Diversity and Limits of Colour Vision in Terrestrial Vertebrates

Abstract: Most terrestrial vertebrates have colour vision, some perceive a less colourful world and others actually discriminate a wider colour spectrum than humans do. Still, we can all make use of the valuable colour information, which is more rigid than just brightness. However, at night when the light is dim, the lack of photons makes colour vision difficult. Nevertheless, some hawkmoths and bees can see colours at night. In my thesis I have studied whether there are any terrestrial vertebrates with the same ability and what adaptation for colour vision they have. My emphasise lies on the arrhythmic horse and a nocturnal gecko. The horse is nowadays well-known to have dichromatic colour vision during the day. In behavioural experiments we found that horses perceive their colour space as a continuum of colours, which is different from how we perceive our trichromatic colour space (Paper 2). The horse is also in possession of one of the largest terrestrial eyes, and a large aperture and a short focal length enhances the signal-to-noise ratio by concentrating the photons on few photoreceptors. Still, the colour vision of horses fails at night. Thus the large eye of the horse does not appear to be adapted for nocturnal colour vision but rather for achromatic vision in dim light (Paper 3). Reptiles have also been proven to have colour vision during the day and we became especially interested in the nocturnal geckos. Due to their evolutionary history, the geckos have only cones in their retina, but they have adapted their cones and their optical system to allow for vision at low light intensities. We show that the eye of the nocturnal helmet gecko is almost 400 times more light-sensitive than our own eye (Paper 4). The adaptations of the cones for vision in dim light made us wonder whether geckos could use colour vision at night. In behavioural studies we found that helmet geckos can distinguish colours even at light intensities similar to dim moonlight (Paper 1). Still, for the nocturnal gecko it is unknown when the colours fade.