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Caro knows why zebras have stripes

Staff writer
Why are giant pandas and zebras black and white?
That’s the sort of question animal behaviorists ask themselves and each other when they’re sitting in a scorching Land Rover in the middle of the Serengeti Plains or having a few beers after a long, boring day in the field.
Not everyone will take the next 10 years thinking up ways to test the various hypotheses to come up with evidence that convinces them, and their colleagues, that they have the right answer. But that’s exactly what Tim Caro, Winters resident and professor of wildlife biology at UC Davis did.
Caro says his interest in animal behavior started at age 3 when his mother gave him “The Observer’s Book of Birds” and he knew he wanted to be a zoologist.
Time passed, he got degrees in zoology and psychology and started a research career focusing on mammals, and particularly questions about why some mammals have very distinct color patterns.
Why are zebras striped? Why do giant pandas have the distinct and charming appearance that makes them the darling of zoos and zoo-goers?
Caro has researched the answers to those questions, and it is an example of the slow ways in which progress is sometimes made in science.
He says that he first thought about the question of why zebras are striped in 2004 when he was doing field work in western Tanzania. He is in Africa about one-third of the year.
He went to Africa a year later with an idea about zebras stripes, the equipment to test it, and spent July, August and September gathering data about that idea. The data showed nothing.
“Year after year, I would think up a new set of experiments to try to get a definitive answer as to the value of stripes to zebras and year after year, I would return to Davis with no results,” said Caro.
One year, he had full-size wooden animal shapes made in Africa, accurately painted to mimic zebra, wildebeest and other animals. He wanted to know if, in dimming light, when lions usually hunt, the “animals” could be readily distinguished by human observers. As reported in an article in Wired magazine, “For a week, as the sun went down, he asked his colleagues (every five minutes) if they could see zebra and other animal cutouts in the dimming light. With a notepad, he’d record their answers, the light conditions, etc. Then he’d ask again, every five minutes, until dark.” He repeated the same experiment at dawn, waking his colleagues just before dawn to make observations. Needless to say, not everyone was cheerfully cooperative.
After many other failed experiments, Caro wondered if the stripes deterred biting flies that carry disease. In order to test this, he had black and white striped pajama suits made for him by a tailor in Dar-es-Salaam. He walked through the lion-infested African plains and had an assistant count the number of flies that landed on him, compared to non-striped clothing. To make sure it wasn’t the tailored pajama suit that was giving that result, he walked through the same territory draped in a zebra hide. As a control, he walked around with a wildebeest pelt draped around him.
Finally these experiments showed some significant data differences.
Back home again, Caro used the resources of the UC Davis library system to map out ranges of biting flies, ranges of zebras and ranges of non-striped horse-like species. Stripes are associated with biting fly abundance. Stripes protect the zebras from fly predation, not from lions.
The paper showing the results of the zebra stripe question was published on April 1, 2014, 10 years after he asked the initial question, said Caro. “Some of my colleagues wondered if I was putting on an April Fool’s joke.”
Strangely enough, Caro’s colleague, Ted Stankowich of Cal State Long Beach, was visiting two days later and Caro suggested they look together at why the Giant Panda has unusual coloration. It only took them and colleagues three years to work out how the panda’s coloration helps it survive — quite a different story from the zebra.
The study, “Why is the Giant Panda black and white?” was published Feb. 28, 2017 in the journal, Behavioral Ecology.
The panda is a bear that does not hibernate. Living solely on bamboo species, it roams during the year from snow zones to tropical forests. It was not practical to investigate this by a field study, so the researchers relied once again, on library and collection resources to do an exhaustive analysis of coat color of various species of bears and other animals, mean annual temperature of habitat, fur depth and color, dark eye markings and glare, analysis of communication due to coloration. Thousands of images were scored, compared, analyzed.
Caro emphasized that the insight that made the study work was the decision to analyze each part of the body as an independent area. The white areas help the mammal hide in the snow. The dark arms and legs help them blend into the shade of forests. Even though neither the white nor black fur is complete camouflage, it is sufficient to keep a predator from seeing the outline of a full body.
The eye markings are possibly used as recognition markers within the species or possibly to signal aggression to other species.
Caro’s research is taking a new turn in the coming year. He and his wife, professor Monique Borgerhoff Mulder who is an evolutionary anthropologist, have relocated from western Tanzania to the island of Pemba, north of Zanzibar in the Indian Ocean. He has started a coloration project with coconut crabs, a large (10 pound) crab that occurs in two color morphs — bright blue and bright red — both in the same population.
As a new venture, they will also be undertaking a project together for the first time. Borgerhoff Mulder is investigating which villages are more successful in preserving their patches of forest on a densely populated island. She hypothesizes that the more collaborative the village is, the more successful they will be. Caro’s work will look into whether or not the forests that are more successful for human needs are also better for animal habitat.
Stay tuned for new scientific findings from this new location and collaborative effort.