet (UV) light, coexist with dangerous predators, but at least some of these cannot detect UV. In these populations of guppies, UV might be used as a private signal. We tested this hypothesis by quantifying the UV and non-UV colouration of male guppies from paired high- and low-predation sites in five rivers. We found evidence in support of the use of UV as a private signal: male guppies living with the predator generally had greater UV reflectance than male guppies living without the predator. Closer inspection revealed differences in this trend among rivers and thus suggested the influence of modifying factors. We also found that the non-UV colour of male guppies does not differ consistently between high- and low-predation environments. This result suggests that a number of factors in addition to predation act on male colour in guppies.
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