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Polar Bear - Ursus maritimus
Polar bears finding it harder to catch enough seals to meet energy demands
Study reveals high metabolic rates, meaning polar bears need a lot of fat-rich prey, but more than half of those studied were running an energy deficit

Date: February 1, 2018
Source: University of California - Santa Cruz

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This is an adult female polar bear on the sea ice wearing a GPS satellite video-camera collar. GPS video-camera collars were applied to solitary adult female polar bears for 8 to 12 days in April, 2014-2016. These collars enabled researchers to understand the movements, behaviors, and foraging success of polar bears on the sea ice.
Credit: Anthony Pagano, USGS

A new study finds polar bears in the wild have higher metabolic rates than previously thought, and as climate change alters their environment a growing number of bears are unable to catch enough prey to meet their energy needs.

The study, published February 2 in Science, reveals the physiological mechanisms behind observed declines in polar bear populations, said first author Anthony Pagano, a Ph.D. candidate at UC Santa Cruz.

"We've been documenting declines in polar bear survival rates, body condition, and population numbers over the past decade," he said. "This study identifies the mechanisms that are driving those declines by looking at the actual energy needs of polar bears and how often they're able to catch seals."

Pagano, who is also a wildlife biologist with the U.S. Geological Survey (USGS), conducted the study as part of his Ph.D. thesis research at UC Santa Cruz, where he has been working with coauthors Terrie Williams and Daniel Costa, both professors of ecology and evolutionary biology.

The researchers monitored the behavior, hunting success, and metabolic rates of adult female polar bears without cubs as they hunted for prey on the sea ice of the Beaufort Sea in the spring. High-tech collars on the bears recorded video, locations, and activity levels over a period of eight to 11 days, while metabolic tracers enabled the team to determine how much energy the bears expended.

The field metabolic rates they measured averaged more than 50 percent higher than previous studies had predicted. Five of the nine bears in the study lost body mass, meaning they weren't catching enough fat-rich marine mammal prey to meet their energy demands.

"This was at the start of the period from April through July when polar bears catch most of their prey and put on most of the body fat they need to sustain them throughout the year," Pagano said.

Climate change is having dramatic effects on the Arctic sea ice, forcing polar bears to move greater distances and making it harder for them to catch prey. In the Beaufort Sea, sea ice starts to retreat away from the continental shelf in July, and most of the bears move north on the ice as it retreats. As the Arctic warms and more sea ice melts, the bears are having to move much greater distances than previously. This causes them to expend more energy during the summer, when they are fasting until the ice returns to the continental shelf in the fall.

In other areas, such as Hudson Bay, most bears move onto land when the sea ice retreats. There, Arctic warming means the sea ice is breaking up earlier in the summer and returning later in the fall, forcing bears to spend more time on land.

"Either way, it's an issue of how much fat they can put on before the ice starts to break up, and then how much energy are they having to expend," Pagano said.

Previous studies had tried to estimate polar bear metabolic rates and energy expenditures based on some assumptions about their behavior and physiology. For example, since polar bears are primarily "sit and wait" hunters, it was thought this would minimize their energy expenditure during hunting. Researchers also speculated that polar bears could lower their metabolic rate to save energy if they were not successful catching seals, Pagano said.

"We found that polar bears actually have much higher energy demands than predicted. They need to be catching a lot of seals," he said.

In the spring, polar bears are mostly preying on recently weaned ringed seals, which are more susceptible to being caught than adult seals. By the fall, the young seals are older and wiser, and polar bears are not able to catch as many. "It's thought that bears might catch a couple per month in the fall, compared to five to 10 per month in the spring and early summer," Pagano said.

USGS researchers have been studying polar bears in the Beaufort Sea area since the 1980s. Their most recent population estimate indicates the polar bear population has declined by about 40 percent over the past decade. It has been difficult, however, for researchers to study the fundamental biology and behavior of polar bears in this very remote and harsh environment, Pagano said.

