Surf or turf: What's on the menu for Arctic polar bears?

February 23, 2015 - 8:05am

Oregon Zoo bears Conrad and Tasul help scientists unlock decades' worth of data

Twenty years ago, biologists could pretty much describe a polar bear's diet in two words: ringed seals.

But as global warming continues to melt the Arctic sea ice — and with it, polar bears' access to the seals — the primary food source of this iconic and imperiled predator is becoming less and less available.

"As polar bears are displaced from their sea ice habitats, their feeding patterns may be changing," said Karyn Rode, a wildlife biologist with the U.S. Geological Survey's Alaska Science Center. "We need to understand how polar bears are responding."

Polar bears are extremely difficult to study in the Arctic, Dr. Rode notes — close behavioral observations over long stretches of time are practically impossible. Now though, a pair of geriatric bears at the Oregon Zoo has given researchers a never-before-available tool for tracking the feeding behavior of their Arctic cousins.

In 2012, polar bear siblings Conrad and Tasul became the first polar bears in the world to voluntarily give blood. This was a significant animal-welfare advance as it reduced stress and allowed the bears to participate in their own healthcare. And, since regular blood sampling was now possible, it also presented a unique opportunity for researchers.

Over the past several years, Dr. Rode and her team have been developing a method for tracing the chemical composition of Arctic polar bears' prey — to see what the bears have been eating and when.

In a controlled setting — with consistent access to blood and hair samples, and the ability to manage and closely monitor the bears' diets — Rode was able determine correction factors to refine this effort, as well as what time window of feeding is represented from a sample.

Keepers managed Conrad and Tasul's feedings — first providing them with a land-based diet for a specified period, and then switching over to seafood.

"We like to think of it as the 'surf and turf' experiment," said Amy Cutting, who oversees the Oregon Zoo's North America and marine life areas. "Conrad and Tasul seemed to enjoy the 'terrestrial pre-trial' diet, as well as the 'marine trial' diet."

Researchers, meanwhile, were able to monitor the chemical traces of each discrete diet by analyzing blood and hair samples. The results were somewhat surprising.

"The time window of diet represented by blood and hair was much longer than we previously thought," Rode said. "Until now, we assumed that blood represented what a bear ate over the past one to two months. But our results suggest those samples represent diet up to six months or more."

"Projects like this provide a unique opportunity for these bears to contribute to the understanding of polar bears in the wild," Cutting said. "Sometimes people think of 'animal research' and they assume that animal welfare is being compromised. But what's exciting to us is the way these collaborations have actually enhanced animal welfare — Conrad and Tasul enjoyed the novelty of the 'surf vs. turf' diet plan this study required — while at the same time providing critical information for researchers."

Rode and other researchers plan to apply this new knowledge to samples obtained from wild polar bears over the past 30 years, helping researchers better understand the feeding ecology of wild polar bears coping with climate change. By tracing the distribution of prey animals in polar bear tissues, researchers can begin to infer the diets of wild polar bears from a single blood or hair sample.

"Our findings at the Oregon Zoo will let us use decades of archived blood samples from wild bears to determine if — and how — diets are changing in response to sea ice loss and other factors. The ability to learn from Conrad and Tasul's behavior and physiology is extremely valuable to understanding polar bears in the wild."

This project, conducted by the USGS Polar Bear Team, is part of the USGS's Changing Arctic Ecosystems research on the effects of climate change on polar bears.