It's not easy to imagine how the world looks through another creature's eyes. After all, we only ever get to see it through our own. That is probably why most people, scientists included, assumed that other animals can only see in shades of grey in the dark.
But a growing number of species are proving us all wrong. From nocturnal geckoes to moths, lemurs and bats, animals of all shapes and sizes are turning out to have colour vision even in near-total darkness. Some researchers now think that colour vision at night might be quite common.
It makes a lot of sense when you think about it. Colour vision is just as useful for finding food, mates and shelter at night as it is during the day. So it stands to reason that nature would try and find a way to make it work.
The trouble is, the laws of physics are stacked against that happening. All eyes in the animal kingdom evolved for the same purpose – to detect photons, the tiny particles that make up a ray of light. So when the sun goes down and there are fewer photons around, they don't work nearly as well.
Our colour receptors stop working when it gets darker than half-moonlight
Seeing colour is especially problematic, because it works by comparing the responses of two or more receptors, each of which are tuned to a different wavelength of light. For example, human eyes have three types of colour receptors. They are tuned to long, medium and short wavelengths of light, which we see as red, green and blue. Comparing the signals from all three receptors allows us to see all the colours of the rainbow. But there needs to be plenty of light if all the receptors are to have enough.
Our colour receptors stop working when it gets darker than half-moonlight. At that point we switch to more sensitive, but colour-blind, receptors called rods. By adding together the output of all of these rods, we can still see, but only in shades of grey.
So how do some animals manage keep their colour vision even with almost no light around? In some cases it's by making extreme modifications to the light-detecting machinery of the eyes.
The first animal found to have colour night vision was the elephant hawkmoth, in a series of experiments by Almut Kelber of Lund University in Sweden, published in 2002.
Helmethead geckoes have lost the rods in their eyes
The hawkmoths have evolved larger lenses in their eyes, and shortened the distance that the light has to travel to get to their colour receptors. This lets enough light in to allow them to detect ultraviolet, yellow and blue on a moonless night, when the only light comes from the stars. Even in this darkness, they can find flowers by colour as easily as their butterfly cousins do during the day.
Two years later, Kelber found colour night vision in another animal, this time a vertebrate with eyes far more similar to our own. But this creature didn't have the luxury of rod-vision to fall back on in the dark.
Helmethead geckoes, like all lizards, lost the rods in their eyes during a long stretch of evolution when their ancestors only came out during the day. That made Kelber wonder how they manage to hunt crickets and other insects, by sight, in the dead of night.
Kelber trained the geckos to avoid grey tweezers holding unappetising salty crickets, and to choose blue tweezers holding a tasty one. She found that they could use their colour vision in this way even in dim moonlight, when the people holding the tweezers couldn't see the difference.
A closer inspection of the geckos' eyes revealed that, with no rods to fall back on, the cones in their eyes had evolved to become more rod-like, longer and more sensitive. Like the hawkmoths, they also had large lenses and a shorter focal distance to cut down how far the light had to travel through the eye.
Evolution works on a "use it or lose it" basis
At this point it looked like the geckos' colour night vision was an accident of evolution. They only have it because their ancestors had lost their colour-blind rods, so when they evolved night vision they only had colour receptors to work with.
For that reason, everyone assumed that mammals, which do have rods, would all be colour-blind in the dark. After all, evolution works on a "use it or lose it" basis, and there is no use hanging onto colour vision if you only come out at night.
But recently, several species of nocturnal lemur have thrown out this assumption, too. The first species found breaking the rule was the aye-aye, a mysterious and rare nocturnal lemur that hasn't come out in the daytime for millions of years. Based on the "use it or lose it" principle, its colour vision should be a distant memory.
But no. In 2011, scientists found that the aye-aye's eyes are tuned to see blue down to near ultraviolet even in dim light. "This could help them find bluish objects, such as the seeds of one of their food trees, in the blue-dominated wavelengths of twilight," says Amanda Melin, who has studied aye aye vision. White flowers or other foods would also look bluish during twilight, she suggests. That includes the blue seeds and fruits of the traveler's palm, which as well as being good to eat are a good spot for catching insects that are attracted to the fruit.
In early 2014 it emerged that several species of nocturnal woolly lemurs also have eyes capable of detecting colour at night. The lemurs' eyes seem to be tuned to pick out a particular shade of green that signals young leaves with the highest protein content. It may be that this gives them an advantage when food is short or they need extra nutrition, such as when they are nursing young.
We should start looking for colour vision in all kinds of nocturnal mammals
Even bats have held onto red and blue colour vision – and not only fruit-eating bats who would benefit from finding ripe fruits. Insect-eating bats seem to have it too, according to a 2009 study by Emma Teeling of University College Dublin in Ireland. Since bats have been nocturnal for around 80 million years, this would suggest that it benefits them in some way.
As a result, Teeling thinks that we should start looking for colour vision in all kinds of nocturnal mammals. We might also have to rethink the assumption that colour vision is a strictly daytime adaptation. "We've shown that carnivorous bats have dichromatic [two colour] vision so we need to think, why?" she says.
And this is where it all gets a bit tricky, because, aside from the hawkmoths, no one knows what any of these animals are using their colour night vision for.
It all depends what is most important for a particular species' survival
The lemur findings seem to suggest that finding food is the driving force. But Huabin Zhao of Wuhan University in China is less convinced. "I don't think colour vision has a link with diet in mammals," he says. "For example, some fruit bats are colour-blind, while some other fruit bats have colour vision."
The best way to settle this is to run careful experiments to figure out what animals can and can't see at night. These are already underway: a team at Duke Lemur Center in Durham, North Carolina, is doing testing aye-ayes. But some animals are proving stubborn. Kelber is testing nocturnal frogs, which have two colour receptors and so in theory might have colour vision. But they have taken a year to learn anything.
Melin suspects that there is no one-size-fits-all answer. It all depends what is most important for a particular species' survival. Species that are more active in twilight and bright moonlight might well keep their colour vision, if it helps them find colourful foods or communicate with group members, she says.
By contrast, Melin thinks species that stick to the darkest places to avoid being eaten might well lose colour vision completely, as approaching predators would still be visible in black-and-white.
In most cases we still don't know if animals can see in colour at night
What's more, there are good reasons for ditching colour vision in the dark and specialising in black-and-white. Rods give a much better measure of the intensity of colour than cones do, so if detail is more important than colour – for instance, trying to see a camouflaged predator hiding among leaves – monochrome might be the best tool for the job.
"There must be some advantage of rods," says Kelber. She says there is genetic evidence that they evolved later than cones, and most vertebrates have since kept them. "So they must be better."
In most cases we still don't know if animals can see in colour at night. Now that the phenomenon has been spotted, we can find out. But the story is a reminder that we should not make any assumptions about how other species experience the world. The truth is, we have very little idea what animals see when they open their eyes.