How did the Zebra get its stripes?

For centuries we have asked questions of evolution. Questions such as why does a Leopard have spots, how did a Camel get it’s hump or perhaps most famous of all: why is a Giraffe’s neck so long? A lot of the time people create “just-so” stories to answer these questions. For example, “it was favourable in terms of evolution by natural selection to be a Giraffe with a longer neck to reach higher branches”. When this theory was tested they found that Giraffes actually spend most their time foraging in low branches. Scientists now believe the long necks are used by males to fight each other for access to females to mate with. (The fact they can also reach higher into fruit bearing trees is an added bonus!) This is called necking. There are plenty of videos on YouTube if you are interested! Most just so stories have now been tested and more scientifically minded and testable hypotheses have been given to describe them. However there is one story that has been puzzling scientists until now: why does a Zebra have its stripes? debate has been going on for around 120 years and provides the first real clues as to why Zebras have such bold patterning. Over this period of time many proposals have been made, including confusing predators, camouflage amongst the grasslands and individual identification amongst a herd. None of these proposals had been tested and there were obvious teething problems to begin with, such as a black and white camouflage for a green and yellow (sandy) background.

However, research indicated that some disease transmitting pests have a preference for black squares rather than black stripes, white squares or white stripes. A team of researchers from the University of Sweden found that horseflies preferred dark coloured horses to lighter coloured, and attributed this to the sensory systems used to detect these animals. Using some plastic horses, vegetable oil and various black and white patterns these Swedish scientists seem to have cracked the story! The team found that the stripes of a Zebra are very unattractive to the disease spreading pests. The reason seems to be because the black and white stripes give off a non-uniform light pattern, compared to the uniform light given off my black squares.

While there are many other factors that may be responsible for repelling these pesky insects, this theory certainly seems plausible. If the selection pressure of disease carrying insects is enough to drive such strong patterning is yet to be discussed. To me it does seem a little far-fetched but the experimental evidence is there to suggest this is correct. If the Zebra’s black and white stripes are a result of the insects this study suggests then it wouldn’t be the most wild and unbelievable feature natural selection has thrown up!

Further reading:
Just so stories by Kipling – (Fun book on Amazon for a penny! –
More info on the Giraffe just so story –
The original paper – Egri, Á., Blahó, M., Kriska, G., Farkas, R., Gyurkovszky, M., Åkesson, S. and Horváth, G. (2012). Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes. J. Exp. Biol. 215, 736–745. (Limited access).