Polar Bears Won't Survive on Birds and Berries
As sea ice declines, some polar bears are foraging for food on land. That won't be enough, says a pair of new studies.
As Arctic sea ice declines as a result of climate change, polar bears -- which traverse sea ice in search of seals, their primary prey -- are in some parts of their range being forced to forage for food on land.
Polar bears have long been known to eat the likes of berries, kelp and birds' eggs to supplement their diet, generally while waiting for sea ice to form in fall or after coming ashore when the ice breaks up in late spring. Now, there's evidence such behavior is becoming more common. Indeed, some bears may be going to extra lengths to procure the precious calories that a nest of bird eggs may provide: In 2010, scientists reported polar bears climbing rock cliff ledges to eat murre eggs in Canada; and the following year, tourists in the Russian Arctic saw a bear clambering down a cliff populated by guillemots, presumably with the same goal in mind.
However, although there has been some speculation that these alternative food sources may mitigate against the impacts of sea ice loss, most researchers have been far more cautious in reaching such conclusions; and a pair of new papers suggests that such caution is merited.
In one, published in the journal Global Change Biology , scientists worked with local Inuit to study the interaction between polar bears and common eiders on Baffin Island. "Inuit are very in tune to this type of environmental change because they hunt eiders for meat, and collect eggs and down from their nests," said study lead author Cody Dey of the Great Lakes Institute for Environmental Research at the University of Windsor in Ontario. However, that traditional hunting may be at risk, Dey and colleagues wrote, because "our research suggests that eiders might nest in different locations to avoid polar bear predation, which could make it harder for local people to harvest eiders."
Dey and his co-authors further found that polar bear predation of eider nests is likely to increase over the next 25 years, and that a large percentage of the nests will be consumed by polar bears in years with earlier sea ice melt.
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However, despite that, Baffin Bay's polar bears will continue to get skinnier.
"When the sea ice is melted, polar bears are forced to fast until it freezes up again," said Dey. "Even if bears are eating eider eggs, they probably can't meet their caloric demand because eider eggs are relatively low in calories (compared to seals). Our models suggest that polar bears can't compensate for the loss of ice-based hunting opportunities by consuming eider eggs."
Another study, in the journal Physiological and Biochemical Zoology, paints, if anything, an even starker picture. Between 2009 and 2014, researchers weighed 142 polar bears that had wandered into the "polar bear capital" of Churchill, Manitoba and were captured and transferred to the town's bear holding facility, colloquially known as the "polar bear jail." The bears were given water but not food, to discourage them from returning to town once they were released.
During their stay, the bears lost about two pounds of body mass a day -- which, said lead author Andrew Derocher of the University of Alberta, is pretty much the same as the weight loss experienced by bears that forage on berries, kelp and goose eggs during the ice-free season along the coast of Hudson Bay. In other words, bears that gain their nutrition from land alone are not demonstrably better off than those that are forced to fast.
"It's not exactly a good news story for the bears," Derocher told the Times Colonist newspaper. "There's always been the question of what could those bears be getting from feeding out there. And, it turns out, not very much."
See Photos: Royal Society Selects Best Animal Photos
Photographer Imre Potyó captured this image of Danube mayflies engaged in a courtship dance. The photo is the overall winner of the Royal Society competition. "At the beginning, females and males fly above the water surface where they copulate," Potyó said. "After that the females begin their upstream-directed compensatory flight, which ends when they deposit their eggs onto the water surface. This shot captures the fantastic energy and chaos of the mayflies' dance and the mood of the night time too."
Credit: Imre Potyó, Royal Society
The winner of the Ecology and Environmental Science category is this photo of a solitary juvenile clown fish (Amphiprion bicinctus) seeking shelter in a bleached bubble-tip anemone in the Red Sea. The photo was snapped following a global bleaching event that has decimated coral reefs worldwide this year. "The lone fish seems like a timely analogy for a generation that may grow up in a bleak future," said photographer Tane Sinclair-Taylor, "without the colorful and diverse coral reefs that we have today."
Credit: Tane Sinclair-Taylor, Royal Society
The winner of the Micro-Imaging category is this photo of an activated carbon grain, which looks like an alien landscape. Photographer María Carbajo Sánchez magnified the grain 5,000 times with an electron microscope to obtain such a close-up view of the tiny object. The source of the carbon was a nutshell.
