Emily ShermanThis summer, you may catch yourself reaching for your favorite pair of sunglasses, whether it’s a sunny day, or you’re feeling like channeling your inner celebrity. Far from just a fashionable accessory, sunglasses can protect your eyes from harmful UV rays that, over time, lead to eye diseases like cataracts and macular degeneration. While we humans have the luxury of a cool pair of shades, many animals don’t. So what alternative eye protection methods does nature rely on?
First, a quick primer on light’s path through a vertebrate’s eye. Light enters through the transparent cornea, where it is bent and focused at just the right angle to pass through the thin slit of the pupil onto the lens. The lens focuses the light again, this time at the back of the eye where bundles of light-detecting neurons in the retina send image-generating signals to the brain.
When light deviates from this path and scatters inside the eye, it sends extra, noninformation-containing light to the retina, reducing the contrast and visible details of an image. While humans can don sunglasses and hats to avoid this light scattering, some animals rely on a different tool: malar stripes.
Malar stripes appear as dark, black stripes immediately beneath the eye and are hypothesized to reduce glare from light reflected into the eye. While these facial markings could serve a number of functions across different species, like camouflage or social signaling, the popular “solar glare hypothesis” proposes that malar stripes improve an animal’s ability to accurately target food objects in bright conditions. Baseball players have adopted a similar strategy. By applying eye black beneath their eyes, players see less of a glare and can better detect details, like an incoming fly ball, in bright conditions.
Cheetahs may have some of the best-known malar stripes in the animal kingdom. Unlike the short, rectangular stripes of eye black on an athlete’s face, malar stripes on a cheetah appear as distinctive dark tear marks that run from the eyes down the side of the face toward the mouth.
Cheetah’s eyes themselves contain more color-detecting photoreceptors (cones) than light-detecting photoreceptors (rods), which means they see more clearly during the day than they do at night. This may be one reason cheetahs are diurnal (i.e. hunt from early morning till late afternoon). Their keen daytime vision enables the cheetah to avoid other predators that hunt at night and catch enough food during the day, which can involve chasing down prey at speeds of up to 70 mph.
At such high speeds, accurately detecting prey is vital. Malar stripes play a key role in the cheetah’s survival by blocking the sun’s glare so they can see their prey clearly in the bright glare of the sun, increasing the odds that they don’t miss out on a midday snack.
In another part of the animal kingdom, the peregrine falcon also boasts its own pair of malar stripes as a strip of dark feathers below the eye. With eyes that are eight times better than a human’s and can spot prey up to 2 miles away, this falcon’s eyes are well-suited for hunting and catching its prey: fast-flying birds. Catching your dinner mid-air is no easy task. The peregrine’s eyes need to be able to quickly discern rapid movements while in motion and adjust its attack accordingly, making sure to avoid predators and obstacles.
Researchers at the University of Witwatersrand and the University of Cape Town were intrigued by the peregrine’s keen eyes and feathery sun protection and were curious why these birds sport malar stripes, while other species of falcons in bright environments don’t have malar stripes at all. They decided to test if the solar glare hypothesis was plausible for peregrine falcons by collecting over 2,000 photos of peregrines from around the world using online repositories. Researchers found a strong positive correlation between a location’s annular solar radiation and the peregrine’s malar stripes’ width, color, and contiguity, meaning that birds living in sunny places had more prominent malar stripes.
While this presented strong evidence for the validity of the solar glare hypothesis, all other species of the genus falco were examined and no correlation was seen between solar radiation and malar stripe prominence. This could imply these markings in other species serve different purposes, like social signaling or camouflage.
Taken together, these findings highlight how these simple stripes enable optimal performance of some species’ complex optical machinery. While they may be the height of fashion in the animal world, the malar stripes could play a more significant role by helping ensure the survival of these and other species.
Peer Editor: Caroline Yu