"We now have the technology to learn how they are moving on the ice, their activity patterns, and their energy needs, so we can better understand the implications of these changes we are seeing in the sea ice," he said.

In addition to Pagano, Williams, and Costa, the coauthors of the paper include USGS researchers George Durner, Karyn Rode, Todd Atwood, and Elizabeth Peacock; Stephen Atkinson of Dugald, Manitoba; and Megan Owen at the San Diego Zoo Institute for Conservation Research. This work was supported by the USGS Changing Arctic Ecosystems Initiative.

Story Source: University of California - Santa Cruz. "Polar bears finding it harder to catch enough seals to meet energy demands: Study reveals high metabolic rates, meaning polar bears need a lot of fat-rich prey, but more than half of those studied were running an energy deficit." ScienceDaily. (accessed February 2, 2018).

Journal Reference:
A. M. Pagano, G. M. Durner, K. D. Rode, T. C. Atwood, S. N. Atkinson, E. Peacock, D. P. Costa, M. A. Owen, T. M. Williams. High-energy, high-fat lifestyle challenges an Arctic apex predator, the polar bear. Science, 2018; 359 (6375): 568 DOI: 10.1126/science.aan8677

Regional declines in polar bear (Ursus maritimus) populations have been attributed to changing sea ice conditions, but with limited information on the causative mechanisms. By simultaneously measuring field metabolic rates, daily activity patterns, body condition, and foraging success of polar bears moving on the spring sea ice, we found that high metabolic rates (1.6 times greater than previously assumed) coupled with low intake of fat-rich marine mammal prey resulted in an energy deficit for more than half of the bears examined. Activity and movement on the sea ice strongly influenced metabolic demands. Consequently, increases in mobility resulting from ongoing and forecasted declines in and fragmentation of sea ice are likely to increase energy demands and may be an important factor explaining observed declines in body condition and survival. 
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Walking is more efficient than thought for threatened polar bears

June 19, 2018, The Company of Biologists

[Image: polarbear.jpg]
Polar Bear (Ursus maritimus) near Kaktovik, Barter Island, Alaska. Credit: Alan Wilson/Wikipedia.

A polar bear plunges into the icy Arctic waters in search of firmer ice; its world, which was once a sea of white, is melting beneath its paws. 'Research has documented declines in polar bear populations in some regions of the Arctic', says Anthony Pagano from the US Geological Survey, explaining that the bears now have to roam further on the receding ice to locate the seals upon which they dine. And, to make their predicament worse, measurements in the 1970s and 1980s suggested that polar bears consume more energy than other similarly sized animals because they have to generate heat to remain warm in the frigid environment and walk long distances to catch food. Knowing how much energy polar bears use just to remain alive is essential if we are to understand how the animals will survive in their dwindling environment, so Pagano and colleague Terrie Williams from the University of California, Santa Cruz, embarked on an ambitious programme of measuring how much energy polar and grizzly bears consume as they amble along. The scientists publish their discovery that polar bears and grizzly bears walk efficiently, consuming the same amount of energy while walking as other large animals, in Journal of Experimental Biology.

'Our conversations with zoos for this study started in 2012', says Pagano, recalling how he and Williams contacted Amy Cutting, Nicole Nicassio-Hiskey and Amy Hash at Oregon Zoo, Portland, USA, and Megan Owen, Tammy Batson and Nate Wagner at San Diego Zoo, USA, as both teams had trained polar bears to participate in husbandry procedures such as providing blood samples for health test. However, Pagano and Williams wanted to measure how much oxygen the 240kg animals consumed to calculate how much energy they were using while walking and the conventional method of placing a mask over the bear's muzzle would not work: 'Big carnivores do not like things on their faces', Williams explains. Instead, Charlie Robbins and Tony Carnahan from Washington State University, USA, built a custom-designed bear-proof metabolic chamber by installing a 3.6m long horse treadmill in a steel-framed chamber constructed from bullet-proof polycarbonate.

