Of Zebra Stripes and Other Questions: Science and Nature at Work (I)
We are closer than ever to answering why zebras have stripes, or are we?
There comes a time in everyone's life when big questions start to haunt us. Why am I here? Are we alone in the universe? What is the meaning of life? Why do zebras have stripes? Well, the good news is we are closer than ever to solving perhaps the most critical of those mysteries. But the process hasn't been easy nor uncontroversial. And for that very reason, it can teach us a lot about how science works.
If you are anything like me, the weirder the animal, the most fascinating it is. Zebras are kind of in an awkward spot: their donkey-like figure is quite familiar to us, but the stripes are genuinely unique. They are striking and beautiful animals, yet their black and white skin patterns leave us pondering. They have been the subject of speculation and jocose origin stories: are they referees of animal sports? Did they tan for too long in half their bodies behind leafy trees? Even cooler is the fact that each stripe pattern is unique to an individual, like a fingerprint.
From plants to big mammals, colour and external characteristics are usually a testimony of many species' evolutionary capabilities. Beautiful flowers, spotted ladybugs, orange felines, they all flaunt features that distinguish them in an ever-changing world. Zebra stripes have always caught our attention because of their high contrast and uniqueness in the African savannas and grasslands. Yet, we are still not sure why they have them.
And there must be a reason why: evolution has taught us that there must be a purpose or at least some logic behind their appearance. It's not just about looks; it's about functionality too.
Telling One Evolution Story
The question "why do zebras have stripes" could be more precisely formulated as "why did they get them in the first place." In a way, evolutionary ecologists work as historians: they tell the story of why something happened. In their work, they rely on a few proven theories, including what we know as mechanisms of evolution: natural selection, genetic drift, mutation, non-random mating, and gene flow. For simplicity, we can focus on natural selection for now.
Zebras evolved away from other Equines (a genus that includes horses and donkeys) over two million years ago. Around that time, random genetic changes and/or gene shuffling produced a line of horse-like animals that tended to have stripes instead of single-colour skins. Well, some of those striped individuals turned out to be better suited (pardon the pun!) for the areas where they lived or the environments they had to face. And thanks to this change, they were able to survive and pass on their genes with more success than others. Today there are three distinct species: the Grévy's zebra, the plains zebra, and the mountain zebra. The question then is, why did stripes made them more likely to live longer and have more babies?
Since the discovery and refinement of the theory of evolution, the why of the stripes has been well contested. Charles Darwin himself proposed one of many theories about it. The most popular explanations we have so far revolve around four general possible causes: to confuse predators or hide from them; to have a better social cohesion; for protection against disease-carrying insects; or as a mechanism to control their body's temperature. It is now a matter of finding out what was the key evolutionary pressure. In other words, why was the stripy species able to not get eaten as much, find more friends and mates, not get sick as often, or stay cooler in hotter areas.
But how do we find out which? Sometimes—and this is, of course, very simplified and reductionistic—, scientists dealing with historical changes are likely to approach a problem like this: First, they observe the traces of past events ("oh, look, those animals have weird stripes!"). Second, a theory is applied ("hmm, they must be like that because of natural selection"). Third, they postulate possible causes for it ("I bet the stripes are good for hiding from predators"). Forth, they look for evidence in favour or against the proposed reason by testing, experimenting, or observing ("Oh, but they seem to be the lions' favourite snack, so maybe not that").
It is now a matter of finding out what was the key evolutionary pressure.
The problem is finding the ultimate proof for a hypothesis is not straightforward, and conclusive evidence (or "smoking guns") is rarely that. Eureka moments in modern science are sadly not that dramatic. Most likely, there will be an accumulation of results and evidence that favours one hypothesis over the others, and the more smoking guns there are, the better for that hypothesis. But that usually leads to more questions that start with "why."
Research on zebra striping has a long history, but for now, we will look at roughly the last 10 years of research. The discoveries made during this time represent a significant leap, but not one without controversy. There is an ongoing debate about the function of the distinctive black-and-white coats, and although one of those hypotheses seems to be getting more traction, the discussion is not over, and we still have a lot to learn.
Recent Research on Stripes
Let's go back to February 2012. Hungarian and Swedish scholars Ádám Egri, Gábor Horváth, Susanne Åkesson, and their colleagues published their research in the Journal of Experimental Biology, stating that stripes were the least attractive pattern for tabanids, also known as horseflies. Tabanids are a big nuisance for zebras: they deliver nasty bites, carry deadly diseases, and distract them from feeding.
To make this claim, the team did a few experiments in a tabanid-infested horse farm near Budapest. One of those tests included horse-shaped figures covered in white, brown, black, or striped plastic. These were coated in a transparent and odourless glue to catch the landed flies, randomly varying the figures' positions for consistency. The results: the striped pretend horse attracted way fewer flies (n=8) than the black (N=562) and brown (N=334), and even the white ones (N=22).
A year later, Martin How (University of Queensland) and Johannes Zanker (Royal Holloway University of London) proposed that those high contrast stripes, when in motion, create a visual effect that confuses or dazzles observers—most likely flying insects and possibly predators during the hunt. This time, however, there wasn't an experiment per se but a simulation. The team created an algorithm that analyzed different parts of the bodies of zebras in motion. The conclusion was that stripes produced highly misleading visual information. The visual effect is similar to when we see a wheel turning in the opposite direction of its actual rotation, or how we see a barbershop pole stripes go up or down when they are actually just rotating (dazzle effect).
A few months later, in April 2014, a study was published in the journal Nature Communications that supported the horsefly deterrence theory. This time, it was not based on experiments or algorithms but on an observational multifactor model. A research team led by Tim Caro of the University of California, Davis, approached the issue systematically in an extensive geographical area. Caro is an authority on animal coloration and zebras stripes in particular and has published numerous papers and books on the subject.
"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," wrote Caro.
In their geographical model, the scientists found a relation with the nuisance of flies. They compared, among other things, the location of breeding grounds of flies to each of the areas where the three species of zebras and other members of the Equus genus graze (including the African wild ass, which only has stripes on its legs). The ecologists measured the animals' stripe patterns in those areas to find a correlation. The association turned out to be very encouraging: "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," wrote Caro.
The vertical striping of zebras seemed to deter flies, but the scientists studying the subject were unsure why. Caro's team highlighted the correlation of the abundance of files in some areas with the characteristic patterns in different zebra species to propose that this was the critical evolutionary pressure. The why, however, needed to be tested further to give this conclusion more weight. In any case, at this point, this study—taken together with the previous results—was making a solid case for the fly deterrence hypothesis.
But strong evidence in favour of one competing hypothesis was on the way. In the second part of this post, we will look at some new research and how scientists debated conflicting results.