For a polar bear, obese is best. This is especially true for polar bears in the Western Hudson Bay population who live in the Seasonal Sea Ice Ecoregion. These bears are forced onto land during the summer when the sea ice melts completely over Hudson Bay; they will stay on land until the sea ice freezes-up again in November or December. This makes early summer an especially interesting time to watch the Polar Bears International Bear Tracker as each bear chooses to do something different on her way home, all in the name of getting and staying fat!
The importance of fat reserves
On land polar bears lose access to their main prey, seals. Polar bears still eat what they can (e.g., eggs, berries, carrion), but terrestrial foods offer little energy compared to the bears’ normal blubber-filled diet. This means they have a long period ahead when they will mostly be fasting (i.e., subsisting on their own body fat) and losing about 1 kilogram of weight per day on land. So, coming back to land as fat as possible is the best strategy to keep healthy as long as possible, especially if freeze-up is delayed.
Being fat now is especially important for pregnant females who hope to den later this year and give birth over the winter. They won’t emerge to hunt again until spring 2022 (about an eight-month fast) so they really need to pack on the pounds before returning to land.
When and where to come ashore
However, due to human-caused climate warming, it is getting harder for polar bears to predict sea ice patterns and how long to keep hunting; we see different strategies arise from the uncertainty. Many of the tracked females are either already on land or have moved toward Wapusk National Park, the region in which they were born and where most (if not all) will choose to spend the summer. This is a good strategy because when the sea ice breaks up, the bears can walk or swim shorter distances back to shore to find a good spot on land to rest. Longer swims can be energetically costly and waste precious body fat.
However, some bears may in fact choose to ride the ice floes down closer to Ontario where ice persists longer, adding a couple of extra weeks of hunting time. Those bears will likely walk a longer distance home over the summer, but the energy balance may work out in their favor. We don’t yet know what spurs polar bears to make such different choices, but it’s something researchers are interested in especially as sea ice continues to change in size, thickness, and patterns across the Arctic. Sea ice can be especially unpredictable during the summer break-up period in June and July.
Shrinking sea ice
The rate of ice loss for June 2021 was faster than average, with the Arctic losing a total of 2.39 million square kilometers (923,000 square miles) during the month. This June, each day we lost an area larger than New Jersey more than we did compared to the same period from 1981-2010. Overall, the June loss of ice since 1979 is equivalent to about three times the size of Texas. It remains to be seen what will happen in Hudson Bay this summer and fall, but we will be there to observe and report.
To help us ensure future generations of polar bears are protected, please join us in our efforts to make climate action a priority by talking about it! Start by checking out these tips and also tune in to our live events on Arctic Sea Ice Day, July 15th, which will also kick off our beluga season (another animal that needs sea ice to survive). Thank you for your support!
Updates on Individual Bear Tracker Bears
X33931, Vienna Zoo, Hope
This spring was especially critical for Hope, who exited her den with a cub or two in tow in March. She had spent the prior eight months fasting, so that first seal kill must have tasted fantastic! After gorging as much as possible to regain some fat stores, and taking care of her offspring in the meantime, Hope must be exhausted but is no doubt trying to eke out as many hunting days as possible on the sea ice. She needs to be careful, though. If she stays out too long that could mean swimming back home to land which is incredibly dangerous for little cubs. We will keep an eye on Hope to when she decides to return to shore and wish her and any cubs safe travels and happy hunting (while it lasts)!
X33928, Munich Zoo, Tenya
Tenya was the first collared bear to arrive back on shore for the summer. After hanging around just off the coast of Ontario for the last few months, she arrived on land in early July in the lower half of Wapusk National Park. This was a good strategy because it meant her trip to shore was short and she didn’t need to expend huge amounts of energy. She may wander a bit before finding a place to settle, or maybe we will see her stop moving fairly soon. She will likely be looking to sustain a pregnancy throughout the next eight months, meaning she needs to find a place to den for this fall and winter. Tenya can hopefully save as much energy as possible during her search since she won’t be eating until next March, when she hopefully emerges with a healthy new cub or two in tow!
X32444, Hannover Zoo, Hope
Since our last update, Hope has looped much farther south and is now off the coast of Wapusk National Park in Manitoba, the region in which she was born and will spend her coming months. This past year, Hope travelled throughout much of the Western Hudson Bay population boundary, no doubt finding the best places to hunt seals wherever she moved. We hope she ate a lot and got as fat as possible because Hope will likely be looking to den this fall and give birth over the winter. Hope is still on the sea ice, but it will be interesting to see how long this lasts. She won’t want to expend a lot of energy getting home to find a den, but she is likely trying to eat as many seals as possible in these final days on ice since she won’t have another blubber meal again until March 2022!
X33110, Rostock Zoo, Vilma
Vilma has always been a big mover with a preferred area, and she stayed true to that all year. Vilma tends to hang out in a region northeast of Wapusk National Park, zigzagging back and forth around her favorite hunting grounds. She has been tracked for multiple years at different times of her life, and we are incredibly grateful for all she has taught us about polar bears. We are sure Vilma is a great teacher to her cubs, too. This bear is likely pregnant and will be hoping to sustain a pregnancy and give birth over the next eight months. This knowledgable mama is squeezing in last days of hunting time as Hudson Bay breaks up, trying to eat as much as possible and gain as much fat as she can before fasting until next spring. By then her collar will have fallen off, but we wish her well and hope to see her again in the future!
X19271, Canada Goose, Aurora
Twenty-two-year-old Aurora is eking out the last few days or weeks of hunting time on Hudson Bay as sea ice breaks up for the summer. She will have mated this past spring, hoping to sustain a healthy pregnancy and give birth this winter, meaning she won’t eat again until March 2022! Hopefully she can cram in some final meals before coming back to land and finding a peaceful den to rest in, likely somewhere in Wapusk National Park, Manitoba. Aurora is quite experienced but probably only has one or two litters left in her lifetime, so fingers crossed she is successful and that this seasoned mom is well prepared for the long months ahead. Good luck, Aurora!
