Night-time in colour – the extraordinary vision of nocturnal animals From hawkmoths to geckos, Adam Manning investigates the science behind colour night vision
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ur sense of sight is something most of us take for granted every day, and we are one of thousands of species of animals that depend on sight to interact with our environment. However, some animals have evolved extraordinary abilities to not only see in the dark, but to see in colour too. Eyes generally work on the same principle in the animal kingdom. Photons of light that have bounced off nearby objects enter our eyes through the pupil and hit photonsensitive cells in the retina, located at the back of the eyeball. These cells then send signals to the brain, which interpret the signal as vision. Most vertebrates possess a dual retina with two types of photonsensitive cells: rods for dim light
particular wavelengths, such as red, blue or green light. By picking up on different wavelengths we are able to differentiate between the different colours of objects. However, in low light conditions, cones are no longer effective at detecting light and we have to rely on our rod cells. This hinders our ability to see in colour in the dark because we only have one type of rod cell that can only detect white
light. Although, this isn’t true for all animals. Researchers have discovered that nocturnal hawkmoths are able to differentiate between “...evolved extraordinary different colours in low abilities to not only see in the light conditions, when dark, but to see in colour too." our human eyes perceive the colours only as vision, and cones for bright light white. They do this by ‘summing’ vision. Depending on the species, the light signals they receive. All many of these animals have one animals sum their visual input to kind of rod and up to four kinds of interpret it, based on where and cones in their retina, but some have how often the light hits in their eye. pure cone or rod retinae. The eyes of It is thought that by summing the invertebrates work in a similar way, light signals they receive in low light, nocturnal hawkmoths are able but they possess different receptors. Cones are used in bright light to cut through background visual and are adapted for receiving signals and see colours clearly. 12 | SATNAV | June 2021
Colour night vision is also apparent in some vertebrates. Through evolution, nocturnal geckos have lost their dual retina and now only have cones which are sensitive to UV, blue, and green wavelengths of light. By not having rods, these nocturnal geckos can still differentiate between colours in dim moonlight and possibly even darker. The cones of nocturnal geckos can still operate in low light conditions because they have evolved to be larger and more rodlike, allowing them to act more similarly to the rods in other vertebrates' eyes while still picking up on colours. By being able to see colour in low light conditions, nocturnal
