Why do zebras have stripes? Possibly to cool off, according to new research. It seems that stripe configuration correlates more strongly with temperature than with anything else—the alternating black and white lines heat the air next to the body at alternate rates, creating eddies of air which cool the zebra. It’s remarkable that it took until now to make headway on this mystery that’s been hiding in plain sight.
Early interest in zebras focused on the nature of stripes rather than the function. A 1936 paper by Angel Cabrera in the Journal of Mammology explores this early research interest in stripes. For these early researchers, stripes were primarily a means of identification, but Cabrera was concerned that early taxonomists, working in museums far from living zebras, were too quick to identify species based on limited specimens.
Cabrera instead relied on personal observation and the input of hunters, wardens, and others experienced with zebra appearances. His conclusion—that no two zebras are alike—was revolutionary and helped unclutter the taxonomy of zebras to three extant species and one extinct.
Interest in the “why” of stripes came later and extends beyond zebras. In 2009, Tim Caro reviewed 5000 instances of black and white markings in mammals, and concluded that in most cases the reason for such markings is mysterious. In most cases bold colors are a warning to predators that an animal is toxic or otherwise makes for poor eating, known as aposematism.
Caro felt that applying aposematism to zebras is a stretch. Unlike most herbivores, zebras will use their teeth as well as their sharp hooves to defend themselves, but really there is nothing particularly unpalatable about a zebra. Camouflage is a possibility since many carnivores are colorblind, but Caro notes that in many mammals it is not clear if the colors actually provide good camouflage or make the animal more conspicuous.
Caro discovers scores of reasons that do not explain zebra coloration, but very few that do. He discounts the camouflage idea for zebras since such patterns provide better camouflage in dense cover rather than the open plains where zebras live. He mentions another theory of zebra stripes, that they play a role in repelling horseflies, but is not enthusiastic. The above thermoregulation theory is dismissed as “bizarre.” Caro finally gives up and classifies the function of zebra stripes as unknown.
Caro is right, it does seem bizarre that stripes can substantially cool a zebra, but apparently it’s plausible. More plausible is that thermoregulation is one of many benefits of stripes. There would need to be a massive evolutionary advantage from the cooling benefits of stripes in order to offset any compromise to camouflage. It seems more likely that all the theories are true to some extent. Zebra stripes probably help conceal and cool the zebra, and possibly repel horseflies as well. The lesson is to keep an open mind. What seems like common sense is not always enough.
