Dazzling stripes are a way of deterring flies. Photo: Getty
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How the zebra got its stripes

A method for dodging predators? A means of social interaction? Or a way of getting rid of flies?

Zebras' stripes have baffled biologists since Charles Darwin. Many hypotheses have been proposed regarding their purpose but, despite hundreds of years of study, there remains disagreement.

In an attempt to end the debate, researchers have pitted various models against each other and systematically analysed data from past studies. Their results reveal the one reason zebras have stripes: to ward off flies.

A handful of ideas regarding zebras’ stripes have found some support among biologists. One proposed that the dark and light bands change how air flows around a zebra’s body and helps in heat management, which could go a long way in the hot tropical areas that zebras live in.

Another proposed the stripes were used by zebras as a way of social interaction. They may use them to identify other zebras and for bonding as a group in the wild.

A third proposal suggested zebras used the stripes as camouflage. While stripes are clearly visible in the day, there some thought that it helped at dawn, dusk, and in the night.

All these ideas were shot down when tested rigorously. Two others, however, remained intriguing.

The first was that the stripes were used to dodge predators. It is called the “motion dazzle hypothesis”, and it suggests predators are confused by zebras' stripes and cannot understand their movement. Research published in the journal Zoology in 2013 used a simulated visual system to show that zebra stripes do interfere with visual perception. But this is a difficult hypothesis to test in the field.

Martin Stevens at the University of Exeter has researched the motion dazzle hypothesis by getting human subjects to catch moving stripy objects on a computer. “It’s an artificial experimental system,” he admitted.

The second proposal was that stripes helped keep flies at bay. Zebras are especially susceptible to biting flies due to their geographic spread. These flies, which include the tsetse fly, stomoxys stable flies, and tabanid biting flies, are particularly prevalent in areas with high temperature and humidity – exactly the areas where zebras are normally found.

Bites from these flies can be nasty and, quite literally, draining. About thirty flies feeding for six hours on just one horse can draw as much as 100mL of blood. Usually the flies can number in the hundreds around one animal.

Zebras have shorter hair than other equids – the family that includes horses, donkeys and zebras – which may also increase their susceptibility to attack. Also, four diseases which are fatal to equids have been found in Africa. This could mean that investing in anti-biting defenses such as stripes is especially important for zebras compared to non-African equids.

It is possible that the dazzle effect acts on flies, rather than larger predators, and deter them from biting. “Stripes clearly have a number of functions,” Stevens said, “and these could be interacting in zebras.”

Revealing maps

In the new research, just published in Nature Communications, Tim Caro and his colleagues at the University of California in Davis, didn’t perform experiments. Instead they used ecological and observational data on zebras' geographical locations and related factors. It is the first time that a comparative approach has been applied to find the reasons for zebras' characteristic colouration. Caro thinks his findings may have nailed the answer at last.

Caro looked at seven species of equids and scored them for number and intensity of stripes. Just to be sure, they tested all five hypotheses regarding zebra stripes' use: camouflage, predator avoidance, heat management, social interaction, and warding off flies. The extent of overlap between the geographic distribution of striped equids with each of these five measures was calculated.

E. greyvi, E. burchelli and E. zebra have stripes on all their bodies. Other equids don’t. Caro, Izzo, Reiner, Walker and Stankowich

“The results were a shock to me,” said Caro. Of these five proposals, only warding off flies had statistical support. He had not expected such a clear-cut answer to the question. As the map shows, the only places where flies and equids live together are areas that are populated by striped equids.

The exact mechanism by which stripes deter flies remains unknown, but experimental studies performed by researchers at Lund University in 2012 have found support for this proposal. They created striped surfaces and stuck glue on them. Based on the number of flies on the surfaces with different thicknesses of stripes, they concluded that these flies stayed away from stripes as thin as those found on zebras.

“As is normal in science you get a solution that asks more questions,” Caro said. It is time to hand the problem over to vector biologists, who can understand the susceptibility of horses to biting flies.

In Darwin’s days, people didn’t consider animal colouration with respect to fitness advantages. “People thought that animal colouration existed simply to please humans or was caused directly by the environment,” Caro said.

Darwin “would be delighted” that researchers are now considering animal colouration as a functional trait, he said. We might not have all the answers regarding zebra stripes – but it seems we may be looking through the right lens.

The ConversationThis article was originally published on The Conversation. Read the original article.

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New Times: David Runciman on networks and the digital revolution

The digital revolution has had two profound effects on how power is distributed – both of which squeeze the state's power.

