Animal colour signals serve important functions in intraspecific interactions, including species recognition, mate choice and agonistic behaviour. An increasing interest concerns ultraviolet (UV) wavelengths, for instance studies on the effect of UV in mating decisions. More recently, some studies also established that UV signals affect intrasexual interactions. We studied the role of UV during aggressive encounters between male three-spined sticklebacks (Gasterosteus aculeatus), a species in which UV has an effect on female and male mate choice and shoaling behaviour. To that aim, we compared the aggressive response of a territorial male to male intruders, either seen in UV-including (UV+) or UV-lacking (UV-) conditions. Our prediction was that, if UV wavelengths are used in male-male competition, a territorial male should show less competitive behaviour towards an intruder representing a lower threat, i.e. the one presented without UV light. Male sticklebacks showed significantly lower levels of aggression towards male opponents lacking an UV component to their coloration than male opponents possessing this colour component. Discrimination was not influenced by a difference in brightness between the UV+ and UV- stimuli. Finally, we present some reflectance-spectrophotometrical data of two skin regions (cheek and abdomen) of the experimental males and analysed relationships between colorimetric variables, body variables and behaviour. Our study emphasises that UV visual cues are of importance in different communicational tasks in the three-spined stickleback.
BackgroundThe use of ultraviolet (UV) signals for communication tasks is widespread in vertebrates. For instance, there is a UV component to mate choice in several species. Nevertheless, it remains unclear how the signal value of the UV wave band compares to that of other regions of the animal's visible spectrum. We investigated the relative importance of UV signals compared with signals of longer wavelengths in the threespine stickleback (Gasterosteus aculeatus), a species using UV wavelengths in female and male mate choice as well as in shoaling behavior. In a choice experiment, female sticklebacks were simultaneously presented with four male visual appearances manipulated by optical filters. Each male lacked one wavelength range of the stickleback's visible spectrum corresponding to the spectral sensitivities of the four cone types. The resulting male appearances thus had no UV (UV-), no short-wave (SW-), no medium-wave (MW-) or no long-wave (LW-) body reflectance.ResultsMales without UV wavelengths and long wavelengths ("red") were least preferred. In contrast, the removal of medium and most notably short wavelengths left male attractiveness to females rather unaffected. Using color metrics, the effects of the four optical filters on stickleback perception of three male body regions were illustrated as quantal catches calculated for the four single cones.ConclusionThe removal of UV light (UV-) considerably reduced visual attractiveness of courting males to female three-spined sticklebacks particularly in comparison to the removal of short-wave light (SW-). We thus report first experimental evidence that the UV wave band clearly outranks at least one other part of an animal's visible spectrum (SW-) in the context of communication. In addition, females were also less attracted to males presented without long wavelengths (LW-) which supports the traditionally considered strong influence of the red color component on stickleback mate choice. Overall, the removal of medium wavelengths (MW-) and especially short (SW-) left male attractiveness for females rather unaffected. Our work suggests that, in addition to long wavelengths ("red"), the UV wave band contains important information for visual mate choice in sticklebacks. Hence, the previously suggested exclusive role of the characteristic red nuptial coloration in visual interactions between reproductively active stickleback conspecifics may be overestimated with UV wavelengths playing a more important role than previously suggested.
Visual signalling can be affected by both the intensity and spectral distribution of environmental light. In shallow aquatic habitats, the spectral range available for visually mediated behaviour, such as foraging, can reach from ultraviolet (UV) to long wavelengths in the human visible range. However, the relative importance of different wavebands in foraging behaviour is generally unknown. Here, we test how the spectral composition of ambient light influences the behaviour of three-spined sticklebacks (Gasterosteus aculeatus) when foraging for live cladoceran Daphnia magna. Although paying particular attention to the UV waveband, we measured the foraging preferences of sticklebacks for prey presented under four different spectral conditions. These conditions selectively removed UV (UV-), short-wave (SW-), mid-wave (MW-) or long-wave (LW-) light from the entire spectrum. The absence of UV and long wavelengths strongly reduced prey attractiveness for G. aculeatus compared with conditions without short-wave and mid-wave light. To control for potential light habitat preferences in the main experiment, we conducted a further choice experiment without prey stimuli. Fish in these trials did not discriminate significantly between the different spectral conditions. When comparing both experiments, it was observed that, although filter preferences for MW-and LW-conditions were virtually consistent, they differed at shorter wavelengths, with a reduced preference for UV-conditions and, at the same time, an increased preference for SW-conditions in the presence of prey. Thus, prey choice seems to be strongly affected by visual information at the short-wave end of the spectrum. The foraging preferences were also mirrored by the chromatic contrast values between prey and the experimental background, as calculated for each lighting condition using a series of physiological models on stickleback perception.
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