The team then transported the 2000kg structure to the polar bears' respective locations, where Nicassio-Hiskey and Hash (Portland) and Batson, Owen and Wagner (San Diego) spent months patiently training the animals to walk on the treadmill. Recalling this period, Pagano says, 'Finding foods that the polar bears would be highly motivated to walk for was challenging'. However, the grizzly bears at Washington State University, USA, were more eager: 'They just bowled right in; they did not care if the treadmill moved fast or slow, all they cared about were the training treats', laughs Williams. Once the bears were comfortable with walking in the metabolic chamber, the team begin measuring the animals' oxygen consumption while filming and recording their movements.

However, when they calculated the amount of energy consumed by the polar bears and grizzlies while sauntering at speeds of up to 4.6km/h, they were surprised that the two species consumed the same amount of energy (2.21kJ/kg m) and no more than similarly sized animals. The polar bears' walking metabolic rate was not intrinsically higher than that of other large mammals, but the team suspect that swimming could be more costly. And when they fitted GPS collars to six wild female polar bears on the Alaskan sea ice, it was clear that they were moving at similar speeds to the captive animals, ambling at around 3.4km/h and rarely breaking into a run, so their movements were as efficient. However, the news wasn't all good: simply standing up was more costly for both species than it is for other large animals, which could impact polar bears detrimentally as their survival teeters on thin ice.

Journal Reference:
Pagano, A. M., Carnahan, A. M., Robbins, C. T., Owen, M. A., Batson, T., Wagner, N., Cutting, A., Nicassio-Hiskey, N., Hash, A. and Williams, T. M. (2018). Energetic costs of locomotion in bears: is plantigrade locomotion energetically economical? J. Exp. Biol. 221, DOI: 10.1242/jeb.175372

Ursids are the largest mammals to retain a plantigrade posture. This primitive posture has been proposed to result in reduced locomotor speed and economy relative to digitigrade and unguligrade species, particularly at high speeds. Previous energetics research on polar bears (Ursus maritimus) found locomotor costs were more than double predictions for similarly sized quadrupedal mammals, which could be a result of their plantigrade posture or due to adaptations to their Arctic marine existence. To evaluate whether polar bears are representative of terrestrial ursids or distinctly uneconomical walkers, this study measured the mass-specific metabolism, overall dynamic body acceleration, and gait kinematics of polar bears and grizzly bears (Ursus arctos) trained to rest and walk on a treadmill. At routine walking speeds, we found polar bears and grizzly bears exhibited similar costs of locomotion and gait kinematics, but differing measures of overall dynamic body acceleration. Minimum cost of transport while walking in the two species (2.21 J kg−1 m−1) was comparable to predictions for similarly sized quadrupedal mammals, but these costs doubled (4.42 J kg−1 m−1) at speeds ≥5.4 km h−1. Similar to humans, another large plantigrade mammal, bears appear to exhibit a greater economy while moving at slow speeds. 
[Image: wildcat10-CougarHuntingDeer.jpg]
Polar bears gorged on whales to survive past warm periods; won't suffice as climate warms

Date:  October 9, 2018
Source:  University of Washington
A new study found that while dead whales are valuable sources of fat and protein for some polar bears, this resource will likely not be enough to sustain most bear populations in the future when the Arctic becomes ice-free in summers.

