Supremely adapted to life on and in the Arctic Ocean, polar bears are now travelling on ever thinner ice.  In May 2006, the polar bear was listed as vulnerable to extinction, the first mammal to be listed due to the effects of global warming.

 

The disappearance of sea ice threatens both the bears and their favourite food, the ringed seal.  Ranging as much as 1000 kilometres north to south the bears follow patterns of ice melt and refreezing. Highly capable swimmers, one female was tracked swimming 100 kilometres continuously over twenty four hours last year, off the east coast of Svalbard.   When swimming, they use their back paws like rudders hanging behind the body while propelling themselves through icy seas with their powerful front paws at up to 10 kilometres an hour.  But excellent swimmers though they are, their preferred habitat is on top of the ice covered Arctic Ocean, where they hunt and mate.

 

Life is very different for males and females.  Males do not hibernate at all and wakefully prowl the Arctic wilderness all winter in search of food.  Pregnant females den in deep snow holes where the tiny, 600 - 700 g, blind cubs are born while their mother sleeps.  Twins are usual, but she can have up to four cubs. The cubs make mewling noises to stimulate their mother to produce milk, and knead her in much the same way as a cat on your lap.

 

Mum emerges from the winter den very hungry in the spring, ( late March or early April) to coincide with the pupping season of the animal’s favourite prey, ringed seals. These high fat, furry morsels are born in snow caves, often above ice ridges where the sea ice has fractured and refrozen.  The emaciated female bear must eat seal pups to restore the body fat and weight lost in feeding her young, and to regain condition to sustain herself.  The cubs are dependent on their mothers for two and a half years, so females only breed every three years.  This slow rate of reproduction makes the population vulnerable.  The breeding season for both ringed seals and polar bears is identical, the two species are highly interdependent and their future on the ice is linked.  The bears need it to hunt on, and the ringed seals need the ice to give birth, rest and moult.  Ringed seals give birth in snow-covered lairs on the sea ice and Brendan Kelly of the University of Alaska has been studying the impact of climate change on their seal pup survival rates. Young seal pups are born and nursed in the lairs.  ‘Early snowmelt is exposing ringed seal pups too early.  This increases their mortality rates from the cold; and from predators like polar bears, arctic fox and birds,’ Kelly explains. Breathing holes in the ice that the seals keep open all winter provide escape routes from hungry bears but the animals are particularly vulnerable when they haul up on the ice to change their fur in spring.

 

 

During the last  survey of bear populations in 1997, it was estimated that there are between 22000 and 27000 animals living around the edges of the permanent ice cap, from Russia to Alaska.  Almost 60% of the world’s population is found in Canada and the most southerly place where bears live all year round is at the same latitude as London; in James Bay, Canada.  Obviously the climate is a little different.  These most southern bears, and those in neighbouring Hudson’s Bay are most at risk due to climate change.  

The western Hudson’s Bay population has been studied continuously since 1981, by Ian Stirling, a scientist at the Canadian Wildlife Service.  His research shows a 17% decline in bear numbers during the last 10 years. ‘Ice is melting there about three weeks earlier than it did 30 years ago,’ says Stirling.  The earlier ice breakup cuts short the prime seal hunting season, critical for the bears to gain condition. ‘For a polar bear, not all weeks are created equal, they are losing three weeks at the best time of the year for feeding on the ice, when seal pups are abundant and bears put on fat that they store for the four lean months that they have to live onshore.’

The absence of ice, and the chance to hunt, means that these bears weigh on average about 15% less than they did 30 years ago.  For an animal that depends on body fat for both insulation, and for females, reserves to convert to milk for growing cubs, this is affecting survival rates. The effects of climate change on polar bear habitat and in particular sea ice formation on this most southerly population gives us strong clues as to the likely future for the rest of the animals. Climate change is happening too fast for the polar bear to adapt, and they don't have time to evolve