We are closer than ever to answering why zebras have stripes, or are we? The most recent research.
The reason behind the unusual black-and-white patterns in zebras has been a matter of debate for many years. In my previous post, we looked at a few of the most recent research publications on striping, including three that support the prevailing hypothesis that its primary purpose is to avoid biting flies. But not all experts fully agree.
A Healthy Controversy
In January 2015, a UCLA team led by Brenda Larison and Thomas B. Smith did a study of geographical areas with a significant presence of zebras, comparing several characteristics of the environment with the intensity of the striping—an approach similar to Caro's 2014 paper. They found that the stripes' quantity and colour are increasingly correlated with the temperature and humidity of the places where plains zebras roam, more than any other factor. The unusual black-and-white pattern's primary function seemed to be related to temperature, not fly repelling.
These results received some media attention, in part because they were on cover of the Royal Society's online journal, Open Science. Larison and Smith highlighted that while other studies focused on a few possible explanations, they included and analyzed more than two dozen factors across the board. Of all the things that might influence stripes, the temperature of an area was the best way to predict the intensity of the striping: in warmer climates, you would find bold lines that cover the entire body; in colder regions, they tend to be lighter, narrower, and scarcer.
"While past studies have typically focused their search for single mechanisms, we illustrate in this study how the cause of this extraordinary phenomenon is actually likely much more complex than previously appreciated, with temperature playing an important role," wrote Smith, a professor of ecology and evolutionary biology at UCLA.
Larison and Smith's publication stirred some controversy in the community. Caro, who had been a firm proponent of the tabanid hypothesis, felt that the new study warranted some criticism. Writing in Open Science as well, Caro remarked that the study had a couple of design flaws that lessened its authority to advocate for one evolutionary pressure over another: the research had concentrated on just one species of zebras (plains zebras), and it couldn't speak of the original reason for the appearance of the unusual trait if it did not include other equines that didn't have stripes. Moreover, the study claimed that the temperature was the crucial factor, but it did not explain how.
In his commentary, Caro called attention to the common ground and how this new research complemented his own in critical areas. For one, findings in both studies disproved previously prevalent hypotheses. None found convincing evidence that the stripes were deterrent to mammal predators, especially lions. Both studies also cast doubt on those experiments that involved stripy objects moving in a computer screen as a proxy for predators' perception. Both also concurred that the stripes did not relate to camouflage in woodland or tree cover and neither found any correlation with tsetse flies' presence (glossinid). Caro also mentioned that his study did not find any correlation between stripes and social interaction (based on group sizes).
It seems the two most promising hypotheses are thermoregulation and deterrence of tabanids.
Larison and Smith couldn't wait to reply. Only a month after Caro's commentary, a response appeared on Open Science. The UCLA team refuted a few points made by Caro, especially the claim that both sets of data supported the tabanid hypothesis more than anything else. For them, Caro did not actually have a valid data set of the tabanid flies' distribution because no such data exists. Instead, what Caro did was to suppose that an area's temperature and humidity implied the presence of the tabanid flies. Caro's paper forced "the reader to take a leap of faith that temperature and humidity are the only factors affecting tabanid fly abundance." Based on both reports, Larison and Smith's call was to accept temperature's relation to striping and to pursue more hypotheses—including thermoregulation and flies—without ignoring that fact. Most of all, they acknowledged that the answer to this mystery was probably not going to be simple or detached from other factors.
The Recent State of Things
In the first half of 2016, Caro collaborated with other scientists and published a couple of papers on the subject and one book. The first paper (co-written with Amanda D. Melin from the University of Calgary) further discredited the camouflage theory by measuring the distance a large predator—a lion, for example—might need to see its prey under certain circumstances. The second article attempted to explain why stripes detracted horseflies based on the polarization of the light reflected off a zebra's coat. In his book, Caro included strong points against thermoregulation: first, the body surface temperature of zebras is not much different from similar mammals in those areas; second, animals in hotter environments tend to have lighter skins; and third, the average temperatures in African areas might not merit such mechanism.
In 2018, Akesson and Horváth—who had presented evidence favouring the tabanids hypothesis—were determined to put thermoregulation to the test. The team covered large metal barrels with skin imitations in different colours: black-and-white stripes, black, white, brown and grey. Then filled the barrels with water and left them out in the sun. They measured the water temperature after some time, finding that although the black barrels were warmer and the white cooler, the striped barrels didn't keep the water cooler than the gray ones.
The fly hypothesis seemed to be getting stronger and more popular, primarily thanks to Caro's prolific efforts. In 2019, Caro and How put a team together to conduct experiments where they analyzed flies' behaviour around zebras and horses in the same environment. Their new study showed stripes don't deter horseflies from a distance, but flies did fail to slow down when approaching zebras, thus preventing successful landings and often bumping into them.
However, in June 2019, a new study revived the thermoregulation theory by revealing a possible temperature control mechanism. Published in the Journal of Natural History, the paper argues that the combination of three elements allows the zebras to cool down more quickly: a particular method for sweating, the way they erect their hairs, and the small-scale convection created by the stripes. Researchers Alison and Stephen Cobb measured the temperature of stripes in living zebras and a dead zebra hide. They noticed a difference of 12 to 15 degrees Celsius between the black and the white stripes on a hot day. Perhaps more surprisingly, the dead animal skin was 16 degrees warmer at the same hour.
So, why such a difference in degrees? The answer: hair and sweat. The zebras were found to raise the hairs on their black parts more than the white ones. Combined with a newly-discovered protein called latherin—which speeds evaporation of sweat—and the temperature difference on contrasting stripes, this creates a "chaotic air movement," enhancing the dissipation of heat by evaporation. "The data and observations in this paper suggest that the primary function of the stripes may be thermoregulation and a secondary benefit, fly-deterrence."
These results may bring a renewed pressure on other theories, but nothing is resolved. Experiments towards the tabanid hypothesis continue. Last August, by dressing horses in different patterned coats, How, Caro, and their team found that the aperture or barbershop effect is not the mechanism behind the flies' confusion. Tabanids had difficulty landing on checked patterns as much as they did on striped ones.
Science and Nature at Work
This is all healthy science in the making. This is what researchers do: they examine the evidence, propose hypotheses, make claims, find faults in their peers' work, put new ideas to the test, and explore promising avenues. Advances in technology and clever ways to approach a problem lead us closer to the truth. But simple things are rare. It is often better to be skeptical the next time a headline blares that a science mystery has been "solved." Processes as complex as the evolution of one species deserve careful study, and we can't expect easy answers.
What direction will the zebra research take in the future? It's still hard to tell. It seems the two most promising hypotheses are thermoregulation and deterrence of tabanids. But perhaps the biggest challenge will be finding ways to test whether evolutionary pressures act alone or in tandem, and how to weigh their individual influences. Imagining that there is no "primary" reason but a confluence of several key driving forces for change might open new research avenues. That's right: it may not always be black and white.