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Polar bears are shown scavenging on the carcass of a dead bowhead whale that washed ashore on Wrangel Island, Russia.
Credit: Chris Collins/Heritage Expeditions

Polar bears likely survived past warm periods in the Arctic, when sea ice cover was low, by scavenging on the carcasses of stranded large whales. This food source sustained the bears when they were largely restricted to land, unable to roam the ice in search of seals to hunt.
A new study led by the University of Washington found that although dead whales are still valuable sources of fat and protein for some polar bears, this resource will likely not be enough to sustain most bear populations in the future when the Arctic becomes ice-free in summers, which is likely to occur by 2040 due to climate change. The results were published online Oct. 9 in the journal Frontiers in Ecology and the Environment.
"If the rate of sea ice loss and warming continues unmitigated, what is going to happen to polar bear habitat will exceed anything documented over the last million years. The extremely rapid pace of this change makes it almost impossible for us to use history to predict the future," said lead author Kristin Laidre, a marine biologist at the UW's Polar Science Center and associate professor in the School of Aquatic and Fishery Sciences.
Polar bears need sea ice to survive because it is an essential platform for hunting seals, their main food source. They travel over the ice, searching for breathing holes or seal birth dens. When the ice breaks up in late spring, polar bears in some populations will fast on land, waiting for the ice to re-form so they can resume hunting.
Still, polar bears are opportunistic feeders and have been observed in multiple locations eating the carcasses of whales that died at sea and washed ashore. The bears can quickly consume and store large amounts of fat, which works in their favor. In some cases, between 40 and 60 different polar bears have been observed feeding on large bowhead and gray whale carcasses and, in 2017, more than 180 bears were seen scavenging on a single dead bowhead whale. Individual bears frequently return to the same carcass over multiple years.
The authors drew upon years of observations in the field to assess the potential importance of whale carcasses and how they might help polar bears survive an ice-free Arctic. It is clear that polar bears persisted through low-ice interglacial periods in the past that resulted from naturally occurring climate cycles. The researchers hypothesized that, to a significant degree, the bears likely survived by scavenging on whale carcasses, storing large amounts of fats when hunting seals was not an option.
"I think this is likely one of the most probable explanations for how polar bears made it through previous warm interglacial periods," said co-author Ian Stirling, former research scientist with the Canadian Department of Environment and an adjunct professor at the University of Alberta, who has studied polar bears for 45 years.
"But when we look at the situation now, ecologically, with respect to food sources, it's a very different picture," Stirling added. "The potential of whale carcasses to bail bears out may still be important in a few areas but, quite simply, their overall availability is going to be substantially less than before humans invaded the Arctic."
The researchers wanted to determine whether enough large whales dying and washing ashore each year could replace seals as a food source for polar bears in some areas. They first calculated how much blubber and meat an average population of 1,000 polar bears would need as a food source each year.
Then, they looked at the abundance of gray and bowhead whale populations -- focusing on the coasts of Chukotka and Alaska -- and estimated the number of potential strandings, factoring in that about 10 percent of whales that die will float to the surface, and only some of those end up on land that is accessible to bears.
Their analysis found that during ice-free summer months, a hypothetical population of 1,000 polar bears would need to eat about eight whales, and during the springtime feast when bears eat more, about 20 whales would be needed to satisfy the same 1,000 bears. In the Chukchi Sea, long-term data collected in Russia indicate that enough whales die and float to shore each year to potentially meet this need, the authors found.
But feeding on dead whales, while possibly critical in historical times, seems unlikely to help most polar bear populations survive a rapidly warming Arctic. The Arctic is home to 19 subpopulations of polar bears, but not every region sees large whales strand and die as regularly as the Chukchi Sea. Additionally, though whale carcasses likely helped polar bears survive in past low-ice periods, the Arctic landscape has changed drastically since then. Present-day whale populations are much smaller due to past human exploitation, and recent human activity in the region such as shipping, coastal communities and offshore industrial activity can further impact polar bears, whales, and the ability of bears to make use of whale carcasses.
"Scavenging on large whale carcasses is probably important for bears in some areas and may buffer them from sea ice loss," Laidre said. "However, carcasses of large whales are not expected to replace seals as nutritional resources as we move towards an ice-free Arctic. In most regions, the environmental changes are too large and the whale carcasses are too few."

Story Source: University of Washington. "Polar bears gorged on whales to survive past warm periods; won't suffice as climate warms." ScienceDaily. (accessed October 9, 2018).

