The human eye is one of evolution's greatest achievements. It can see tiny dust specks and huge mountains, near or far, in full colour. Backed by the processing power of our brains, it can also pick out movement and help us identify the people we love just by their faces.
One of the eye's best tricks is so good, you don't notice it. When you step from the brightly-lit outdoors into a dimly-lit building, the light level drops massively, but your eyes adjust almost immediately. That's because they have evolved to work even when there isn't much light around.
But other species are even better in dim light. Try reading a newspaper in the gloom of twilight, and the black letters will merge with the white background into a smoky greyness that your eyes cannot decipher. However, hand the paper to a cat and it would be fine, or at least it would if it knew how to read.
But even cats, despite their habit of hunting at night, are far from the most light-sensitive animals around. The creatures with the ultimate night vision have evolved amazing adaptations to capture the tiniest flickers of light. Some can even see when, according to our best understanding of physics, there is nothing to see at all.
To compare how much light animals need to see by, we'll be using lux - the amount of light per square metre. Human eyes work well in bright sunlight, when the illumination can be well over 10,000 lux. We can also see, albeit dimly, in just 1 lux, which is about what you get on a dark night.
Domestic cat (Felis catus): 0.125 lux
Cats can see in just one-eighth of the illumination we need. That's because, while their eyes are basically similar to ours, they have a few tweaks that give them the upper paw at night.
Cats' eyes have larger lenses than humans'
Both human and cat eyes have three basic parts: a hole that lets light in, called a pupil; a lens that focuses the light; and a screen on which images are formed, called the retina. In a cat's eye, all three have been modified.
Whereas human pupils are circular, a cat's pupil is elliptical. It can narrow to a slit during the day and open wide for maximum illumination during the night. Human pupils can open and close, but not to such an extent.
Cats' eyes have larger lenses than humans', which can collect more light. They also have a reflective layer behind the retina called the tapetum lucidum. It's this layer that makes cats' eyes glow in the dark, because incoming light goes through the retina and then bounces right back out. In the process, the light passes through the retina twice, so the retina has two chances to absorb it.
Cats have also changed the makeup of their retinas. There are two types of light-sensitive cells there. Cones are colour-sensitive but only work in bright light, while rods only do black-and-white but work in dim light. Humans have lots of cones, giving us rich colour vision in the day, but cats are big on rods. They have 25 rods to each cone, compared to four rods to each cone in humans.
Cats also pack in 350,000 rods per square millimetre in their retina, compared to the measly 80,000-150,000 that humans have. What's more, in cats, the signals from about 1500 rod cells are pooled into each neuron that leads out of the retina. That means weak signals can be added up to make a detailed image.
There is a downside to this. Cats' day vision is similar to that of humans with red-green colour blindness. They can distinguish blue from other colours, but cannot tell red, brown and green apart.
Tarsiers (Tarsiidae): 0.001 lux
Tarsiers are tree-dwelling primates from South East Asia. They have probably the largest eyes, relative to their body size, of any mammal. A tarsier's body, excluding the tail, is about 9-16 cm. Meanwhile its eyes are 1.5-1.8 cm across, and occupy almost the entire head.
The one downside of the huge eyes is that tarsiers cannot move them
They mostly eat insects, and hunt for prey early in the morning and late in the evening, typically in light levels between 0.001 and 0.01 lux. Scrambling through the treetops, they must spot small, well-camouflaged prey in near-darkness, and leap between branches without falling.
They manage all this with eyes similar to our own, though obviously rather larger. Tarsiers' enormous eyes can allow a lot of light in, and large muscles surrounding the pupil regulate the amount of light that enters the eye. An equally large lens focusses images onto a retina packed with rods: more than 300,000 rods per square millimetre, similar to the retina of a cat.
The one downside of the huge eyes is that tarsiers cannot move them. To compensate, their heads can turn 180 degrees.
Dung beetles (Onitis sp.): 0.001-0.0001 lux
Wherever there is rich animal manure, chances are you will find dung beetles. These beetles fly to the freshest dung pile and start living in the pile, rolling dung into balls to take away or tunneling under the pile to make a larder. Dung beetles of the genus Onitis fly at different times of the day, and night, to look for dung.
Dung beetle eyes are very different from ours. Insects have compound eyes, made of individual units called ommatidia packed together in different ways.
