5/22/2017 2:00:41 PM
How Can Scientists Tell What Polar Bears Eat?
Have you ever wondered how scientists can estimate what polar bears are eating? It would be great to follow polar bears around all day and track their foraging patterns (Dr. Ian Stirling and his assistants did just that back in the 1970s and 80s, observing from a tower with binoculars), but that’s tricky. So instead, scientists have developed a few ways to analyze diet, including one method that allows our research team to estimate what a polar bear is eating by taking a small sample of their fat, breaking it down, doing some math, and voila—coming up with an estimate of their diet!
Here’s how and why it works: polar bears are what they eat. When a polar bear eats a seal (or scavenges on a whale carcass), it incorporates the seal’s fat into its own fat stores. When biologists are handling polar bears for various monitoring projects, they take a small biopsy sample of fat from the rump of each bear.
The first step is to acquire a small sample of polar bear fat during a routine monitoring project. Photo copyright Dr. Gregory Thiemann.
Scientists place the sample in a vial and send if off to the lab, where we follow a recipe-like protocol of extracting the lipid from the fat, then measuring the fatty acids: 71 fatty acids in all! Based on this, our research team can determine each individual bear’s own fatty acid signature, which reflects the bear’s diet over the past few months. The set-up allows us to process 12 polar bears’ fat samples in a day’s work.
This tiny biopsy sample is all we need to estimate a polar bear’s diet. Photo copyright Dr. Gregory Thiemann.
If this isn’t cool enough, we then use a fancy technique, called quantitative fatty acid signature analysis, that looks at the fatty acid signature of a polar bear using a mathematical model. This allows us to estimate the proportions of different prey species in a bear’s diet. For example, for an individual polar bear, the model may estimate the diet is: 60% ringed seal, 15% bearded seal, 20% beluga whale, and 5% walrus.
Polar bears live across the circumpolar Arctic, in areas with differences in the physical sea ice conditions, and variation in the availability, accessibility, and abundance of prey species. What will a polar bear eat in these different situations? Are bears eating more beluga in one area than another area? Are polar bear diets shifting over time? If we can pinpoint what bears are eating in different spatial areas, and over time, we can better understand the complex relationship between the sea ice, polar bears, and their prey.