Akshat Rathi

The only reason zebras have stripes is to ward off flies

April 2, 2014April 18, 2014 Akshat Rathi1 Comment

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.

Now, how do I get rid of these ants?
dkeats, CC BY

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.

Written with Angela White. This article was originally published on The Conversation. Header picture credit: eoghann, CC-BY-NC.

The greatest mass extinction may have been the doing of microbes

April 1, 2014April 5, 2014 Akshat RathiLeave a comment

The worst time to be alive in Earth’s history is unarguably the end-Permian, about 250 million years ago. It is the period when the greatest-ever extinction event recorded took place, killing 97% of all species, an event so severe it has been called The Great Dying.

This event has generally been blamed on massive volcanic eruptions that took place at the same time. But now, in a new analysis, researchers at the Massachusetts Institute of Technology (MIT) argue that the mass extinction event may have been instigated by microbes. These microbes led to a perturbation of the carbon cycle that caused environmental shocks, such as global warming and ocean acidification. The shocks wiped out species in great numbers over a period of tens of thousands of years – a blip on geological scales.

Felt like the end of time

The end-Permian extinction, which took place about 250 million years ago, is the most severe of five known mass extinction events. It killed off the last of the trilobites – a hardy marine species that had survived two previous mass extinction. While land plants survived, almost all forests disappeared. Worse of all, it is the only known extinction event where even insects weren’t spared.

For an event of this size to take place, a lot of things would have had to go wrong. At the time the world was made up of a single supercontinent called Pangea. This large landmass, by altering the dynamics of how carbon is cycled with subducting plates, may have pushed global temperatures to the highest they had ever been.

Then, over the course of about a million years, huge eruptions in Siberia created basalts that cover an area that was about seven times the size of France. This may have pushed the environment past a tipping point by sending even more carbon dioxide into the atmosphere. That would have caused the oceans to acidify, killing more marine life, and heat up, releasing frozen methane. The upshot of all this would have been a “runaway” climate that kept heating up and removing more oxygen from the environment.

The mighty microbe

But Daniel Rothman of MIT thinks that the numbers don’t add up. “The changes in the carbon cycle globally are difficult to reconcile with only volcanic activity in Siberia,” he said.

His calculations, just published in the Proceedings of the National Academy of Sciences, were hinting that something else must have caused the runaway event. One hypothesis was that microbial life may have been responsible for that.

“This hypothesis is not as outrageous as it seems. After all, about 2.4 billion years ago, it was microbes in the form of cyanobacteria that gave our atmosphere all of its oxygen,” Rothman added. That period, called the Great Oxygenation Event, also killed most organisms that were adapted to the lack of oxygen and began one of the longest cold periods in Earth’s history. So microbes can certainly have global impact.

With colleagues at MIT, Rothman looked at the evolutionary history of Earth and spotted the rise of a particular type of microbe that occurred around the time of the Great Dying. That microbe, called Methanosarcina, had the ability to digest organic matter to produce methane. (Molecular biologists at MIT have shown that Methanosarcina evolved this ability thanks to the transfer of a single gene from the Clostridia class of bacteria.)

Rothman knew that the chemical process involved in creating the methane relied on the metal nickel. He went looking for evidence that Methanosarcina was thriving at the time in the sedimentary layer of the Meishan region of China. If the environment at that time had any more nickel than normal, then the sediments would hold the record of it.

Rothman chose the Meishan region to look for nickel because it is a particularly well-studied region. Its sedimentary layers have been used to mark and standardise different periods of Earth’s geological history, and they span the period of the Great Dying.

The search was successful. There was indeed a higher amount of nickel in the sediments deposited during that period. Methanosarcina would not have just been effective at creating methane – they would have flourished.

The nickel, Rothman suggests, would have been added to the oceans, where Methanosarcina lived and grew, by the continuous volcanic activity occurring in Siberia. The growing amount of nickel, transported by ocean currents, would have allowed more Methanosarcina to convert organic matter into methane, which would be converted to carbon dioxide through reactions with oxygen. This would have meant increased global temperatures and acidification of the oceans. The latter would have combined with the loss of oxygen (used up in creating the carbon dioxide) to accelerate the extinction in the oceans. And the dead organisms would have provided Methanosarcina with more organic matter to digest.

In short, a microbial innovation may have tipped over the balance to cause the Great Dying.