Credit: María Carbajo Sánchez, Royal Society
An eagle ray swimming over the reef with its prey was the subject of the Evolutionary Biology category winner. "Eagle rays have evolved very long tails, but this is the longest that I have ever seen," said photographer Nick Robertson-Brown.
Credit: Nick Robertson-Brown, Royal Society
Shooting from the inside of a large mammal carcass with the help of a long trigger wire, photographer Jonathan Diaz-Marba captured the moment when a griffon vulture searched inside the dead animal's ribcage. The photo is a runner up in the Behavior category. "My fear was that these huge birds could vandalize the expensive photographic equipment, but I had to take the risk," Diaz-Marba said.
The birds nest in colonies on cliffs undisturbed by humans, and they fly over huge open areas searching for food. "I chose an area with many magpies, whose presence gives the vultures a good cue of where to feed," he said.
Credit: Jonathan Diaz-Marba, Royal Society
This image of a trainworm (Myrianida pinnigera) took runner up in the Evolutionary Biology category. "Its front end, the trainworm's engine, is followed by a row of carriages called 'stolons' that increase in size towards the worm's tail end," said Frederik Pleijel, who took the photo. "The carriages are the worm's swimming sexual organs."
"When the trainworm is mature, the last carriage in the train lets go and detaches," he said. "It swims up the water column to reproduce."
Credit: Frederik Pleijel, Royal Society
Photographer Tegwen Gadais captured "Gentoo penguins seemingly 'decorating' their nest with guano" in this photo, taken on the island of South Georgia in the southern Pacific.
"Once the eggs have been laid, each parent will take turns incubating them, relieving themselves by lifting their tails away from the nest and creating the long streaks seen in the picture," Gadais said.
The photo was a runner up in the Ecology and Environmental Science category.
Credit: Tegwen Gadais
A runner up in the Micro-Imaging category, this photo shows an African house snake (Boaedon fuliginosus) one day after its mother laid her eggs. Photographer Tyler Square noticed that many of the features at this early developmental stage, such as muscle segments and a chambered heart, are shared with other animals.
Credit: Tyler Square, Royal Society
Looking more like jellyfish or subjects of an Andy Warhol painting, the objects in this special commendation photo are actually carbon nanotubes grown in a pillar formation. "The metal disks that make up the jellyfish bodies are made by 'sputtering' charged aluminum and iron ions onto a surface to deposit a thin film of the metals," said photographer Clare Collins.
Credit: Clare Collins, Royal Society
A special commendation went to this human-like photo of Japanese macaques huddling to stay warm during the winter. "When they huddle in these small groups it's called in Japanese saru-dango; saru means 'monkey' and dango is a skewer of Japanese sweet dumpling made from rice flour," photographer Alexandre Bonnefoy said. "These groups are composed only of members of the same family."
"This behavior is not observed everywhere in Japan, but only in (a) few groups," he said. "It's a cultural behavior peculiar to the monkeys in Shodoshima and in Nagano, where this photo was taken, where there is a hot spring which the monkeys bathe in."
Credit: Alexandre Bonnefoy
A special commendation also went to this photo of colorful butterflies gathering on the head of a caiman. "A number of minerals are a scarce resource throughout Amazonia," said Mark Cowan, who took the photo. The behavior allows insects like butterflies to have access to salt, even if it is on the head of a large predatory reptile.
"This particular phenomenon where butterflies and bees congregate on the heads and around the eyes of caimans and turtles has been documented before," he added, "but what is unique here is the simultaneous number of butterfly species and the way in which each species is associated with its own kind."
Credit: Mark Cowan, Royal Society
The winner of a special commendation, publisher's choice, goes to this photo of a superb fan-throated lizard (Sarada superba) native to the northern Western Ghats of India. "About two decades ago, a large part of this plateau was converted into one of Asia's largest wind farms," photographer Prasenjeet Yadav said. "This has resulted in drastic changes in the ecology of this charismatic lizard species."
The lone lizard appears to be contemplating the seemingly inescapable human-caused changes to its habitat.
Credit: Prasenjeet Yadav, Royal Society