27 February 2015

Zebra Stripes

Welcome back. Did you see where they solved the mystery of why zebras have stripes? No, I don’t mean the study published in 2013 that solved the mystery. No, not the study published last April that solved the mystery. I’m referring to the study published last month that solved the mystery.
Zebra (mother and foal?) in
Namibia (Photo by Ellen Haack)

Those three studies offered different explanations for how stripes might have evolved, but when they were published, the media announced that each had finally solved the mystery. Well, the media did hedge a bit on the last because that study’s title included “a problem with too many solutions.” And there’s no shortage of older studies on the topic. Charles Darwin and Alfred Russel Wallace were debating the issue 120 years ago.

2013 -- Optical Illusion

When I saw and filed away the 2013 paper two years ago, I was only thinking it would be fun to write a blog post on How the Zebra Got its Stripes, which, as I suspected, turned out to be a children’s book.

How the Zebra Got its Stripes
(Little Golden Book)
For that study, investigators from Australia’s University of Queensland and the UK’s University of London examined photographs and videos of zebras using a motion-detection algorithm that analyzed signals generated by different parts of the zebra’s body when the animals moved.

Their simulations demonstrated that the visual signals generated by moving stripe patterns could be highly misleading to an observer. The signals correspond to the wagon-wheel effect (see Temporal Resolution Addendum) and the barber-pole illusion (visually misperceive the direction of motion). The investigators concluded that the stripes create visual illusions when zebras move--especially two or more zebras--that confuse biting insects and possibly predators.

2014 – Biting Flies

The study published last April took a different approach. Although only other zebras might notice or care, zebra stripes vary (e.g., number, thickness, length). Collaborating researchers from the University of California at Davis, National Institutes of Health and California State University thought that variation might provide insight into natural selection.

They matched the variation in striping of zebra, horse and ass species and subspecies with the geographic ranges of both the animals and selected environmental variables (e.g., woodland areas, large predators, temperature and biting flies).

Using multifactor models, they tested the foremost explanations posed as evolutionary drivers for zebra stripes: camouflage, predator avoidance, heat management, social interaction and biting fly avoidance. The only explanation for which they found significant associations was with biting flies.

2015 -- Temperature

For the most recent study, investigators from the University of California, Los Angeles, Germany’s University of Tübingen and Princeton examined the correlation between 29 environmental variables and stripe characteristics of plains zebra across their range in Africa. Temperature was the best predictor of stripe variation.

Similar to the 2014 report, they found no association with predator avoidance. Unlike that report, however, they also found no association with biting fly avoidance. Moreover, they judged that the approach taken to incorporate biting flies in the 2014 analysis lacked rigor, being based on environmental conditions that favored the flies' propagation, such as temperature and humidity, rather than on actual fly distribution data.

Although temperature stood out in the 2015 study, the investigators noted that temperature’s association with striping may in fact be indicative of different biological processes. They concluded that the evolutionary drivers of striping are probably diverse and complex.  
How Zebras Got Their
Stripes (Usborne)

Wrap Up

In support of that final point, it should also be noted that a statistical correlation between two variables, such as that between temperature and zebra stripes, is indicative of a relationship between the two but provides no evidence of cause and effect.

Bottom line, I’m sorry if you were hoping for a 100%, that’s it! answer to the so-called mystery, but I hope you found these studies of interest. Thanks for stopping by.


2013 study in Zoology journal and article on BBC Nature News website:
2014 study in Nature Communications and example articles:
2015 study in Royal Society Open Science journal and example articles:
Children’s books:
How the Zebra Got its Stripes
How Zebras Got Their Stripes

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