Marc Reichow at the University of Leicester remains sceptical of these results. He argues that there is no evidence that the increased nickel came from Siberian volcanoes. Rothman agrees that current data cannot identify the source of the nickel.

“This is an interesting hypothesis, but I think that Great Dying was the doing of many ‘kill mechanisms’ rather than just a single mechanism suggested here,” Reichow said.

There is also doubt over the exact period in which Methanosarcina actually evolved. Current techniques for estimating its origins based on DNA sequence differences have a huge error margin, which means it could have been well before or after the Great Dying.

Rothman concedes that there are limitations. “We believe that volcanism alone could not have caused this extinction event. Instead, what we have done is broadened the conversation by suggesting that it is possible that microbes may have caused it to happen.”

“The implications for today are that there many ways in which natural fluctuations can happen in Earth’s carbon cycle. When studying the changes happening to the carbon cycle now, we should try to take into consideration as many of those as possible to make future predictions.”

This article was originally published on The Conversation. Image credit tjt195 (CC-BY-NC).

New dwarf planet found sneaking through the inner Oort cloud

March 27, 2014April 18, 2014 Akshat Rathi1 Comment

A new, planet-like body has been found on the outer edges of the solar system. This object, called 2012VP₁₁₃, is the second body of its class found since the identification of the dwarf planet Sedna in 2003. It joins an exclusive club composed of some of the strangest objects in the solar system.

The observable solar system can be divided into three regions: the rocky planets including the Earth and asteroids of the inner solar system, the gas giant planets, and the icy Kuiper Belt objects, which include Pluto. The Kuiper belt stretches from beyond Neptune, which is at 30 astronomical units (one astronomical unit, AU, represents the distance between the Earth and the sun), to about 50AU.

Sedna and 2012VP₁₁₃ are strange objects, because they reside in a region where there should be nothing, according to our theories of the solar system formation. Their orbit is well beyond that of Neptune, the last recognised planet of the solar system, and even beyond that of Pluto, which differs from planets because of its size, unusual orbit, and composition. (Pluto, once considered a planet, is now considered the lead object of a group of bodies called plutinos.)

The closest Sedna, which is 1000km-wide, gets to the sun is about 76AU and for 2012VP₁₁₃, which is 450km-wide, that distance is 80AU. Their orbits are also at weird inclinations compared to most other solar system objects.

The results of the discovery have been published in Nature. Chadwick Trujillo of Gemini Observatory in Hawaii, who was also involved in finding Sedna, and Scott Shepherd of the Carnegie Institution for Science, who found 2012VP₁₁₃ with Trujillo, propose that these objects are members of the inner Oort cloud.

The Oort cloud is a hypothetical region that is thought to stretch outwards beyond the Kuiper belt. Beyond 5000AU, the Oort cloud expands out into a sphere centred on the sun. We have no direct evidence that the Oort cloud exists, but indirect evidence comes in the form of comets with extremely elongated orbits.

Stephen Lowry at the University of Kent said: “The orbital properties of these two objects are so very different from that of the Kuiper belt objects that it wouldn’t be wrong to suggest they may be part of the inner Oort cloud.”

The fact that these objects exist is remarkable, since they exist in a region where material is thought to have been too sparse for them to form. Current thinking is that they actually formed in the giant-planet region, and that their orbits may carry the signature of whatever events caused them to scatter to such distances. It is hoped that this discovery will lead efforts to find other objects.

David Rothery of Open University said: “This is a remarkable discovery, but it is not entirely surprising. When they found Sedna, there was hope that they would find others in that region.”

But the fact that it took Trujillo, who was involved in the original team that found Sedna, more than ten years to find Sedna’s neighbour speaks to the challenge of discovery. “The farther you get from the sun, the less sunlight falls on these objects, which makes the task of locating them harder,” Lowry said.

“Worse still,” Lowry continued, “the eccentric orbits of these objects means that there is very tiny window in which they can be observed from even the most powerful telescopes on Earth. What is needed to find these objects is not just technology but persistence.” For example, Sedna gets as close as 76AU away from the sun, but at its farthest it is nearly 1000AU. Its orbital period is about 11,400 years, which means it spends lots of time too far out to be detected.

While 2012VP₁₁₃ and Sedna provide some information about the inner Oort cloud, to say any more, scientists are going to need more than two data points. Next generation instruments such as the Subaru telescope in Hawaii and Large Synoptic Survey Telescope in Chile may hold the answers.

First published on The Conversation. Images by NASA.