What Do Polar Bears Do in Spring?

After the cold, dark hardship of winter, the return of spring to the Arctic is like a rebirth, as life springs forth anew. Male polar bears are keen to find mates, mothers who gave birth in wintry snow dens emerge with their cubs, and polar bears of all ages take advantage of the spring seal bounty to accumulate as many calories as possible before the sea ice melts again in the summer.

By the standards that most of us experience, the return of spring in the Arctic does not immediately presage warm temperatures.

“In the Beaufort and Chukchi Seas, you historically see temperatures of minus 10 or minus 20 Fahrenheit through March into April, with the occasional day dropping to minus 30,” explains Geoff York, senior director of research and policy for Polar Bears International.  “Conditions start warming considerably by the time we get deep into April. And by warming considerably, I mean they rise to slightly above freezing.”

But compared to winter, when temperatures can be 20 or more degrees colder still, that’s a big improvement. More important than the ambient temperature, however, is the return of sunlight, which sparks an explosion of productivity that begins in the sea ice.

The bounty begins with sea ice

Ice floes in the Arctic contain as many as 200 different species of microscopic plants called diatoms, which over the course of the winter slide through small fissures in the ice to the underside of the floes. With the return of sunlight, the sun’s rays penetrate the ice, warming the water beneath, melting the bottom of the floes and releasing the diatoms into the water’s surface. Bathed in light, they photosynthesize and multiply, and are eaten by only slightly larger animals called zooplankton.

The zooplankton in turn are eaten by fish, the fish by seals, and the seals, of course, by polar bears – including the mothers and young cubs that are emerging into the daylight after spending the last couple of months in warm, dark dens that the mothers built in snowbanks. So great is the smorgasbord that spring provides that not only does it allow polar bears to fatten up after the preceding lean months, but it also provides them with enough nourishment to see them through the harder times that lie ahead.

“You really get that entire system waking up from a slumber to produce high-fat organisms,” says York. “Moms and cubs are coming out of dens across the Arctic and needing to find food – like a spring bloom, but of polar bear cubs instead of flowers. But those blooms need food. On top of that, you have all the bears that are trying to capitalize on the brief spring buffet. The feasting they do then allows them to hit the shore, or stay on the sea ice as summer hunting slows down, with enough calories to make it through till next spring.”

Ringed seals and easy hunting

For polar bears, the biggest component of that spring buffet – indeed, the item that they pile high on their plate to the effective exclusion of everything else – is ringed seals. Like polar bear moms, ringed seal mothers build dens in which to give birth and nurse their young; unlike those of polar bears, ringed seal dens are built on the pack ice, over a breathing hole that the mother seal maintains in the ice. And whereas polar bear mothers generally give birth between November and January, depending on whereabouts in the Arctic they are, ringed seal pups are born in March or April – just as hungry mothers and their cubs are emerging from their own dens in search of food.

As a result, the spring sea ice can in places resemble a kind of bear bacchanalia as polar bears sniff out the seals’ dens and smash through them in search of the pups inside. The carnage continues later in the spring, as surviving seals are obliged to lay on the ice for lengthy stretches at a time while they undergo their spring molt.

But, York points out, it isn’t just about the seals. The sea ice itself is in its most polar bear-friendly state in spring months: extensive, but with plenty of leads and fissures granting access to the water below.

“As the ice becomes more active and starts to persistently stay open, and leads persistently form, you have seals focusing in on those open areas, and polar bears focusing in on the seals that are using those lead systems,” he explains. “And then returning beluga whales are using those systems to migrate, and eiders are using them as part of their migration. So as soon as you start to get those persistent patches of open water, life takes advantage. And that definitely has direct links to polar bear movements and behavior.”

Mating season

Not every bear is laser-focused on accumulating calories in the form of seal blubber, however. One subset of the population has more pressing priorities in spring. Guided by scents that females leave behind in their paw prints that indicate they are in estrus, male polar bears set out in search of a mate.

“Male bears will emaciate themselves during breeding, and treat mating as a primary activity,” says York. “It's definitely a kind of hormonal takeover, if you will, in terms of prioritizing how they spend their time. ‘Should I eat, or should I find a mate?’ It seems like in certain times of the spring, it's mate, mate, mate.”

A changing Arctic 

All the above is how spring has historically unfolded for polar bears. In a warming Arctic, however, change is afoot, and not for the benefit of polar bears or other Arctic wildlife. Recent research has confirmed that on average spring sea ice is melting three to nine days earlier than previously, giving polar bears less time to accumulate the calories they need to see them through the less-productive months ahead – a problem exacerbated by the fact that fall freeze-up is on average occurring a similar number of days later, meaning that polar bears may have as much as 18 days fewer on the ice each year.

That isn’t the only issue.

Some parts of the Arctic are seeing less snowfall – which, explains York, is a problem not just for bears but for seals.

“So, in some places, like Svalbard, ringed seals are increasingly being forced to pup on the surface of the ice without any snow cover,” he says. “And as a result, seal pups are being picked off not just by polar bears, but by foxes and seagulls and ravens, driving their reproduction in some areas down to zero.”

There’s more. Warming springs don’t just mean less snowfall. They can also bring rain.

The extra weight of rain, which swiftly freezes, on top of snow can cause disaster for polar bear mothers that have not yet emerged from their maternal dens, York explains, as the den roofs can collapse on them.

“And in places like East Greenland and Svalbard, where polar bears den in fairly steep terrain, it increases the risk of avalanches,” he adds. "One more threat that historically wasn't much of a thing.”

Spring remains the season of bounty for polar bears and much else in the Arctic. But until and unless we take drastic steps to reduce global warming, that bounty will diminish, and polar bear lives, already tough to begin with, will continue to get much harder.

Kieran Mulvaney is a freelance writer who has written extensively about polar bears and the Arctic for publications including National Geographic, The Guardian, and The Washington Post. A native of Bristol, England, he lives in Bristol, Vermont.