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Basically, rods interpret light, and cones interpret colors. Animals that are truly colorblind completely lack cones, and animals that can only see during the day completely lack rods—but these extremes are uncommon, and most animals have a combination of cones and rods.
So how do animal eyes differ from human eyes?
1. Cats and dogs don"t see as many colors but have a wider field of vision and see better at night
Whereas humans have three color-receptor cones in our eyes, dogs only have two—they"re missing the one that detects red. So it"s true that dogs don"t see as many colors as us, but they"re not colorblind; it"s just that they only see shades of blue and yellow. This is true for cats as well, and actually for most mammals in the world.
We might see more colors than they do, but dogs and cats have more rods than us, meaning that they"re better able to see at night. And before you start bragging, keep in mind that humans aren"t at the top of the color-vision chain, either.
2. Some insects can see in ultraviolet
Like I said, humans have three color-receptor cones in our eyes, and we tend to think that we can see all the colors there are to be seen. But actually, there are other animals that have more cones and see far more colors than we do.
Bees and butterflies, for example, have four color-receptor cones. They can see an amazing spectrum of colors, including ultraviolet colors. (On the other hand, they don"t see much in the way of detail.) The photoreceptors in their eyes that make this possible are important, because many types of flowers have ultraviolet patterns on their petals—these work like a runway strip for a plane, allowing the bees and butterflies to zero in on the nectar they want to eat.
These extra color receptors might also be useful for warning coloration (which is when animals display bold patterns or colors to ward off predators—for a refresher, check out last year"s Pop Sci post about animal coloration).
3. Some snakes use thermal sensing to "see"
Pythons, boas, rattlesnakes, and other members of the snake family known as pit vipers are able to see in infrared, which means that they "see" in heat signatures (also known as thermal sensing). They have evolved special pits located between their eyes and nostrils that are able to sense minute temperature changes—meaning that it can be pitch black out, but a snake will know if a warm body approaches. (Check out the video above for a demonstration of this.) Their infrared vision is so accurate, they can detect prey at distances of up to one meter and can detect temperature changes as precise as 0.002 degrees centigrade.
4. Some animals (unlike humans) can see colors at night
The color receptor cones in human eyes stop working when it gets darker than half-moonlight. By using the rods in our eyes, rather than the cones, we can still see, but only in shades of gray. This made sense for humans evolutionarily, but other animals still need to be able to see colors at night.
Take the gecko, for instance: their eyes have evolved to be up to 350 times more sensitive to color at night than ours. This is important for geckos, because their eyes actually have no rods at all—so instead, the cones in their eyes have evolved to become more rod-like: longer and more sensitive.
Some other animals that can see color at night are elephant hawkmoths, which can find flowers by color as easily as their butterfly cousins do during the day, and several species of nocturnal woolly lemurs, which can pick out a particular shade of green that scientists believe signals young leaves with the highest protein content.
5. The mantis shrimp sees more colors than any other animal
Finally, we come to the king of the color-seeing kingdom: the mantis shrimp.
As compared to humans" measly three color-receptive cones, the mantis shrimp has 16 color-receptive cones, can detect ten times more color than a human, and probably sees more colors than any other animal on the planet. (!!) They can see in ultraviolet, infrared, and even polarized light. Not only that, but their eyes are on separate stalks and are able to move independently of each other, meaning they"re able to keep an eye watching out for predators and prey in two different directions at once—and in more colors than we can even conceptualize.
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Besides having the most incredible color vision on the planet, mantis shrimp are also terrifyingly deadly: their club-like appendages folded beneath their bodies (like a praying mantis"s) can strike at the speed of a .22 caliber bullet—or 50 times faster than the blink of an eye.