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by Tim Caro
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ACCORDING to Rudyard Kipling’s Just So Stories, the zebra got its stripes by standing half in the shade and half out, “with the slippery-slidy shadows of the trees” falling on its body. In Zebra Stripes, Tim Caro, a professor of wildlife biology at the University of California, Davis, sets out to test all the hypotheses explaining this most mysterious yet obvious phenomenon.
A bit of thought reveals the scale of the challenge: if having stripes is such a good thing, why do no other animals living on the African savannah, or non-zebra members of the horse family, Equidae, have stripes? Something, or some things, very specific to zebra ecology must have driven the evolution of this apparent adaptation, and will explain its absence in closely related species, and in species that share its niche.
The challenge isn’t simply to come up with an explanation that “makes sense”(science would remain at Just So level were that the case) but to create predictions that flow from that explanation, and then to test them experimentally.
Caro lists dozens of theories, most of which boil down to five common factors: camouflage (protection from predators); warning coloration (zebras can bite); communication (social behaviours); temperature regulation (stripes may help resist the heat); and ectoparasites (biting flies might not like stripes).
Each of these and their many variants is explored in turn, as Caro describes how he tested them, and how virtually all – including the generally favoured camouflage theory – fail. In the end, the data converge on a single explanation: the role of biting flies (this is not a spoiler – the media pounced on it in 2014 after Caro’s paper was published in Nature Communications).
As it turns out, the presence of stripes in zebras entirely overlaps with the distribution of certain species of biting fly. For reasons that are still unclear, flies dislike landing on striped surfaces. It is possible that the wiring of the fly eye means they cannot see stripes of some widths. Zebras may need to deter flies because their hair is shorter and thinner than that of other savannah animals so they are more susceptible to bites.
Zebra Stripes isn’t a piece of popular science writing, but an entrancing monograph that revels even in the minor details – from descriptions of how Caro used Photoshop to measure the stripes, through the number of paces he took in the bush wearing a zebra pelt to record how many insects settled on him, right down to the nitty-gritty of the statistical tests he used to make sense of his data.
This isn’t in the slightest bit boring or hard to understand, as Caro’s logical approach guides the reader. At every step, he pulls aside the curtain, revealing to the public exactly how science is done.
Caro’s dogged exploration of every possible explanation, his determination to extract testable predictions and then compare those with reality, are all reminiscent of Charles Darwin in On the Origin of Species. Like Origin, Caro’s book is also one long argument – not with the reader, but with the data, as he tussles with the annoying facts. In the end, the reader is compelled by the weight of evidence, which converges strongly on the flies.
In the right hands, this book could change lives. Had I read something like this at 18, I would have tried to join Caro’s research group, or at least have a career emulating his work. It’s too late for me, but I predict that this marvellous book will encourage a new generation to get into the field and tackle evolutionary biology’s remaining enigmas, with or without the help of Kipling.
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