The Building Blocks of Polar Bears

The new frontier of polar bear research gets back to basics

Despite the fact that polar bears are inherently difficult to study, researchers have, over the last few decades, managed to learn a great deal.

We know their geographical range, their preferred diet, the vast distances they can wander, the normal number of cubs in a litter and more. We have a rough estimate of their global population, we divide them (for now) into 20 distinct populations and recognize that as the climate warms and sea ice diminishes, polar bears — at least in parts of their range — are already having less reproductive success, becoming nutritionally stressed and declining in numbers.

What still is not fully understood, however, is how polar bears function on a granular level: not just how entire populations change over long periods of time, but how bears respond individually over shorter timeframes in response to their environment. To Dr. John Whiteman, chief research scientist at Polar Bears International (PBI), that’s the new frontier of polar bear research, and it’s one in which PBI is intimately involved.

“Population level monitoring has substantiated the strong link between polar bear abundance and sea ice conditions,” Whiteman says. “But it takes an incredible amount of effort, and even when you are able to do that, it leaves open the question of: What is the mechanism? So sea ice did this in year X, and in the following year, the population did this, but why? Exactly how?”

Understanding with greater precision how individual polar bears interact with their environment is, Whiteman continues, key to understanding variations in how populations respond when that environment changes.

One key to answering these questions is a field of study known as energetics. As its name implies, the aim is to establish how much energy a polar bear uses in a range of activities, from walking to resting, versus how much it gathers from eating seals and other food. In theory, with enough data, it is possible to figure out how much energy a bear would lose by, for example, being forced ashore by melting ice a week earlier than usual, what that might mean for its physical condition and how that translates to the population were it to happen to multiple bears for multiple years.

Breaking it down

The first step is to determine how much energy a polar bear uses when it is simply resting. And the easiest way to do that is with a bear in captivity.

Dr. Anthony Pagano of the U.S. Geological Survey and colleagues sought to do just that in a 2018 study of metabolic rates in Southern Beaufort polar bears. Working at the San Diego Zoo, they measured the oxygen consumption of an adult female bear and found that she exhibited “a higher resting metabolic rate than was previously appreciated.”

What that suggests is that polar bears already have the odds stacked against them because they need a great deal of energy simply to keep the heart beating, blood pumping and organs functioning. Which means that they are likely to be especially vulnerable as longer summers with more ice-free days limit their ability to find food.

The next step is to measure energy consumption in wild polar bears — by monitoring their movements, tracking their weight loss over time or by quantifying the loss of an injected, harmless tracer called doubly labeled water to track energy expenditure.

The challenge then is to combine these data sets — existing and extensive satellite data on polar bear movements and newly-acquired information on energetics — to create a clearer image of how individual bears respond to environmental changes and how this can be scaled up to the population level.

New frontier

At the forefront of such work is PBI research fellow Dr. Louise Archer, who this year has published two studies that show how effective it is to combine the two approaches.

The first, with colleagues Pagano and Stephen Atkinson, examined the body condition of polar bears at two different points in the spring up to 39 days apart and then modeled what that revealed about their eating habits. Several bears showed a significant weight difference — some heavier, some lighter — between the two captures, highlighting that, even during their peak hunting season, polar bears tend to either feast or fast. They are gorging or not eating at all. The study, published in Arctic Science, also showed that the window in which feasting takes place is apparently shorter than previously believed.

The second study, published in Science and written with several Canadian colleagues, developed an “energy budget model” for polar bears. The model tracks the energy balance of an individual polar bear across its entire lifetime, accounting for incoming energy from feeding and energy spent on movement, growth and reproduction, amongst other processes. The study then compared those model simulations to four decades of historical data from the Western Hudson Bay polar bear population. Not only did it underline a quantifiable link between sea ice and bears, it also showed that cubs are by far the most vulnerable to climate change, and their survival significantly impacts the population.

Polar Bears International is continuing to support the work of Archer and other researchers, as they continue to drill into the minutiae of what makes polar bears tick. Not only is it exciting that scientists are still gaining fundamental insights into polar bears, but understanding the basics could unlock new ways of understanding populations across the Arctic, providing guidance to help ensure their continued survival.