Inquisitive minds can now relax.
A group of Southern California researchers have discovered many evolutionary factors that contribute to how and why the zebra is striped.
California State University, Long Beach Biological Sciences Assistant Professor Theodore Stankowich Wildlife and Biology Professor Tim Caro from UCLA, discussed new discoveries compared to another study that a UCLA professor conducted in their article published last week in online journal Royal Society Open Science.
The Daily 49er talked with Stankowich about the findings.
Why do zebras have stripes?
The question of why zebras have stripes is a really old question. It’s been an ancient question that has been around for hundreds of years because it is such an unusual trait to have and no one has ever really understood what effect the stripes have on their lifestyle and others around them and predators and parasites. So, our goal with our original paper that we published in 2014 was to understand what to compare the hypothesis to why zebras have stripes.
Those hypotheses were things like to help them camouflage in woodland environments. They have deterred predatory attacks because they were warning colors. They have a social function where they help individuals recognize each other. Some people have shown that they have an anti-parasite function where they make the animals less invisible to biting flies that carry diseases and suck their blood.
Our original paper did a large analysis where we compare these hypotheses across all the zebras and horses, and what we found was that the strongest predictor of striping was high exposure of the biting flies that cause these diseases. And we published that back in January of this year. Another research group from UCLA published a different paper on one species of zebras. The plains zebra – they found that individuals were more intensely striped in areas where there were warmer temperatures and greater amounts of rainfall.
What information did the UCLA paper publish?
They concluded that stripes have an effect of thermoregulation of keeping them cool. These are very hot environments and some ideas of striping actually keep the animals cooler than they would normally be. So, they published this paper and put these ideas forward. The paper we just published last week or two was a comment on that paper. Their results support our hypothesis with biting flies. Because of their combination of temperature and rainfall very much mimics our variable we used for activities of biting flies like hot humid temperatures for a lot of the year, as to breed and reproduce. So, their results support our conclusions as well.
Second of all, their idea about temperature regulation… there really is no good mechanism for it. There’s no good way to think about why stripes cool you off. Some people said that having black and white alternating stripes creates little eddies of air, vortexes of air because heat rise hot black stripes and reflective white stripes. There hasn’t been any evidence of that and that only works on certain parts of the body. If the animal were moving that would disrupt all of them. So, there’s no good mechanism for their hypothesis, whereas our hypothesis flies, there has been numerous flies don’t land on striped fur. They can’t see it as well. It messes with the polarization of their eyes. That is the current exchange we recently had.
What was the initial study?
Our initial study was done with looking at images of zebras and looking at where they live. We did a lot of analysis where we quantified how intensely striped their bodies were by their flank, butt, neck, head, legs. So we broke it down into different regions of their body. That was our response variable. We got a lot of info of how much each species ranged and is overlapped with conditions where temperatures were really hot, or humid. Or how much range is overlapped with different predators like lions, hyenas or how much their range – how big the size of the group they live in. By mosquitos and flies, so every hypothesis we wanted to test we had possible factors that could support the hypothesis.
We put all these factors into the hypotheses and allowed them to compete with each other. The biting fly activity is one of the biggest factors in striping. In areas where high temperatures and humid conditions for half the year, intense biting exposure, they almost all have leg stripes and body stripes which indicate that it’s a strong, best predictor of striper of all those factors. Some others predicted them as well, but not as strong as the combination of temperature and humidity. It was a very analytic study where we looked at images, maps of temperatures where these guys lived and where the predators, flies lived.