To human eyes, the world at night is a formless canvas of grey. Many nocturnal animals, on the other hand, experience a rich and varied world bursting with details, shapes, and colors. What is it, then, that separates moths from men? Moths and many other nocturnal animals see at night because their eyes are adapted to compensate for the lack of light. All eyes, whether nocturnal or not, depend on photoreceptors in the retina to detect light particles, known as photons. Photoreceptors then report information about these photons to other cells in the retina and brain.
The brain sifts through that information and uses it to build up an image of the environment the eye perceives. The brighter the light is, the more photons hit the eye. On a sunny day, upwards of 100 million times more photons are available to the eye than on a cloudy, moonless night. Photons aren’t just less numerous in darkness, but they also hit the eye in a less reliable way. This means the information that photoreceptors collect will vary over time, as will the quality of the image.
In darkness, trying to detect the sparse scattering of randomly arriving photons is too difficult for the eyes of most daytime animals. But for night creatures, it’s just a matter of adaptation. One of these adaptations is size. Take the tarsier, whose eyeballs are each as big as its brain, giving it the biggest eyes compared to head size of all mammals. If humans had the same brain to eye ratio, our eyes would be the size of grapefruits. The tarsier’s enlarged orbs haven’t evolved to make it cuter, however, but to gather as much light as possible.
Bigger eyes can have larger openings, called pupils, and larger lenses, allowing for more light to be focused on the receptors. While tarsiers scan the nocturnal scene with their enormous peepers, cats use gleaming eyes to do the same. Cats’ eyes get their shine from a structure called the tapetum lucidum that sits behind the photoreceptors. This structure is made from layers of mirror-like cells containing crystals that send incoming light bouncing back towards the photoreceptors and out of the eye. This results in an eerie glow, and it also gives the photoreceptors a second chance to detect photons. In fact, this system has inspired the artificial cats’ eyes we use on our roads. Toads, on the other hand, have adapted to take it slow.
They can form an image even when just a single photon hits each photoreceptor per second. They accomplish this with photoreceptors that are more than 25 times slower than human ones. This means toads can collect photons for up to four seconds, allowing them to gather many more than our eyes do at each visual time interval. The downside is that this causes toads to react very slowly because they’re only receiving an updated image every four seconds. Fortunately, they’re accustomed to targeting sluggish prey.
Meanwhile, the night is also buzzing with insects, such as hawk moths, which can see their favorite flowers in color, even on a starlit night. They achieve this by a surprising move – getting rid of details in their visual perception. Information from neighboring photoreceptors is grouped in their brains, so the photon catch of each group is higher compared to individual receptors. However, grouping photoreceptors loses details in the image, as fine details require a fine grid of photoreceptors, each detecting photons from one small point in space.
The trick is to balance the need for photons with the loss of detail to still find their flowers. Whether eyes are slow, enormous, shiny, or coarse, it’s the combination of these biological adaptations that gives nocturnal animals their unique visual powers. Imagine what it might be like to witness through their eyes the world that wakes up when the Sun goes down.