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| Why do zebras have stripes? Scientists have the answer | Why do zebras have stripes? Scientists have the answer |
| (about 5 hours later) | |
| The zebra’s striped coat is simultaneously | The zebra’s striped coat is simultaneously |
| extraordinary and stunning. So wondrous, in fact, that many people have imagined | extraordinary and stunning. So wondrous, in fact, that many people have imagined |
| it to be evidence of God’s infinitely artistic hand. Over the years, there have been many more rational explanations, but that all-important scientific consensus | |
| has remained elusive. | has remained elusive. |
| Charles Darwin certainly found the zebra’s | Charles Darwin certainly found the zebra’s |
| stripes to be a conundrum. In The | stripes to be a conundrum. In The |
| Descent of Man, he dismissed the idea they could act as camouflage, citing William Burchell’s | Descent of Man, he dismissed the idea they could act as camouflage, citing William Burchell’s |
| observations of a herd: | observations of a herd: |
| Their sleek ribs glistened in the sun, and | Their sleek ribs glistened in the sun, and |
| the brightness and regularity of their striped coats presented a picture of | the brightness and regularity of their striped coats presented a picture of |
| extraordinary beauty, in which probably they are not surpassed by any other | extraordinary beauty, in which probably they are not surpassed by any other |
| quadruped. | quadruped. |
| Although both males and female zebras are | Although both males and female zebras are |
| similarly striped, Darwin hedged that “he who attributes the white and dark | similarly striped, Darwin hedged that “he who attributes the white and dark |
| vertical stripes on the flanks of various antelopes to sexual selection, will | vertical stripes on the flanks of various antelopes to sexual selection, will |
| probably extend the same view to the … beautiful zebra.” In other words, the | |
| stripes help males and females make sensible choices about whom they mate with. | |
| Alfred Russel Wallace begged to differ. “It | Alfred Russel Wallace begged to differ. “It |
| is in the evening, or on moonlight nights, when they go to drink, that they are | is in the evening, or on moonlight nights, when they go to drink, that they are |
| chiefly exposed to attack,” he wrote in Darwinism. | chiefly exposed to attack,” he wrote in Darwinism. |
| “In twilight they are not at all conspicuous, the stripes of white and black so | “In twilight they are not at all conspicuous, the stripes of white and black so |
| merging together into a grey tint it is difficult to see them at a little | merging together into a grey tint it is difficult to see them at a little |
| distance.” | distance.” |
| There are other possibilities too. Perhaps | There are other possibilities too. Perhaps |
| the stripes act as some kind of zoological barcode, allowing one individual to | |
| recognise another. It has been suggested they could somehow help with | |
| thermoregulation. Or perhaps they are there to deter parasitic flies. | thermoregulation. Or perhaps they are there to deter parasitic flies. |
| Tim Caro of the University of California, | Tim Caro of the University of California, |
| Davis, has puzzled over contrasting colouration in mammals before. | |
| Now, in a new study published in Nature | Now, in a new study published in Nature |
| Communications this week, he and his colleagues have focused their attention on the | Communications this week, he and his colleagues have focused their attention on the |
| zebra. | zebra. |
| They take a completely original approach, stepping back from one species of zebra and attempting to account for the differences in patterning across different | They take a completely original approach, stepping back from one species of zebra and attempting to account for the differences in patterning across different |
| species and subspecies of zebras, horses and asses. Is there anything about the | species and subspecies of zebras, horses and asses. Is there anything about the |
| habitat or ecology of these different equids that hints at the function of | |
| stripes? | stripes? |
| “I was amazed by our results,” says | “I was amazed by our results,” says |
| Caro. “Again and again, there was greater striping on areas of the body in | Caro. “Again and again, there was greater striping on areas of the body in |
| those parts of the world where there was more annoyance from biting flies.” | those parts of the world where there was more annoyance from biting flies.” |
| Where there are tsetse flies, for instance, the equids tend to come in stripes. Where there aren't, they don't. | Where there are tsetse flies, for instance, the equids tend to come in stripes. Where there aren't, they don't. |
| The idea that flies don’t like | The idea that flies don’t like |
| stripes dates back at least to 1930. Since then, there have been several studies that | stripes dates back at least to 1930. Since then, there have been several studies that |
| have provided experimental support, with flies preferring to alight on all-black or all-white surfaces rather than on stripes. The authors also stress the burden of | have provided experimental support, with flies preferring to alight on all-black or all-white surfaces rather than on stripes. The authors also stress the burden of |
| blood sucking insects: both tsetse flies and horseflies are the vectors for | blood sucking insects: both tsetse flies and horseflies are the vectors for |
| significant and often-fatal diseases in horses; they are probably also capable | significant and often-fatal diseases in horses; they are probably also capable |
| of draining a significant amount of blood (several hundred millilitres in a day, apparently). | of draining a significant amount of blood (several hundred millilitres in a day, apparently). |
| The next step will be to come up with | The next step will be to come up with |
| experiments that explore the vision of biting insects such as tsetse flies and | |
| horseflies and a convincing explanation for why they seem to have such an aversion to stripes. If these are forthcoming, a scientific consensus may yet emerge, saving zoologists of the future from a whole lot of zebra-based head-scratching. | horseflies and a convincing explanation for why they seem to have such an aversion to stripes. If these are forthcoming, a scientific consensus may yet emerge, saving zoologists of the future from a whole lot of zebra-based head-scratching. |
| Caro T et al (2014) The function of | |
| zebra stripes. Nature Communications. Published 1 April 2014. doi:10.1038/ncomms4535 | zebra stripes. Nature Communications. Published 1 April 2014. doi:10.1038/ncomms4535 |