What's Black and White and Read All Over?

Well it’s not a zebra being bitten by flies; it’s the newspapers that are reporting about zebras. I’m sure many of you have seen the story by now that scientists have finally answered the question about why zebras have stripes. Just check out some of the headlines:

  • Scientists Solve Mystery of Why Zebras Have Stripes (E Canada Now)
  • Why Do Zebras Have Stripes? Mystery Solved, Scientists Say (NBCNews.com)
  • Why do zebras have stripes? Scientists have the answer (The Guardian)
  • Scientists solve the riddle of zebras' stripes (UC Davis)
 Zebras in Botswana displaying what used to be one of nature's greatest mysteries. Now if only we could figure out how they get their mohawks to stay like that... Image Credit: Paul Maritz, Wikimedia commons.

Zebras in Botswana displaying what used to be one of nature's greatest mysteries. Now if only we could figure out how they get their mohawks to stay like that... Image Credit: Paul Maritz, Wikimedia commons.

In case you haven’t read the story, I’ll give you the summary. A team of researchers from the University of California, Davis set out to answer the question of why zebras have stripes. Scientists have come up with a lot of different hypotheses over the years, but no one can agree on which one is right. Here are the five that the UC Davis team evaluated.

  1. The vertical stripes help to provide camouflage in tall grass.
  2. The stripes confuse predators with motion dazzle (when a group of zebras are moving together, it will be difficult for a predator to target one individual.
  3. They serve as a visual cue or identifier (like a barcode) that helps with sexual selection.
  4. The black and white help the animals to regulate heat.
  5. The stripes deter biting flies.

All of these hypotheses have been individually tested over the years, and scientists have been debating them since the time of Charles Darwin and Alfred Russel Wallace. What makes the new study different is that they tested every hypothesis at once. They looked at the ranges of zebra species (and striped equids) and their subspecies, and how those ranges overlap with habitat types, predators, climate, biting fly ranges, etc. They found that the different patterns of the subspecies correlated the most with the fly distributions. The article is here (though sadly behind a paywall), if you want to know more.

I’m not actually here to talk about zebras, though. I want to give you some insight into science and science reporting. Take another look at those headlines. Now, compare them to this excerpt from the publication’s abstract:

For subspecies, there are significant associations between our proxy for tabanid biting fly annoyance and most striping measures … and between belly stripe number and tsetse fly distribution, several of which are replicated at the species level. Conversely, there is no consistent support for camouflage, predator avoidance, heat management or social interaction hypotheses.

The researchers never mention anything about solving the mystery, or proving any hypotheses. They say that they found “significant associations” for one and “no consistent support” for the others.

Why can't they be sure?

To explain why they avoided any bold statements, I'll need to back up and give you a short lesson about the scientific method. Science is all about asking questions, thinking of possible answers, and testing those answers. We call those potential answers hypotheses, and we test them through experiments.

 P.T. Barnum's infamous Fiji Mermaid. Perhaps the closest anyone has ever come to the real thing.  Image credit: Wikimedia Commons

P.T. Barnum's infamous Fiji Mermaid. Perhaps the closest anyone has ever come to the real thing. Image credit: Wikimedia Commons

There’s something important to remember about a hypothesis: it can never be proven true, but it can be proven false. In more scientific terms, our observations (experimental results) will either support or refute a hypothesis. Here’s an exaggerated example: mermaids do not exist. Now, in order to definitively prove that statement we would have to observe every point in the ocean simultaneously. We can’t do that, so I cannot be absolutely certain that mermaids are fictional. At the same time, I’m pretty confident that they don't exist because no one has ever captured or documented one. In other words, every scientific exploration of the ocean to date has supported my hypothesis that mermaids do not exist. There are, of course, a multitude of other reasons for not believing in mermaids; it’s just a fun thought experiment.

Back to zebras and their stripes. If we test the five hypotheses with experiments, we’ll get results that either support or refute them. It turns out that every one of them has been supported by experiments in some way or another. Well, now what? What’s a scientist to do with conflicting hypotheses? Sure, we can debate and argue, but that doesn't always resolve the issue. Darwin and Wallace started the zebra debate in the 1800s, and it has gone on ever since. What Tim Caro and his colleagues did was test all the hypotheses at once. They found the strongest support for number five; zebras have stripes because it deters biting flies.

So that settles it, right? We have our final answer. Sorry folks, it's just not that easy. Despite the newspaper headlines, the issue is not completely resolved. There are more questions to be answered (particularly how the stripes work), more experiments to run, and maybe even other hypotheses that haven't been considered yet. That’s why scientists use phrases like “significant associations”, or “results indicate that…” It’s rare to read a scientific paper that declares “without a doubt” that their hypothesis is correct. It’s also not uncommon to read suggestions or ideas from the researchers about future research.

I’m not saying that the reporters did anything wrong. Headlines are meant to grab a reader’s attention (they certainly piqued my curiosity), but there is a valuable lesson here. When you read a bold headline, take a look at the article (or the research paper behind it, if you can). You’ll probably notice that the language used to describe the results is not as certain. It may seem trivial for a story about zebra stripes, but when we’re talking about vaccination, avian flu, radioactive waste, or any number of other serious topics, the phrasing of the results, the news report, and the headline become very important. Just a simple change in the words can turn "Scientists find evidence of a possible correlation between cell phone use and brain tumors" into "Scientists link cell phones to brain cancer."

I'll leave you with my favorite headline about the story. Not surprisingly, it comes from National Geographic. 

Why Do Zebras Have Stripes? New Study Offers Strong Evidence

Something to keep in mind when you read any headlines about new scientific findings.