In other words, an organism with a slit, multifocal pupil could maintain better color vision at both low and high levels of light.Īnimals with rounded pupils almost never have these different focal zones (there are some exceptions in certain species of snakes and rodents) because, obviously, the broader zones would be blocked in high-light conditions.Ĭolor is important specifically for predators because it allows them to contrast a hidden prey animal against its surroundings. But when the pupil constricts in a vertical way, all three zones of the pupil still have some exposure. When the pupil constricts in a circular way, the outermost (black) zone of the pupil is completely blocked, meaning that in bright conditions an animal would be unable to see colors with longer wavelengths. The orange represents the animal’s iris, while the black and gray rings are different focal zones within the animal’s pupil. By bending the light at different angles in different focal zones, the fish can get all colors to hit its retina at the same time.
In fish eyes, this detriment can be corrected by having a pupil with different focal lengths- i.e., different sharpness in the way light is angled towards the fovea.
Think of how much harder it is to make out the colors of an object in the dark. This isn’t much of a problem when there’s lots of light, but when the light is very dim, it can result in a loss of color vision. Because these wavelengths are different, each color reaches the pupil at a different speed. This is based off of studies of fish eyes, which are adapted to highly variable underwater light conditions.Ĭolors are produced by light bouncing off of objects, and each color represents a different wavelength, from the long infrared to the very short ultraviolet. So what advantages, other than dramatic dilation, does the vertical slit pupil allow? Recently, a theory has been proposed that the slit pupil aids predatory animals in seeing color in different light conditions. In fact, round pupils provide the clearest nighttime images when compared to all other shapes. I like to call that the “where is your god now” effect. Many nocturnal species have round pupils which are actually quite good at contracting to very small sizes. This explanation has never been perfect, however. This allows the eyes of nocturnal creatures, which are designed to take in much more light than those of diurnal creatures, to close down and protect themselves during the daytime. Slit pupils allow the iris to contract or expand more dramatically- a human’s round pupil can expand to allow light to be 10-fold more intense compared to its smallest size, while a cat’s pupil expands to 135-fold intensity. Slit pupils are found most often in animals with eyes that are exposed to highly variable light conditions- i.e., nocturnal or crepuscular animals. The simplest and most basic theory looks at an animal’s functional anatomy. So what advantages do slit pupils confer? There are actually a number of different theories right now. It’s functionally very similar to the aperture on a camera.Īgain, in all these groups, slit pupils evolved separately. In bright light, the terrestrial vertebrate pupil contracts (becomes smaller) to reduce the amount of light coming in conversely, in dim light the pupil expands to increase the amount of light coming in. Of course, no two scientists seem to agree on exactly what these advantages are.Įssentially what the pupil does is allow light to pass through the eye and onto the retina at the back of the eye. With it, there are a whole bunch of different ways that animals can shape their vision- and their pupil- to their advantage. Without it, our eyes would be functionless.
Light passes differently through water than it does in air, not to mention that now we have to worry about our lenses- which have to be moist to properly function- drying out.īut the focus (ha ha) today is on the pupil, the transparent bit inside the iris that allows light to enter the eye. The eye itself is kind of a weird misshapen organ, particularly in land animals where it has had to compensate for, you know, the fact that it originally evolved in the water. There are a lot of different pupil shapes among vertebrates (and some invertebrates, too).
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