While most of us take it for granted that we can perceive the world around us through our eyes; in all its colors, richness of detail, and movement; this is not self-evident for all animals on this planet. For those active at night, the sparse light makes vision, which depends on the abundance of light, problematic. What is it about the night that makes animals chose it over the day, despite the challenges it holds for them? The general hypothesis is that animals turned to night creatures to avoid the crowd of other animals active during the day – and thus have less competition for their food sources, and fewer predators that might consider them food in turn. Hence, overcoming the challenges of a nocturnal life is worth it for higher chances of finding food sources and lower chances of being eaten.

But it is not just animals active at night that need to tackle the challenge of sparse photons for their sense of vision. Animals living in the oceans also face this problem: as one travels deeper and deeper in the ocean, more and more light is absorbed, until below depth of 1000m no sunlight remains. The deep sea, however, is not entirely deprived of light, as in its depths light is created by animals through chemical reactions, a phenomenon called bioluminescence. Because of this, even deep sea creatures such as the fearsome looking dragonfish or the giant squid have evolved eyes, as well as many of the same adaptations that nocturnal animals have, to see this sparse light.

What is so special about the sense of vision that so many animals have evolved adaptations to be able to use vision as one of their primary senses when active in dim light? The sense of vision provides an animal with a rich body of information about the environment: at each instance, an animal’s eye can capture all its surroundings (in some animals almost 360 degrees are covered by the two eyes), with great detail (big obstacles such as trees, and small features such as patterns on flowers or other animals can be discriminated). By comparing signals from the two eyes, animals can work out how far away and how big objects are (depth perception).  Furthermore, the perception of color adds additional information (such as whether a fruit is ripe or rotten). 

All this information can be gathered with great positional certainty at one glance (unlike smell, where an animal often has to move to find an odor source), without having to directly contact objects (as is necessary for the sense of touch) and without the objects having to make a sound (as for hearing). Because of this, vision is the primary sense for many animals, including humans, and camera vision is also used by many autonomous vehicles and robots.

Learn more about the human eye with this TED-Ed Lesson: A journey through the human eye.

While it is true that vision gives animals a great richness of information and animals active in dim light go to great lengths to maintain it, many of them also rely on their other senses to complement vision - often more strongly than relatives active in bright light. Rats and cockroaches, for example, explore their night time environment with touch sensors – whiskers in the case of rats, and antennae in cockroaches. Many night active moths and some birds (such as the kiwi and the kakapo) rely more strongly on olfaction than their diurnal relatives and sniff out their food sources and mates. Hearing is another sense that can be of great value in a low light environment: barn owls, for example, can locate a mouse using only the sound that reaches their ears. A few animal species have gone even further and evolved senses that can indeed replace the sense of vision, to navigate their environment safely at night, such as bats and their ability to echolocate. By producing a sound, and analyzing the echo that reflects back from the bat’s environment, they obtain a precise image of the objects around them, and can thus fly in complete darkness, when their eyes do not see anything. Another example would be weakly electric fish and their ability to electrolocate objects in their environment using self-generated electric fields.

While we humans are not primarily night active creatures, we also have some specializations for seeing in dim light. But what is more, we have developed technology that helps us see in dim light. These devices use sensors that are much better at capturing even the tiniest amount of photons than our eyes, while others capture the heat radiating from objects (such as animals, humans or cars). Many devices make use of a part of the light spectrum that our eyes cannot see: infrared light. They essentially light up the nocturnal scene with infrared light (though not visible for our eyes, and the eyes of most animals), then capture it with special sensors and convert it into an image our eyes can detect. Yet, you can get very far exploring the night using your natural visual abilities, and a healthy portion of curiosity.

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