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Night-time in colour - the extraordinary vision of nocturnal animals

From hawkmoths to geckos, Adam Manning investigates the science behind colour night vision

Our 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 photon-sensitive 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 photon-sensitive cells: rods for dim light vision, and cones for bright light vision. Depending on the species, many of these animals have one kind of rod and up to four kinds of cones in their retina, but some have pure cone or rod retinae. The eyes of invertebrates work in a similar way, but they possess different receptors.

Cones are used in bright light and are adapted for receiving 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 different colours in low light conditions, when our human eyes perceive the colours only as white. They do this by ‘summing’ the light signals they receive. All animals sum their visual input to interpret it, based on where and how often the light hits in their eye. It is thought that by summing the light signals they receive in low light, nocturnal hawkmoths are able to cut through background visual signals and see colours clearly.

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 rod-like, 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 geckos, hawkmoths, and other animals are able to differentiate objects in their surroundings at night. These animals are better equipped to navigate their environment, find food sources or avoid predators. Many other unstudied animals may also have this incredible ability and give us a fascinating taste of how evolution influences even the smallest details of life, even if most of us take it for granted every day.

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From Issue 22: the Dark Side of Science

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