Vistar Insights

The Eyes Have it: How Humans and Animals See

Tuesday, June 02, 2015

Think of the last time you saw the sun set. What were the colors like? What happened to them as that massive, glowing orb sank into the horizon?

It’s hard to believe, but there are certain animals that see this beautiful event in much more detail than humans.

Our eyes contain millions of light-sensitive cells called rods and cones. Rods allow us to process light and motion. Cones allow us to see color. Human eyes contain three types of color-receptors – blue, green and red. Our red receptor not only helps us to see the color red, but also other colors that are made from red.

Dogs have only green and blue receptive cones. They’re only able to process blue, green and a small amount of yellow.

Butterflies and non-nocturnal birds, however, have five receptive cones! They can process blue, red and green, plus a whole multitude of colors humans cannot see or even describe!

For example, when a female grosbeak is looking to mate, she chooses her partner based on the reflective quality of the male bird’s feathers. Male grosbeaks with brighter, more reflective plumage are more likely to be chosen as mates. Female grosbeaks are brown and male grosbeaks are bright, electric blue. Scientists have measured the peak feather wavelength (where the most light will be reflected) under an instrument called a spectrophometer. For grosbeaks, peak wavelength falls in the ultraviolet range. The intensity of the males’ color varies from bird to bird. Humans cannot detect this nuance in color, but female grosbeaks can – and it’s often the deciding factor in choosing a mate.

The mantis shrimp is king of the color receptive cones. They don’t have just have a few extra cones – they have sixteen.

Exactly how does a mantis shrimp see?

Mantis shrimp are equipped with six parallel strips in the midband (the middle section) of each eye. The first four strips contain eight different types of light-sensitive cells. These cells contain pigments that can detect different wavelengths of light, causing the mantis shrimp’s visible spectrum to extend into infrared and ultraviolet. With the help of filters, each individual cell can be tuned according to light conditions.

The fifth and sixth rows of the midband contain color-receptive cones that specialize in detecting polarized light. Humans are normally oblivious to polarized light, except when we see it in the glare that reflects off water and glass. We use polarized filters in cameras and sunglasses to block out this light.

The mantis shrimp is also the only animal whose eyes contain cells that can detect light that travels in the shape of a helix as it spins clockwise and counter-clockwise. We now have the technology to detect this, but the mantis shrimp has been able to do this for as long as 400 million years.

Back to the sunset. The whole display, as gorgeous and poetic as it may be to us, is actually only derived from combinations of the three different color-receptive cones that we have. Imagine what it would look like to a mantis shrimp!