Tigers are the largest species of feline on the planet and one of the most fearsome predators in the world. They are known for their very recognizable striped red-orange fur, a coloration that is useful for camouflage.
This might be obvious when tigers are hunting through the dry undergrowth. Their coloration allows them to be hardly visible even to keen human eyes. But in a predominantly green forest, you'd think a bright orange cat that can be almost 4 meters (13 feet) long would be quite visible. Researchers from the University of Bristol have published a new study explaining why this is not the case.
Tigers' favorite meals are deer, boars, and other ungulates. These animals, like most mammals, are dichromats. They have just two types of functioning color receptors in the eye, meaning they are red-green blind. They struggle to distinguish between green tones and red-orange tones. For them, tigers will match the surrounding forest pretty well.
Humans, on the other hand, are trichromats, meaning we have three color receptors that allow us to reliably differentiate between orange and green, hence tigers appearing orange to us.
The researchers explain in the Journal of the Royal Society Interface how they employed a machine learning algorithm to simulate what the world looks like to dichromats, to work out the optimal colors for camouflage and concealment in animals. They concluded that as long as your predator or prey can’t distinguish between orange and green, there’s no need to develop an emerald hue to hide in a forest.
“Based on our results and given that most non-human mammals have dichromatic color vision that is unable to reliably differentiate orange and green, it seems that there is little benefit to actually become green if the receiver is dichromat," the researchers wrote in their conclusion. "Hence predators (eg tigers), whose main prey is other mammals (eg deer), experience little evolutionary pressure to evolve green coloration from a trichromatic perspective.”
Red pigments are common in mammals' colorations while greens aren’t, so it is not surprising that no mammal has felt the evolutionary pressure to evolve some tone of jade in their fur or skin. Sloths can sometimes appear green, but that’s due to the symbiotic relationship between a green algae that lives on the Central and South American animal’s fur, giving it some added camouflage.
The researchers were curious why evolution hasn't pushed prey mammals to become trichromats, like us and other apes. After all, primates are not the only creatures that have three types of color receptors. Marsupials have three, and birds and fish use four pigments for vision. Until that happens, though, tigers have no reason to change their stripes.