UC Davis researchers explain why zebras have stripes.
The pioneers of the theory of natural selection, Charles Darwin and Alfred Russel Wallace, began the discussion of black and white coloration in mammals over a century ago. The debate has lasted over 120 years and has made little progress in finding a systematic approach to provide reasons for this color pattern.
For the last decade, UC Davis wildlife, fish and conservation biology professor Tim Caro and his team have had little luck researching in the fields of Katavi, Tanzania as to why zebras have stripes — until now.
“The idea of explaining why animals and plants have coloration is something that we all take for granted,” Caro said. “In this lab we try to do large-scale phylogenetic analyses, where we look at the coloration of [an animal] and try to relate those external coloration patterns to the kind of environment that they live in, or the prey that they hunt. There’s something else going on with zebras that is not as easily explicable [as other questions of mammal colorization].”
Researchers’ understanding of animal coloration is rapidly developing, although only having been introduced in the field of evolutionary ecology in last 15 years.
Beginning about two and a half years ago, Caro mapped the pattern of striping across seven equine species and 23 subspecies. Scientists projected five hypotheses which were involved in the debate, including the concepts that the stripes are a form of camouflage against predators, a form of visual confusion against predators, a method of heat management, a form of social complexity and a method to fend off biting flies.
“It wasn’t until we visited the library here [at Davis] that we suddenly realized that we can put these into a common statistical model that pits one hypothesis against another,” Caro said. “You have these different hypotheses that are rather difficult to sort out because they all suggest the same area in which zebras live.”
Caro tested the five hypotheses on this statistical model to see which was most plausible.
“If you thought it was something to do with temperature, you’d expect the stripes to be found in very hot places…if it were something to do with social interaction, […] you might expect to find that zebras live in much more complex societies,” Caro said. “When you throw all those things into one large statistical model, there’s only one thing that came out everytime, and that is the extent of tabanid biting flies.”
Caro’s undergraduate researcher at the time, UC Davis wildlife, fish and conservation biology alum Hannah Walker compiled the information from UC Davis and four other museums, which included the Los Angeles Museum of Natural History and the California Academy of Sciences.
Walker now works as a graduate student at California State University, Long Beach (CSULB) with another member of the research group, Ted Stankowich, a professor in the department of biology at CSULB. Walker continues to study the colorization of black and white animals, namely in pandas and skunks, with both Caro and Stankowich separately.
“[Those museums are] where I took those measures, the graphs that are in the paper are from those. I did that kind of portion of data collection,” Walker said. “I also do a lot of research on the morphology of the biting flies […] and I did a compilation of the types of diseases that they carry and how susceptible equids are to those diseases and then how susceptible other non-equid species [would be].”
Tabanid flies, commonly known as horse or deer flies, are understood through Caro’s research to be abundant where the patterns of black and white striping are seen.
“[Tabanid flies] like humid […] and warm environments. [By] using environmental proxies to try and understand tabanid distributions, [we got] where the hotspots of abundance of biting flies are, and they match perfectly with where you find stripes,” Caro said. “[With that] you can discard these other hypotheses.”
Since 1981, multiple studies have shown that Tabanid flies avoid landing on flytraps and other surfaces that are black and white. According to Caro, the lab is trying to answer two questions: what aspects of striping might be visible or invisible to these biting fly visual systems, and what’s so terrible about biting flies in the first place?
That branch of Caro’s study has continued on to the present. Visual physiologist and UC Davis neurobiology, physiology and behavior professor Ken Britten was recruited to assist in answering the first question of a fly’s involvement in a zebra’s ecology.
He was able to suggest measurements for Caro to take in the fields of Tanzania, in order to calculate the polarization of the stripes. This work has revealed that the amount of polarization of the black stripes matches that of the white.
“[Horvath, a previous researcher,] has taken measurements of [tabanids] and polarization measurements of horses … [and] makes a very good case that…Tabanids are attracted to polarized light,” Britten said. “The measurements of the reflection taken in Hungary show that dark horses have more polarization reflection than light horses do, and that led to the specific hypothesis that stripes on zebras are some way of combatting or diluting the attractive force of the polarized light signature for the tabanids.”
According to Britten, the eye of the Tabanid fly has polarization sensitivity in the very middle, rather than at the bottom of the eye like other flies; this creates a unique relationship between this striped equid and its pest.
Britten suggests that the branch of the study of polarization axes in tabanid flies is still in the works, and much more research can be done to understand the tabanids’ lack of attraction to the stripes.
In January, a study from the University of California, Los Angeles (UCLA) took a different approach in answering why zebras have stripes. This study suggests that the stripes, with alternating dark and light colors, are a form of thermal regulation.
“In their analysis, in one species of zebra, the more striped individuals were found in hotter climates, so they put forward the idea that black and white striping has somehow to do with setting up convection currents of air along the back of the animals,” Caro said. “Currently […] the weighted evidence shows that it’s biting flies, until they can show evidence that striping in some way sets up air currents to cool zebras. [It] seems highly improbable, because air currents don’t work if the zebra works or it there’s any wind, it has to be absolutely still.”
However, Caro recognizes that both studies are interpreting the same information and are close to approaching a correct answer.
“These studies are showing the same thing. Both the paper we brought out a year ago, [in which] we found an association with humidity and temperature and striping, and they found an association with temperature and also humidity […] and interpreted that to being a way for thermal regulating,” Caro said. “We interpreted it in a way of avoiding fly attack. So what we’re trying to do in [response to this was] bring the two studies together and say ‘really, we’re showing the same thing.’”
Caro, who spends half of his time on conservation efforts, believes that developing research that sparks the public’s interest pushes people to think more frequently about ecological issues. For Walker, educating children on why animals have specific colorations is one for the most important developments of these studies. Britten expects that a study of what attracts biting flies may lead to an understanding and even prevention of the transmission of tropical diseases.
“Everyone cares why zebras have their stripes; it’s such a spectacular adaptation. […] When Tim’s last paper came out, the phone was just ringing off the hook for a week,” Britten said. “People will have their own specific [reasons], but the question of why zebras have stripes is generally interesting — people are really excited about this stuff.”