Why polar bears walk around with brightly colored dots on their fur

To fill the missing gap in understanding polar bear hair growth, researchers are staining sections of their fur with colored dots to visually track progress.

In a study led by scientists from the University of Washington and the US Geological Survey (USGS), several polar bears participate in the pelt survey. The research is being conducted in collaboration with the San Diego Zoo, Louisville Zoo, Assiniboine Park Zoo in Canada, Detroit Zoo, Oregon Zoo, Old Dominion University, Aarhus University and Center College.

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Jenny Stern, a University of Washington graduate student who is co-leading the fur dye study, spoke to The Weather Network in February 2022 about the ongoing research. The main elements scientists want to address are how quickly their hair grows and when it occurs, she said.

“It’s quite shocking that we don’t have that information, but [really] that was the inspiration. We’re missing this really fundamental question, which is fundamental to much of our research on wild polar bears,” Stern said. “A group of polar bear scientists came together and designed this study to answer it.”

© C.Breiter/Assiniboine Park Zoo - color patches with crimp plate 3

© C.Breiter/Assiniboine Park Zoo – color patches with crimp plate 3

(C. Broader/Assiniboine Park Zoo)

After being colored with a colored dot, the bears receive a label tracer (non-toxic biomarker) through their food. Scientists can then analyze where the tracer is located in the hair to determine growth rate and seasonal timing of hair expansion, along with the factors affecting those metrics, such as dietary intake, nutritional status, height, age, gender and environmental conditions.

“Even if you go to a salon and get your hair colored, eventually your hair will grow back and you’ll start to see your roots,” Stern said. “We use the same principle where we measure the hair every few weeks and see how much their roots are showing how much of the hair is undyed.”


A total of eight polar bears were used in the study over the past few years, but not all had purple spots — some were mottled brown or black. The bears involved in the investigation are located at the Point Defiance Zoo and Aquarium, the San Diego Zoo, the Assiniboine Park Zoo, and the Detroit Zoo.

“Zoos provide an incredible opportunity to ask questions about polar bears and ask questions that we can’t answer with their wild counterparts,” Stern said.

She then explained via email that the dye color chosen in the study was not important. “It just needs to be dark enough to create contrast to be able to track new hair growth. The purple is actually a blue-black hair dye that looks very purple when applied to transparent hair,” Stern said.

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Understanding coat growth patterns allows scientists to better explain stress levels, pollution levels and nutritional status using hair samples collected from polar bears in the wild.

“When we went into this, we didn’t know if coloring her hair and using this label tracer would be reliable methods of determining hair growth rate. We see that it is,” Stern said. “Hair is an extremely important tissue for learning about polar bears. They are really difficult to access [since] They live in very remote areas that are difficult to reach…”


At the time of the interview with The Weather Network, Stern said researchers had already seen some differences in how hair starts to grow.

Polar bear with dyed fur/San Diego Zoo Wildlife Alliance

Polar bear with dyed fur/San Diego Zoo Wildlife Alliance

Polar bear with colored fur. (San Diego Zoo Wildlife Alliance)

“All of this information that we’re getting from the hairs, ranging from how stressed the bear is, what the bear has eaten, how many pollutants and impurities are in the bear’s body… we’re going to be able to create a time frame for that.” and say when they eat certain things or when they’re most stressed,” Stern said.

“I’m just so grateful to be able to work with scientists, zoo keepers and people at all levels to answer these really big questions that many people have been asking for a long time.”

Thumbnail courtesy of C. Breiter/Assiniboine Park Zoo.

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