Limits to the salience of ultraviolet: Lessons from colour vision in bees and birds

Ultraviolet is an important component of the photic environment. It is used by a wide variety of animals and plants in mutualistic communication, espe cially in insect and flower inter-relationships. Ultraviolet reflections an d sensitivity are also becoming well considered in the relationships betwee n vertebrates and their environment. The relative importance of ultraviolet vis a vis other primary colours in trichromatic or tetrachromatic colour s paces is discussed, and it is concluded that ultraviolet is, in most cases, no more important that blue, green or red reflections. Some animals may us e specific wavebands of light for specific reactions, such as ultraviolet i n escape or in the detection of polarised light, and other wavebands in sti mulating feeding, oviposition or mating. When colour vision and, thus, the input from more than a single spectral receptor type are concerned, we poin t out that even basic predictions of signal conspicuousness require knowled ge of the neuronal wiring used to evaluate the signals from all receptor ty pes, including the ultraviolet. Evolutionary analyses suggest that, at leas t in arthropods, ultraviolet sensitivity is phylogenetically ancient and un dergoes comparatively little evolutionary fine-tuning. Increasing amounts o f ultraviolet in the photic environment, as caused by the decline of ozone in the atmosphere, are not likely to affect colour vision. However, a case for which ultraviolet is possibly unique is in the colour constancy of bees . Theoretical models predict that bees will perform poorly at identifying p ure ultraviolet signals under conditions of changing illumination, which ma y explain the near absence of pure ultraviolet-reflecting flowers in nature .