Abstract
When night falls, at light intensities between 0.01 and 0.001 Cd/m2, humans and most other vertebrates are left colour-blind, because they rely on their single type of rod sensitive enough to allow for vision under dim light conditions. This makes sense as rods are specifically adapted to collect photons and keep noise levels low. Colour vision demands opponent mechanisms between cones, which result in lower signal-to-noise ratios. However, geckos, being nocturnal lizards and lacking rods, have three types of cone and thus the precondition for colour vision. The same it true for many nocturnal insects. Nocturnal geckos, as well as nocturnal hawkmoths and bees can use three spectral types of photoreceptor for true colour vision in starlight. To be able to do so, they sacrifice spatial and temporal resolution in favour of colour vision. Besides the three groups for which nocturnal colour vision has been proven behaviourally, other animals may have nocturnal or dim light colour vision. Possible candidates include toads and frogs with two rod types, other nocturnal insects, spiders and some deep-sea fish. Nocturnal colour vision is useful because the colour of light changes vastly between late twilight and nocturnal moon- or starlight. The changes of the illumination colour make achromatic vision much less reliable compared to colour vision. Mammals and birds, in contrast, sacrifice colour vision in dim light and invest in high sensitivity using rods.