AboutZoos, Since 2008


Polar bears will starve due to loss of sea ice, in the end…

pub­lished 21 July 2015 | mod­i­fied 21 July 2015

Polar bear on pack ice of Arctic SeaPolar bears are unlikely to phys­i­o­log­i­cally com­pen­sate for extended food depri­va­tion asso­ci­ated with the ongo­ing loss of sea ice, accord­ing to one-​of-​its-​kind research con­ducted by Uni­ver­sity of Wyoming sci­en­tists and oth­ers, and pub­lished on 17 July in the jour­nal Science.

We found that polar bears appear unable to mean­ing­fully pro­long their reliance on stored energy, con­firm­ing their vul­ner­a­bil­ity to lost hunt­ing oppor­tu­ni­ties on the sea ice — even as they sur­prised us by also exhibit­ing an unusual abil­ity to min­i­mize heat loss while swim­ming in Arc­tic waters,” says John White­man, the UW doc­toral stu­dent who led the project.

The loss of sea ice in the Arc­tic, which is out­pac­ing pre­dic­tions, has raised con­cern about the future of polar bears, lead­ing to their list­ing as a glob­ally threat­ened species under the U.S. Endan­gered Species Act in 2008. The bears depend on hunt­ing seals on the sur­face of the sea ice over the con­ti­nen­tal shelf, most suc­cess­fully from April to July. In parts of the polar bears’ range, the length­en­ing period of sea ice retreat from shelf waters — caused by increas­ing tem­per­a­tures — can reduce their oppor­tu­ni­ties to hunt seals, lead­ing to declines in bear nutri­tional condition.

Some ear­lier research sug­gested that polar bears could, at least par­tially, com­pen­sate for longer sum­mer food depri­va­tion by enter­ing a state of low­ered activ­ity and reduced meta­bolic rate sim­i­lar to win­ter hiber­na­tion — a so-​called “walk­ing hiber­na­tion.” But the new research shows that the sum­mer activ­ity and body tem­per­a­ture of bears on shore and on ice were typ­i­cal of fast­ing, non-​hibernating mam­mals, with lit­tle indi­ca­tion of “walk­ing hibernation.”

White­man and his col­leagues con­cluded in the Sci­ence publication:

This sug­gests that bears are unlikely to avoid dele­te­ri­ous declines in body con­di­tion and, ulti­mately, sur­vival, that are expected with con­tin­ued ice loss and length­en­ing of the ice-​melt period.

The study

The researchers reached that con­clu­sion by cap­tur­ing more than two dozen polar bears, implant­ing tem­per­a­ture log­gers and track­ing their sub­se­quent move­ments on shore and on ice in the Arc­tic Ocean’s Beau­fort Sea, north of Alaska and Canada, dur­ing 20082010. The unprece­dented effort, logis­ti­cally sup­ported by the U.S. Geo­log­i­cal Sur­vey (USGS) and funded by the National Sci­ence Foun­da­tion, USGS, U.S. Fish and Wildlife Ser­vice (USFWS), as well as the Envi­ron­men­tal Pro­tec­tion Agency, required the assis­tance of numer­ous per­son­nel, mul­ti­ple heli­copters and deploy­ment of the U.S. Coast Guard ice-​breaker, the Polar Sea.

Many col­leagues — even some on our research team — doubted whether the study was pos­si­ble, until we actu­ally did it,” says Merav Ben-​David, the UW pro­fes­sor who devel­oped the research plan along with Pro­fes­sor Hank Har­low, an eco-​physiologist and col­league in the Depart­ment of Zool­ogy and Phys­i­ol­ogy, and Steve Amstrup, pre­vi­ously with the USGS and cur­rently the chief sci­en­tist at Polar Bears Inter­na­tional. “This project was logis­ti­cally so intense that it may never be replicated.”

Polar bear track­ing:

(Source: Uwyo Wyoming YouTube channel)

At the same time, the sci­en­tists found that polar bears use an unusual phys­i­o­log­i­cal response to avoid unsus­tain­able heat loss while swim­ming in the cold Arc­tic waters. To main­tain an inte­rior body tem­per­a­ture that allows them to sur­vive longer and, nowa­days, more fre­quent swims, the bears tem­porar­ily cool the out­er­most tis­sues of their core to form an insu­lat­ing shell — a phe­nom­e­non called regional heterothermy.

This regional het­erothermy may rep­re­sent an adap­ta­tion to long-​distance swims, although its lim­its remain unknown,” wrote the sci­en­tists who, in an ear­lier pub­li­ca­tion in 2011 in the jour­nal Polar Biol­ogy noted that one of the bears in the study sur­vived a nine-​day, 400-​mile swim from shore to ice. When recap­tured seven weeks later, the bear had lost 22 per­cent of her body mass, and her cub died.

By shed­ding light on poten­tial mech­a­nisms that facil­i­tated that bear’s sur­vival dur­ing her long swim, as well as the over­all metab­o­lism and activ­ity of bears, the cur­rent study “pro­foundly con­tributes to under­stand­ing the value of sum­mer habi­tats used by polar bears in terms of their ener­get­ics,” Har­low says. Amstrup adds, “It fills a gap in our oth­er­wise exten­sive knowl­edge of polar bear ecol­ogy and cor­rob­o­rates pre­vi­ous find­ings that the key to polar bear con­ser­va­tion is arrest­ing the decline of their sea ice habitat.”

(Source: Uni­ver­sity of Wyoming news release, 16.07.2015)

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