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For polar bears, it’s sur­vival of the fattest

pub­lished 21 March 2013 | mod­i­fied 08 March 2014

Polar bear hudsonbayOne of the most southerly pop­u­la­tions of polar bears in the world — and the best stud­ied — is strug­gling to cope with climate-​induced changes to sea ice, new research reveals. Based on over 10 years’ data the study, pub­lished in the British Eco­log­i­cal Society’s Jour­nal of Ani­mal Ecol­ogy on 20 March, sheds new light on how sea ice con­di­tions drive polar bears’ annual migra­tion on and off the ice.

Lead by Dr Seth Cherry of the Uni­ver­sity of Alberta, the team stud­ied polar bears in west­ern Hud­son Bay, where sea ice melts com­pletely each sum­mer and typ­i­cally re-​freezes from late Novem­ber to early Decem­ber. “This poses an inter­est­ing chal­lenge for a species that has evolved as a highly effi­cient preda­tor of ice-​associated seals,” he explains. “Because although polar bears are excel­lent swim­mers com­pared with other bear species, they use the sea ice to travel, hunt, mate and rest.”

Polar bears have adapted to the annual loss of sea ice by migrat­ing onto land each sum­mer. While there, they can­not hunt seals and must rely on fat reserves to see them through until the ice returns.

Climate-​induced changes that cause sea ice to melt ear­lier, form later, or both, likely affect the over­all health of polar bears in the area. Ulti­mately, for polar bears, it’s sur­vival of the fattest
Dr Seth Cherry, Uni­ver­sity of Alberta »

Dr Cherry and col­leagues wanted to dis­cover how ear­lier thaw­ing and later freez­ing of sea ice affects the bears’ migra­tion. “At first glance, sea ice may look like a bar­ren, uni­form envi­ron­ment, but in real­ity, it’s remark­ably com­plex and polar bears man­age to cope, and even thrive, in a habi­tat that moves beneath their feet and even dis­ap­pears for part of the year. This is an extra­or­di­nary bio­log­i­cal feat and biol­o­gist still don’t fully under­stand it,” he says.

From 199197 and 2004-​09, they mon­i­tored move­ments of 109 female polar bears fit­ted with satel­lite track­ing col­lars. They tagged only females because males’ necks are wider than their heads, so they can­not wear a col­lar. Dur­ing the same period, the team also mon­i­tored the posi­tion and con­cen­tra­tion of sea ice using satel­lite images.

“Defin­ing pre­cisely what aspects of sea ice break-​up and freeze-​up affect polar bear migra­tion, and when these con­di­tions occur, is a vital part of mon­i­tor­ing how poten­tial climate-​induced changes to sea ice freeze-​thaw cycles may affect the bears,” he says.

The results reveal the tim­ing of polar bears’ migra­tion can be pre­dicted by how fast the sea ice melts and freezes, and by when spe­cific sea ice con­cen­tra­tions occur within a given area of Hud­son Bay.

Accord­ing to Dr Cherry: “The data sug­gest that in recent years, polar bears are arriv­ing on shore ear­lier in the sum­mer and leav­ing later in the autumn. These are pre­cisely the kind of changes one would expect to see as a result of a warm­ing cli­mate and may help explain some other stud­ies that are show­ing declines in body con­di­tion and cub pro­duc­tion.”

Recent esti­mates put the west­ern Hud­son Bay polar bear pop­u­la­tion at around 900 indi­vid­u­als. The pop­u­la­tion has declined since the 1990s, as has the bears’ body con­di­tion and the num­ber of cubs sur­viv­ing to adult­hood.

Because polar bears’ main food source is seals, and these are hunted almost exclu­sively on sea ice, the longer bears spend on land, the longer they must go with­out energy-​rich seals. “Climate-​induced changes that cause sea ice to melt ear­lier, form later, or both, likely affect the over­all health of polar bears in the area. Ulti­mately, for polar bears, it’s sur­vival of the fat­test,” says Dr Cherry. He hopes the results will enable other sci­en­tists and wildlife man­agers to pre­dict how poten­tial climate-​induced changes to sea ice freeze-​thaw cycles will affect the ecol­ogy, par­tic­u­larly the migra­tion pat­terns, of this iconic species

(Source: British Eco­log­i­cal Soci­ety Press Release, 20.03.2013)

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