Convergent Evolution: Malar Stripes & Eye Black

Note: Influenced by Eye Black: What Works for Football Players Works for the Cheetah



Look at these two pictures. Do you see any similarities?

Figure 1. Falcon

Figure 2. Cheetah

  In case you didn't catch it, both the cheetah and the falcon have dark 'tear stains' running from their eyes down to their cheeks. How and why did this characteristic come about? They are both different species. In this post I will go over a concept called convergent evolution and why you see similar traits in unrelated species. Convergent evolution is "the independent evolution of similar features in species of different lineages. Convergent evolution creates analogous structures/traits that have similar form or function but were not present in the last common ancestor of those groups." A simple example of this would be flight. Birds, bats, and insects all have independently evolved the capacity of flight. Even though they acquired this structure independently, their separate ancestors may have lived in very similar environments (more predators on the ground, less resources in one location). Individual ancestors of the bird, bat, and insect population who could fly were able to survive compared to the one's who couldn't which led them to reproduce more and eventually the ancestral bird, bat, and insect species evolved with the adaptation of flight.

Cheetah, Falcons, and Gazelle: Malar Stripes

 When looking at the big cats, the "tear drops" are unique to cheetahs. These tear stains or malar marks help the cheetah see in sunny environments (while hunting especially). The marks help by reducing glare and keeping the sun out of their eyes. Cheetahs are primarily diurnal hunters that spot prey at long distances (can see up to 3 miles) and rely more on vision than other senses. Not only do cheetahs have an eye retinal fovea that take an elongated shape - which influences how they view their surroundings, but the cheetah's malar stripes also enhance and improve their vision, ultimately effecting their hunting strategies.

  The color black absorbs a lot of light and gives off way less visible light than it absorbs (emits a lot of UV and infrared). In the cheetah's case, the black tear stains absorb sunlight, protecting their eyes during hunting and can act as an anti-glare surface. "In physics, a black body is a perfect absorber of light, but, by a thermodynamic rule, it is also the best emitter." A method to keep heat/glare from the sun away is by using black paint (football/baseball) or having black tear stains near the eyes for cheetahs. The black stains on a cheetah improves their vision by reducing contrast sensitivity and absorbing light/wavelengths that produce glare.

  Cheetahs are also compared to birds of prey in how great their vision is. Falcons also have malar stripes/tear stains, which help increase the accuracy of their vision because they also must be able to spot prey from large distances in sunny conditions that produce glare. Malar stripes are not only important for hunting, but general vision in sunny environments. The darker the malar stripe the more light it absorbs and the more thermal radiation it emits (i.e. Infrared so it's both the best absorber and the best emitter).

  Cheetahs prey on animals that also have the malar stripe: gazelles and Impala. A hypothesis that has been stated is co-evolution; an arms race. Cheetahs evolved the malar stripes which helped them become better hunters, so gazelle and other grass-feeding animals (that the cheetahs prey on), may have evolved the tear stains to look out for predators. Since cheetahs can see very well from long distances, the gazelle who had malar stripes may have been able to spot them in a sunny, 'full of glare' environment and were able to survive and reproduce in comparison to the ones who didn't have this mutation/trait. Thus, becoming an evolutionary trait in gazelle and impala.

Figure 3. Gazelle with malar stripes 

'Eye Black' in Sports

Now did football and baseball get the "eye black" from cheetahs?

Figure 4. Athletes who play in sunny outdoor conditions are seen wearing "eye black" (football, baseball, sometimes soccer).

 Who knows! It could've been that or it could've been that only a certain thing works for a certain situation. Athletes who have to catch, throw, hit something and are in a sunny environment will experience glare from the sun. That's not good if a team is relying on someone to catch a baseball or a football. Wearing eye black may tone down the glare one experiences. The black stripes can enhance contrast sensitivity by improving the eye's ability to distinguish between light and dark colors. In other words, increased contrast means an individual can see in greater detail. This is exactly what occurs with cheetahs when they hunt. When tracking a ball going at a very high speed, one must eliminate any distractions. One of the greatest distractions is the sun.

  In conclusion, different, unrelated species may have acquired analogous traits most likely due to similar environments. In this post, I compared the convergent evolutionary trait of malar stripes seen in both cheetahs and falcons (includes some other birds of prey) who are diurnal, long distance hunters, occupying sunny environments. The gazelle (one of the cheetah's prey) also has malar stripes which is more likely due to an arms race or co-evolution. Since both the gazelle and cheetah share the same sunny, glared environment, it is more beneficial for the gazelle to have these stripes to reduce glare which may assist them in spotting predators (especially cheetahs) from a distance.

  Football players and baseball players are sometimes seen with 'eye black' that may act similarly to the malar marks of cheetahs and falcons. Considering that black is a great absorber and emitter, using a sort of black paint may help reduce glare for individuals who need to catch, pitch and hit.