How Do Polar Bears Use Sea Ice to Travel?

When you look at the Polar Bear Tracker on the Polar Bears International website, bear movement can seem deceptively simple: bears appear to walk in straight lines across a stable white sheet of sea ice. In reality, travelling across a frozen ocean is far more complex. The ice beneath a bear’s feet is constantly drifting, cracking apart, and reshaping itself, while sea ice conditions, wind, prey availability, and the location of important habitats all influence how and where polar bears move.

Drifting with the ice

Not only are polar bears moving — ocean currents and winds are causing the sea ice to move too. This is called sea ice drift. For example, in Hudson Bay, average sea ice drift values across the winter range from around 3-5 km per day . This means if a polar bear is resting on the ice for a whole day, they might move without expending energy walking! Conversely, this could hurt a polar bear’s progress as they travel. If bears are moving in the opposite direction from the ice drift, sea ice drift can act as a “treadmill” — the drift can offset their progress so they stay nearly stationary in geographical location. Either way, the locations seen on the Polar Bear Tracker are the net result of polar bear movements plus sea ice drift.

Actively choosing particular ice habitats

Understanding how bears move on ice is only part of the puzzle. Understanding where bears move on ice is another. Why do polar bears choose specific habitats over others and how do scientists make sense of this?

One common tool used to answer this question are habitat selection functions. This is a statistical framework that allows scientists to evaluate which habitat characteristics matter most to a species. It also indicates how strongly the bears prefer or avoid different habitat characteristics. One study found that in the Western Hudson Bay subpopulation of polar bears, sea ice concentration (a measure of how much sea ice is in a certain region) and distance to denning areas were both important variables in determining where a polar bear would be located. Studies in the Barents Sea and Western Hudson Bay showed that ocean depth, distance to open water, and distance to shorelines also affected the bears’ decisions.

The time of year matters too. For example, in Hudson Bay, during the spring ice break-up season, bears might select locations closer to shore because they know the last pieces of ice will likely melt in those regions. However, habitat preferences are not a “one-size-fits-all” rule for polar bears. Age, reproductive status, and learned behaviors all can all impact how individual bears choose locations on sea ice.

Using winds and scents

Research has found that polar bears often travel crosswind, particularly at night, when most hunting occurs. Travelling crosswind is the optimal method for locating smells travelling on the wind, because it more effectively increases the area searched than travelling directly into or with the wind. Once a bear smells something interesting, it can turn upwind to locate the source of the odor. This finding suggests that movement decisions are influenced not only by large-scale habitat features, but also by the environmental information available to bears at any given moment. So a movement path that appears indirect on the Bear Tracker may actually reflect a bear responding to scent cues carried by the wind, rather than simply travelling toward a destination.

Energetic cost?

Whether a bear is following scent cues on the wind, seeking hunting habitat, or moving toward better sea ice, travel always comes with a cost. The Arctic is a constantly changing landscape, and the amount of energy required to move across it depends heavily on the condition of the ice itself.

While these bears are strong swimmers, swimming is roughly 5 times more energetically costly than walking at the same speed. As a result, highly fractured ice landscapes require polar bears to expend more energy to cross as they must alternate between walking and swimming.

Hudson Bay freezes over every fall and melts completely every spring. Polar bears in this area migrate on and off the sea ice every year in response to this ice cycle, encountering fractured sea ice landscapes during migration. Dr. Jody Reimer and Rylie Gagne at the University of Utah are developing a method to predict the most energetically efficient path for a polar bear to cross a patchy landscape of sea ice and water, depending on the configuration and geometry of the ice.

Are bears actually following these optimal paths? This question is surprisingly difficult to answer, due to one of the limitations of telemetry data: its time resolution. Telemetry data typically picks up the location of collared bears every couple of hours. This means that while scientists know the starting point and ending point of a polar bear's movement over that period, they don’t know the exact route they’ve taken. Ongoing research aims to bridge this gap, using drones to trace polar bear tracks. This will allow us new insights into their step-by-step movements across variable ice and snow conditions.

A complicated path

While a polar bear's path on a map may look like a simple line connecting one location to the next, in reality, every step is shaped by a moving landscape. Bears travel across sea ice that drifts with winds and currents, navigate around fractures and open water, respond to changing prey opportunities, and make movement decisions that scientists are still working to understand. The tracks displayed on the Polar Bear Tracker represent much more than a journey from point A to point B: they are the result of a polar bear navigating one of the most dynamic environments on the planet.

Citations

Griffen, B. D. (2018). Modeling the metabolic costs of swimming in polar bears (Ursus maritimus). Polar Biology, 41(3), 491–503. https://doi.org/10.1007/s00300-017-2209-x

Klappstein, N., Togunov, R., Reimer, J., Lunn, N., & Derocher, A. (2020). Patterns of sea ice drift and polar bear (Ursus maritimus) movement in Hudson Bay. Marine Ecology Progress Series, 641, 227–240. https://doi.org/10.3354/meps13293

Lone, K., Merkel, B., Lydersen, C., Kovacs, K. M., & Aars, J. (2018). Sea ice resource selection models for polar bears in the Barents Sea subpopulation. Ecography, 41(4), 567–578. https://doi.org/10.1111/ecog.03020

McCall, A. G., Pilfold, N. W., Derocher, A. E., & Lunn, N. J. (2016). Seasonal habitat selection by adult female polar bears in western Hudson Bay. Population Ecology, 58(3), 407–419. https://doi.org/10.1007/s10144-016-0549-y

Togunov, R. R., Derocher, A. E., & Lunn, N. J. (2017). Windscapes and olfactory foraging in a large carnivore. Scientific Reports, 7(1), 46332. https://doi.org/10.1038/srep46332