Journal Reference:
  1. Kristin L Laidre, Ian Stirling, James A Estes, Anatoly Kochnev, Jason Roberts. Historical and potential future importance of large whales as food for polar bears. Frontiers in Ecology and the Environment, 2018; DOI: 10.1002/fee.1963
Polar bears (Ursus maritimus) are expected to be adversely impacted by a warming Arctic due to melting of the sea‐ice platform from which they hunt ice‐breeding seals. We evaluated the hypothesis that scavenging on stranded large whale carcasses may have facilitated polar bear survival through past interglacial periods during which sea‐ice was limited by analyzing: (1) present‐day scavenging by polar bears on large whale carcasses; (2) energy values of large whale species; and (3) the ability of polar bears, like the brown bears (Ursus arctos) from which they evolved, to quickly store large amounts of lipids and to fast for extended periods. We concluded that scavenging on large whale carcasses likely facilitated survival of polar bears in past interglacial periods when access to seals was reduced. In a future, ice‐impoverished Arctic, whale carcasses are less likely to provide nutritional refuge for polar bears because overharvesting by humans has greatly reduced large whale populations, carcass availability is geographically limited, and climate‐induced sea‐ice loss is projected to occur at a more rapid pace than polar bears have experienced at any previous time in their evolutionary history.

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  • Claudiu Constantin Nicolaescu
First tally of US-Russia polar bears finds a healthy population

November 14, 2018, University of Washington

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The striped area on the left shows the Chukchi Sea polar bear subpopulation's range. Sea ice reaches south to the dotted line in winter, and retreats to the solid black line in summer. The right shows a closeup of the study area off Alaska's coast. White circles show where polar bears were tagged between 2008 and 2016. Credit: Regehr et al./Scientific Reports

Not all polar bears are in the same dire situation due to retreating sea ice, at least not right now. Off the western coast of Alaska, the Chukchi Sea is rich in marine life, but the number of polar bears in the area had never been counted. The first formal study of this population suggests that it's been healthy and relatively abundant in recent years, numbering about 3,000 animals.

The study by researchers at the University of Washington and federal agencies is published Nov. 14 in Scientific Reports, an open-access journal from the Nature Publishing Group.

"This work represents a decade of research that gives us a first estimate of the abundance and status of the Chukchi Sea subpopulation," said first author Eric Regehr, a researcher with the UW's Polar Science Center who started the project as a biologist in Alaska with the U.S. Fish and Wildlife Service. "Despite having about one month less time on preferred sea ice habitats to hunt compared with 25 years ago, we found that the Chukchi Sea subpopulation was doing well from 2008 to 2016.

"Sea-ice loss due to climate change remains the primary threat to the species but, as this study shows, there is variation in when and where the effects of sea-ice loss appear. Some subpopulations are already declining while others are still doing OK."

Of the world's 19 subpopulations of polar bears, the U.S. shares two with neighboring countries. The other U.S. subpopulation—the southern Beaufort Sea polar bears, whose territory overlaps with Canada—is showing signs of stress.

"The southern Beaufort Sea subpopulation is well-studied, and a growing body of evidence suggests it's doing poorly due to sea-ice loss," Regehr said.

Polar bears were listed as threatened under the U.S. Endangered Species Act in 2008 due to the threat of declining sea ice, which the animals depend on for hunting, breeding and traveling. But the new study suggests that such effects are not yet visible for bears in the remote waters that separate Alaska from Chukotka, Russia.

"It's a very rich area. Most of the Chukchi Sea is shallow, with nutrient-rich waters coming up from the Pacific. This translates into high biological productivity and, importantly for the polar bears, a lot of seals," Regehr said.

Previous studies by the State of Alaska show that ringed and bearded seals have maintained good nutritional condition and reproduction in the Chukchi Sea region, Regehr said.

"Just flying around, it's night and day in terms of how many seals and other animals you see," Regehr said. "The Chukchi Sea is super productive and the Beaufort Sea is less so."

