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Polar bear diet changes as sea ice melts

pub­lished 23 Jan­u­ary 2014 | mod­i­fied 19 April 2015

Three-​part study shows that some Hud­son Bay polar bears are switch­ing prey, mix­ing plant and ani­mal food sources as they sur­vive in chang­ing envi­ron­ment.

Polar bear hudsonbayA series of papers recently pub­lished by sci­en­tists at the Amer­i­can Museum of Nat­ural His­tory sug­gests that polar bears in the warm­ing Arc­tic are turn­ing to alter­nate food sources. As Arc­tic sea ice melts ear­lier and freezes later each year, polar bears have a lim­ited amount of time to hunt their his­tor­i­cally pre­ferred prey – ringed seal pups – and must spend more time on land. The new research indi­cates that at least some polar bears in the west­ern Hud­son Bay pop­u­la­tion are using flex­i­ble for­ag­ing strate­gies while on land, such as prey-​switching and eat­ing a mixed diet of plants and ani­mals, as they sur­vive in their rapidly chang­ing environment.

… we’re find­ing that they [polar bears] might be more resilient than is com­monly thought
Robert Rock­well, research asso­ciate Depart­ment of Ornithol­ogy, Amer­i­can Museum of Nat­ural History »

“There is lit­tle doubt that polar bears are very sus­cep­ti­ble as global cli­mate change con­tin­ues to dras­ti­cally alter the land­scape of the north­ern polar regions,” said Robert Rock­well. “But we’re find­ing that they might be more resilient than is com­monly thought.”

Polar bears are listed as a threat­ened species under the United States Endan­gered Species Act and are clas­si­fied as “Vul­ner­a­ble” with declin­ing pop­u­la­tions on the IUCN Red List of Threat­ened Species™. Cli­mate warm­ing is reduc­ing the avail­abil­ity of their ice habi­tat, espe­cially in the spring when polar bears gain most of their annual fat reserves by con­sum­ing seal pups before com­ing ashore for the sum­mer. The new work, led by Rock­well and Linda Gormezano, a post­doc­toral researcher in the Museum’s Divi­sion of Ver­te­brate Zool­ogy, exam­ines how polar bears might com­pen­sate for energy deficits from decreas­ing seal-​hunting opportunities.

In the first paper, pub­lished in the Sep­tem­ber 2013 issue of the jour­nal Polar Biol­ogy, the researchers pro­vide, for the first time, data and video of polar bears pur­su­ing, catch­ing, and eat­ing adult and juve­nile lesser snow geese dur­ing mid-​to-​late sum­mer, when the geese are replac­ing their pri­mary flight feathers.

In the sec­ond paper, pub­lished online on 28 August 2013 in the jour­nal Ecol­ogy and Evo­lu­tion, researchers used polar bear scat to show that the diet of at least some of the bears has shifted from what it was 40 years ago, before cli­mate change was affect­ing the Hud­son Bay low­lands. Today’s polar bears are prey­ing more on cari­bou as well as on snow geese and their eggs.

Research con­ducted at Spits­ber­gen showed sim­i­lar for­ag­ing devel­op­ment in polar bears. This video shows suc­ces­sive vis­its by polar bears to a bar­na­cle goose colony, west coast of Spits­ber­gen, Sval­bard. Bears walk from nest to nest to eat the eggs, leav­ing behind the upset nest own­ers. Other bird species that are pre­dated on this video are eider and glau­cous gull. The video sum­marises 20 hours of polar bears feed­ing in the colony through­out the 2012 breed­ing season:

(Video: Youtube; Author: Jouke Prop; The video speed is 5x nor­mal. The field work was by Jouke Prop, Eva Wolters, Tom van Spanje, Oebele Dijk and Thomas Oudman.)

In the final paper in the series, pub­lished on 21 Decem­ber 2013 in the jour­nal BMC Ecol­ogy, the researchers show that polar bears are, with a few excep­tions, con­sum­ing a mixed diet of plants and ani­mals. The pre­dom­i­nance of local veg­e­ta­tion in col­lected scat sug­gests lit­tle move­ment among habi­tat types between feed­ing ses­sions, indi­cat­ing that the polar bears are keep­ing energy expen­di­ture down.

Taken together, the research indi­cates that dur­ing the ice-​free period, polar bears are exhibit­ing flex­i­ble for­ag­ing behav­iour. This behav­iour likely derives from a shared genetic her­itage with brown bears, from which polar bears sep­a­rated about 600,000 years ago.

“For polar bear pop­u­la­tions to per­sist, changes in their for­ag­ing will need to keep pace with climate-​induced reduc­tion of sea ice from which the bears typ­i­cally hunt seals,” Gormezano said. “Although dif­fer­ent evo­lu­tion­ary path­ways could enable such per­sis­tence, the abil­ity to respond flex­i­bly to envi­ron­men­tal change, with­out requir­ing selec­tive alter­ations to under­ly­ing genetic archi­tec­ture, may be the most real­is­tic alter­na­tive in light of the fast pace at which envi­ron­men­tal changes are occur­ring. Our results sug­gest that some polar bears may pos­sess this flex­i­bil­ity and thus may be able to cope with rapidly chang­ing access to their his­toric food supply.”

Amer­i­can Museum of Nat­ural His­tory press release, 21.01.2014)

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