In insects that fly during the day, each ommatidium has a sleeve of pigment that absorbs excessive light, ensuring they don't get dazzled. It also isolates each ommatidium from its neighbours. But in nocturnal insects, this pigment sleeve is missing. That means the light collected by many ommatidia can be pooled and focused onto a single photoreceptor, boosting the sensitivity.
Onitis dung beetles include both types. Day fliers have pigment sleeves, evening fliers pool information from ommatidia, and night fliers pool information from twice as many ommatidia as the evening fliers. The eyes of the nocturnal O. aygulus are 85 times more sensitive than the eyes of the day flying O. belial.
Social sweat bee (Megalopta genalis): 0.00063 lux
But that rule doesn't always apply. Some insects can see in extremely dim light, despite having visual systems that are clearly built for day vision.
Eric Warrant and Almut Kelber of Lund University in Sweden have found that some bees have sleeves of pigment separating their ommatidia, yet they do a fantastic job of flying around and finding food at night. For instance, they showed in 2004 that Megalopta genalis can find its way around at light intensities 20 times dimmer than starlight.
Being active at night has two advantages to bees: there is plenty of nectar and pollen from nocturnal flowers, and fewer predators and parasites. But it also means being able to see flowers, and to find your way back to the nest when you're done feeding, in the dark. Yet these bees' eyes are set up to see in the day.
They have done it by rewiring their eyes. Once the retinas absorb light, neural circuits transmit the information to the brain. This is another step in the process where signals can be added together to boost the image.
M. genalis has specialised neurons that link ommatidia into groups. That means the signals from all the ommatidia in the group are lumped together before being sent to the brain. This makes the resulting image blurrier, but considerably brighter.
Carpenter bee (Xylocopa tranquebarica): 0.000063 lux
This carpenter bee from the Western Ghats of southern India goes one better. It can even fly on moonless nights, when light intensities are even lower. "They can fly in starlight, on cloudy starlit nights and even when there is a lot of wind," says Hema Somanathan of the Indian Institute of Science Education and Research Thiruvananthapuram.
When they encounter a sudden flash of light, these bees drop down
Somanathan has found that the ommatidia of X. tranquebarica have unusually wide openings, and its eyes are large relative to the size of its body. These modifications will all increase the amount of light harvested.
But they are not enough to explain its excellent night vision. It may also be grouping its ommatidia just like M. genalis.
They aren't limited to flying by night. "I have seen X. tranquebarica fly during the day, when their nests are disturbed by predators," says Somanathan. "When they encounter a sudden flash of light, these bees drop down, unable to process the high amount of light. But they still recover and start flying again."
It looks like these bees have the most sensitive night vision of any animal. But as of 2014, one other animal is in the running.
American cockroach (Periplaneta americana): less than one photon per second
We can't directly compare cockroaches to the other animals, because their vision has been measured in a different way. But it is clear that their eyes are extraordinarily sensitive.
In experiments published in 2014, Matti Weckström of the University of Oulu in Finland and his colleagues recorded how individual light-sensitive cells within a cockroach's ommatidia responded to extremely low light levels. They impaled these cells with tiny electrodes made of glass.
The cockroach is quite marvellous
Light is made up of unimaginably tiny particles called photons. The human eye needs at least 100 photons to come in at once in order to detect anything. But the light-sensitive cells in the cockroach could respond to a grating moving from side to side, even when each cell was only getting one photon of light every 10s. In other words, the cells still responded when they were receiving less than one photon of light per second.
The cockroach has 16,000–28,000 green-sensitive photoreceptors per eye, and Weckström's study suggests that signals from hundreds or even thousands are pooled under low light conditions. This dwarfs the pooling in cats' eyes, where 1500 rods work together in dim light. The extent of pooling in the cockroach is "massive", as Weckström writes in the paper, and appears to be unparalleled.
"The cockroach is quite marvellous. Less than a photon per second," says Kelber. "That is the ultimate night vision."
But in one respect, the bees may still trump them: cockroaches don't fly in the dark. "Flight control is much more difficult, as the animal moves faster and it is more dangerous to bump into things," says Kelber. "In that sense, the nocturnal carpenter bee is the most special. It can fly and forage in moonless nights and still see colours."