The area also has an abundance of whale traffic, and when carcasses wash up on shore the polar bears can feed on them in summer, when the sea ice melts and a portion of the Chukchi Sea subpopulation waits on land for the ocean to refreeze.

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Hundreds of Chukchi Sea polar bears spend the ice-free months on Wrangel Island, including this adult female and cub photographed in fall 2017. Credit: Eric Regehr/University of Washington

Recent ecological observations had suggested that Chukchi Sea bears are doing well. A study led by co-author Karyn Rode, at the U.S. Geological Survey, showed the top predators have similar amounts of body fat as 25 years ago, a good indicator of their overall health.

The current study is the first assessment of the subpopulation size using modern methods. It estimates just under 3,000 animals, with generally good reproductive rates and cub survival.

As a federal wildlife biologist based in Alaska until 2017, Regehr and colleagues gathered the data by tagging roughly 60 polar bears in most years from 2008 to 2016. He flew by helicopter over the area just north of Alaska's Seward Peninsula, looking for tracks on the sea ice. The helicopter then would follow tracks in the snow to locate the bear and use a tranquilizer dart to sedate the animal. Over about an hour, researchers would weigh the bear, collect biological samples, apply individual tags and, in some cases, attach a GPS transmitter.

All the data were incorporated into a new model designed to estimate population size for large carnivores that are highly mobile and whose territory spans a large region. The authors made the model publicly available in the hope that it will be used on other populations or species.

"Polar bears can travel thousands of miles in a year. But with the GPS tags, we can see when a bear leaves our study area but is still alive, because it's moving. This information is key because there are bears that we see once and never see again, and to get a good population estimate you need to know if these animals died or just moved to a new area," Regehr said.

For the first time, the model also considered local and traditional ecological knowledge collected by the North Slope Borough of Alaska from Native hunters and community members who have generations of experience with polar bears.

"It was important to bring our science together with the observations and expertise of people who live in polar bear country year-round and understand the animals in different ways," Regehr said.

A joint U.S.-Russia commission is responsible for management and conservation of the Chukchi Sea subpopulation. In response to the new assessment, the commission raised the sustainable level of subsistence harvest, which is nutritionally and culturally important to Native people in Alaska and Chukotka.

"These polar bears move back and forth between the U.S. and Russia, so it's very much a shared resource," said Regehr, a longtime scientific advisor to that commission.

As part of his current research at the UW, Regehr spends about two months each year working with Russian scientists on Wrangel Island in the Arctic Ocean, where hundreds of Chukchi Sea polar bears spend their summers.

"These findings are good news for now, but it doesn't mean that bears in the Chukchi Sea won't be affected by ice loss eventually," Regehr said. "Polar bears need ice to hunt seals, and the ice is projected to decline until the underlying problem of climate change is addressed."

Journal Reference:
Eric V. Regehr et al, Integrated Population Modeling Provides the First Empirical Estimates of Vital Rates and Abundance for Polar Bears in the Chukchi Sea, Scientific Reports (2018). DOI: 10.1038/s41598-018-34824-7

Large carnivores are imperiled globally, and characteristics making them vulnerable to extinction (e.g., low densities and expansive ranges) also make it difficult to estimate demographic parameters needed for management. Here we develop an integrated population model to analyze capture-recapture, radiotelemetry, and count data for the Chukchi Sea subpopulation of polar bears (Ursus maritimus), 2008–2016. Our model addressed several challenges in capture-recapture studies for polar bears by including a multievent structure reflecting location and life history states, while accommodating state uncertainty. Female breeding probability was 0.83 (95% credible interval [CRI] = 0.71–0.90), with litter sizes of 2.18 (95% CRI = 1.71–2.82) for age-zero and 1.61 (95% CRI = 1.46–1.80) for age-one cubs. Total adult survival was 0.90 (95% CRI = 0.86–0.92) for females and 0.89 (95% CRI = 0.83–0.93) for males. Spring on-ice densities west of Alaska were 0.0030 bears/km2 (95% CRI = 0.0016–0.0060), similar to 1980s-era density estimates although methodological differences complicate comparison. Abundance of the Chukchi Sea subpopulation, derived by extrapolating density from the study area using a spatially-explicit habitat metric, was 2,937 bears (95% CRI = 1,552–5,944). Our findings are consistent with other lines of evidence suggesting the Chukchi Sea subpopulation has been productive in recent years, although it is uncertain how long this will continue given sea-ice loss due to climate change.
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  • Claudiu Constantin Nicolaescu
Quote:Garry Brown in his book Great Bear Almanac (1993) is citing a story from the 1991 book No Room for Bears by Frank Dufresne. The following is the original Dufresne's account:

