During the spring feasting season, polar bears pack on the weight and young cubs, while still nursing, taste seal fat and meat for the first time. Copyright Mike Lockhart.

3/22/2015 1:56:01 PM

Spring on the Sea Ice

Alysa McCall

It's that time of year again when birds are chirping, flowers are blooming, and polar bears are hunting!

While polar bears eat a variety of foods, the ringed seal, a relatively small arctic seal species, is considered their main prey. At an average weight of 150 lbs., ringed seals are big enough to be a good meal for a male polar bear but small enough that females and subadults can consume them without exerting too much energy. Ringed seals are the only seal species that maintains breathing holes in the sea ice using their teeth and claws. This unique behavior allows them to occupy the vast, stable areas of sea ice that form along the land during the winter, known as shorefast ice, which is where they give birth in the early spring.

The Perfect Prey

Ringed seals have their pups within a snow cave (called a subnivean lair) that the mother seal digs over one or more of her breathing holes. This snow cave offers her pup some protection from predators and the unforgiving weather. In Hudson Bay, ringed seal pups are born from mid-March to mid-April, suckle for about six weeks, and are weaned before the ice breaks up in June. While nursing, pups spend about half their time in their lairs on top of the ice, and half underwater diving.

Seal pups are the perfect prey: vulnerable, naÌøve, and high-calorie. Polar bears will crush the roof of snow lairs to try to find pups inside, while also scanning the sea ice for oblivious pups and tired adults. The high concentration of pups and adults at this time of year allows polar bears to gorge themselves as much as they can, a period called hyperphagia. By eating massive quantities of food in a short time period, polar bears gain much of the body fat that they will need to survive on over the summer. Dr. Nick Pilfold's latest blog post delves deeper into this critical time of year and shows just how much seal population dynamics can influence polar bears.

This feeding period may be especially exciting for the polar bear cubs that were born this January. These cubs-of-the-year (coys) are still getting used to being on the sea ice, and, while they are still feeding on their mother's milk almost exclusively, they are also sneaking licks and bits of mom's meals and beginning to develop a taste for seals.

The Next Generation

As if this season could get more exciting for polar bears, it's also mating time! Two-year-old cubs will be weaned and sent on their way to become self-sufficient while their mothers and other solitary females will be waiting for a male to find them via their footprints.

Even though they mate in the spring, female polar bears have delayed implantation, which means the zygote won't implant until the fall—and only if the female has enough body fat to support a cub. If things go according to plan, pregnant females will binge on seals this spring, gain plenty of weight, and enter maternity dens in the fall. They will return to the sea ice with their coys next February or March to binge again.

Where the Bears Are

It's hard to know what is exactly happening out on Hudson Bay right now, but all of these possibilities are exciting! We're able to get a glimpse into the world of polar bears by watching PBI's Bear Tracker, which has been tracking collared bears from the Southern and Western Hudson Bay polar bear populations all winter. As usual, it's impossible to make general statements when it comes to polar bear movements: no two bears are alike.

The northernmost bear on the Bear Tracker at the moment is from the Southern Hudson Bay population. She has traveled well over 1400 km (870 miles) with her two cubs since November, ending up a long way from home! Conversely, at least two SHB polar bears have stayed in the much smaller James Bay area all year, traveling a much shorter distance and only in a small area. James Bay bears are the most southern out of all the polar bears in the world and can have some of the smallest home ranges; there have been suggestions that they should become their own population, but so far they are still considered part of the SHB population. Several SHB bears that did leave James Bay last fall have been sticking fairly close to the east coast, just off of Quebec, all year. At times they have been clumped in the same small area (likely because of a food source), but eventually they spread out again and go their own way.

In the adjacent WHB polar bear population, three bears have been hanging out in the same general area northeast of Wapusk National Park for weeks. There may be some seal activity there as their movements have been small lately. Another WHB bear is in the south-central region of the Bay, far from the others; she has travelled 1530.47 km (950.99 mi) throughout western Hudson Bay so far this year with her cub. Unfortunately, another WHB bear who had been been heading right across Hudson Bay toward Quebec recently dropped her collar; we will ask scientists to keep a look out for her next fall.

No matter where the bears are this time of year, we hope they are finding plenty of seals and that the seals are having a productive year. Climate changes in Hudson Bay are expected to affect ringed seal reproduction and survival: warmer temperatures and rain in the spring melt snow and leave pups exposed, and very cold temperatures can make the shorefast ice too thick to maintain breathing holes. Changes in ice movement patterns may also make it harder for males to find mates, impacting polar bear reproduction. You can help ensure a better future for polar bears, seals, and the arctic ecosystem by becoming involved in our Save Our Sea Ice campaign and signing (and sharing) our Petition for Polar Bears, which asks world leaders for meaningful action on climate change.

New research suggests that polar bears may find each other during mating season by following scented trails on the sea ice left by footpads.These trails may be disrupted in as sea ice melts in a warming Arctic. © Daniel J. Cox/Natural Exposures

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