Left-wing political parties exist to use the power of the state to rectify unjust distributions of power in society. What has gone wrong with this project? First, the political parties bit. Established parties everywhere are struggling to seem relevant to most people’s everyday concerns: they look increasingly like the tired relics of a more hierarchical age. The exception, of course, is the current Labour Party, which has opened itself up to become the biggest mass-membership party in Europe. But the trade-off has been to move away from seeing the acquisition of power as its primary purpose. These days parties can only really draw people in by offering to be vehicles for the expression of political resentment and disenchantment. But that is no way to rectify the causes of their resentment; neglecting the challenge of power usually ends up making things worse.

However, this is just a symptom of the wider problem, which is the changing nature of power. Technology lies at the heart of it. The digital revolution has had two profound effects on how power is distributed. First, it has empowered individuals, by providing them with unprecedented access to information, tools of communication and the means of expression. This is power exercised as choice: we all now have multiple ways of registering our likes and dislikes that never existed before.

Second, the digital revolution has empowered networks, creating vast new webs that span the globe. Some of them, such as Facebook, are close to being monopolies. We end up joining the networks that other people have joined, because that’s where the action is. This gives a small number of networks an awful lot of power.

Both of these developments are deeply problematic for the power of the state. The proliferation of choice makes citizens much harder to satisfy. Many of us have got used to micromanaging our lives in ways that leaves government looking flat-footed and unresponsive, no matter how hard it tries. At the same time, states face global networks that they have no idea how to control. International finance is one of these: money is information and information now has too many different ways to flow. States are getting squeezed.

The paradox is that the same forces that are squeezing the state are also giving impetus to left-wing politics. There are huge imbalances of power being created in networked societies. The monopolists are hoovering up money and influence. Personal connections count for more than ever, now that networked connections have become ubiquitous. Education is turning into a way of pulling up the drawbridge rather than moving up the ladder. One temptation for the left is to assume that the evidence of injustice will sooner or later outweigh the disabling effects of these social forces on the state. That is part of the Corbyn gamble: hang around until people are sufficiently pissed off to start demanding social-democratic solutions to their problems.

I don’t think this is going to happen. There is nothing to suggest that popular dissatisfaction will find its way back to the state as its best outlet. It will be channelled through the networks that are making the life of the state increasingly difficult.

The other temptation is to think that the left can achieve its goals by bypassing conventional social democracy and channelling its own ambitions into network politics. This is the other side of the Corbyn gamble, or at least the view of some of the people who have attached themselves to him: a new politics is coming that uses digital technology to mobilise fleet-footed networks of activists who can generate change without going through the cumbersome and time-consuming process of winning general elections. That also looks pretty wishful to me. These networks are just another vehicle for expressing personal preferences. They don’t have any means of changing the preferences of people who think differently. You need to win power to do that.

The state’s power is being squeezed by networks of empowered individuals, but these networks don’t have the kind of power necessary to do the redistributive work of the state. What is the left to do? It needs to try to find value in the fact that the state is not just another network. The right does this instinctively, by talking up the state’s security functions and championing ideas of sovereignty and national identity. But that does nothing to address the deleterious effects of living in a modern networked society, where we are swamped by personal choice but impotent in the face of corporate and financial power.

Rather than trying to harness the power of networks, the left should stand up for people against the dehumanising power of Big Data. The state isn’t Google and should not try to pretend to be. We don’t need more choice. We don’t need more efficiency of the kind that digital technology is endlessly supplying. We need protection from the mindless bureaucratic demands of the new machine age: the relentless pursuit of information, regardless of the human cost. There are limits to what the state can do but it retains some real power. It still employs real human beings; it educates them and provides them with welfare. It should do what is in its power to make the work tolerable and the education meaningful, to provide welfare in ways that don’t leave people at the mercy of faceless systems. The left needs to humanise the state.

At the moment, too much energy is being spent trying to humanise the party. We are told that people are tired of robotic, careerist politicians; they want unspun versions of people like themselves. But robotic politicians aren’t the problem; the coming age of robots is. While the party tries to feel more comfortable with itself, the effects of a networked society are running rampant. Acquiring the power of the state is still the best way to fight back. It doesn’t matter if that has to be done in an ugly, mechanised, artificial way, by careerist politicians with whom we wouldn’t choose to spend our personal time. Better an ugly, artificial politics than an ugly, artificial world. 

David Runciman is a professor of politics and the head of the department of politics and international studies at Cambridge

This article is part of a New Times collection of the future of the left. Read the other pieces here.


This article first appeared in the 22 September 2016 issue of the New Statesman, The New Times