Nanook fight anything when hungry," declared Pooshuk, and grinned as another example came to mind. Once while hunting bowhead whales in the crumbling June ice with brass harpoon gun, he and his oomiak crew members had seen a polar bear jump on the back of a surfacing whale, go down with it, and come up again still trying to bite a mouthful of blubber off the forty-ton behemoth

Dufresne Bonus: Because they had no choice but to hold their compass course, Jim and his seven Eskimos plodded steadily toward the things barring their way to Point Barrow until they'd narrowed the distance enough to make positive identification. "The dark object was not a seal," stated Trader Jim. "It was the carcass of a huge bowhead whale foundered a quarter mile off the beach line. The white creatures were bears. We counted more than a hundred of them tearing chunks of blubber off the whale. Some were lying around on the ice sleeping off their blubber jags; some were walking along the whale's back; some others had eaten a hole into the belly of the whale big enough so they were walking inside and coming out the whale's mouth." Jim shook his head as if to clarify the scene. "It was the damndest sight you ever saw!"

Credited to brobear who helped me find this account.
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  • Claudiu Constantin Nicolaescu
Polar bears (Ursus Maritimus), the most evolutionary advanced hibernators, avoid significant bone loss during hibernation

Quote:Some hibernating animals are known to reduce muscle and bone loss associated with mechanical unloading during prolonged immobilisation,compared to humans. However, here we show that wild pregnant polar bears (Ursus maritimus) are the first known animals to avoid significant bone loss altogether, despite six months of continuous hibernation. Using serum biochemical markers of bone turnover, we showed that concentrations for bone resorption are not significantly increased as a consequence of hibernation in wild polar bears. This is in sharp contrast to previous studies on other hibernating species, where for example, black bears (Ursus americanus), show a 3-4 fold increase in serum bone resorption concentrations posthibernation,and must compensate for this loss through rapid bone recovery on remobilisation, to avoid the risk of fracture. In further contrast to black bears, serum concentrations of bone formation markers were highly significantly increased in pregnant female polar bears compared to non-pregnant,thus non-hibernating females both prior to and after hibernation. However, bone formation concentrations in new mothers were significantly reduced compared to pre-hibernation concentrations. The de-coupling of bone turnover in favour of bone formation prior to hibernation, suggests that wild polar bears may posses a unique physiological mechanism for building bone in protective preparation against expected osteopenia associated with disuse,starvation, and hormonal drives to mobilise calcium for reproduction, during hibernation. Understanding this physiological mechanism could have profound implications for a natural solution for the prevention of osteoporosis in animals subjected to captivity with inadequate space for exercise,humans subjected to prolonged bed rest while recovering from illness, or astronauts exposed to antigravity during spaceflight.© 2008 Elsevier Inc. All rights reserved.
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  • Claudiu Constantin Nicolaescu, Taipan
Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat
Polar bears (Ursus maritimus) prefer to live on Arctic sea ice but may swim between ice floes or between sea ice and land. Although anecdotal observations suggest that polar bears are capable of swimming long distances, no data have been available to describe in detail long distance swimming events or the physiological and reproductive consequences of such behavior. Between an initial capture in late August and a recapture in late October 2008, a radio-collared adult female polar bear in the Beaufort Sea made a continuous swim of 687km over 9days and then intermittently swam and walked on the sea ice surface an additional 1,800km. Measures of movement rate, hourly activity, and subcutaneous and external temperature revealed distinct profiles of swimming and walking. Between captures, this polar bear lost 22% of her body mass and her yearling cub. The extraordinary long distance swimming ability of polar bears, which we confirm here, may help them cope with reduced Arctic sea ice. Our observation, however, indicates that long distance swimming in Arctic waters, and travel over deep water pack ice, may result in high energetic costs and compromise reproductive fitness.

First (23 August 2008) and second (26 October 2008) capture locations (circles with white crosses) of adult female polar bear 20741 in northern Alaska. Also provided are place names indicated in the text and the complete travel route of 20741
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  • Claudiu Constantin Nicolaescu, Taipan
Quote:What is a polar bear?

Evolutionarily speaking, polar bears (Ursus maritimus) are brown bears (Ursus arctos). That might seem counterintuitive, but modern biologists classify species according to their evolutionary history. Organisms that are more closely related are grouped together. In this system, only clades — groups of organisms that contain all the descendents of an ancestor — are named. When we look at the family tree of bears, we can see that not only are polar bears most closely related to brown bears, but they actually fall within the brown bear clade. There is no clade that includes all the brown bears and excludes the polar bears. From an evolutionary perspective, polar bears are simply a unique and highly specialized sort of brown bear!
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  • Claudiu Constantin Nicolaescu, Taipan
Study reveals new genomic roots of ecological adaptation in polar bear evolution

by University of Massachusetts Amherst

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Polar bears survive in the Arctic on a diet of primarily seal. Credit: US Fish and Wildlife Service

Scientists from the University of Massachusetts Amherst, Vanderbilt University and Clark University have shed new light on the genomic foundation of the polar bear's ecological adaption by pinpointing rapid changes in the bear's gene copy numbers in response to a diet shifting from vegetation to meat.
In a paper published Monday, June 17 in the Proceedings of the National Academy of Sciences and chosen for the cover of PNASVol. 116, issue 27, John G. Gibbons and Ph.D. student Shu Zhao of UMass Amherst, David C. Rinker of Vanderbilt and Natalya K. Specian of Clark discuss the first population-level study to characterize genome-wide patterns of copy number variation (CNV) in the polar bear and brown bear.
CNV refers to differences among individuals in the number of copies of a particular portion of the genome, and the study's results suggest this variation played an important role in the adaptation of polar bears to the Arctic.
"This research addresses a big-picture evolutionary question of how diet shapes the genome," explains Gibbons, assistant professor of food science.
Since the brown or grizzly bear (Ursus arctos) and polar bear (Ursus maritimus) diverged less than 500,000 years ago, the polar bear has evolved unique traits to adapt to the Arctic climate and ecology, such as a camouflaging coat of pigment-free fur. Previous population genomic studies of polar bears and brown bears analyzed single-nucleotide polymorphisms, or changes in a single base pair in a DNA sequence.
"CNVs were traditionally harder to detect so they weren't always analyzed," Gibbons says. "With recent advances in DNA sequencing technologies over the last 15 years or so, computational approaches to detect and quantify CNVs from genomic data have been developed. The polar bear data gave us a nice opportunity to fill in the gap."
Using available raw whole-genome DNA sequence data, the team of researchers compared differences in gene copy numbers among 17 polar bears, nine brown bears and two black bears. "Polar bears and brown bears are excellent models for exploring the impact of natural selection on CNV," the researchers write, "because they inhabit vastly different habitats yet are so recently diverged that they remain capable of producing fertile hybrid."
Gibbons says he and his colleagues hoped to see differences in gene copy numbers related to the bears' diet, and they did. Brown bears are omnivores, primarily consuming vegetation, while polar bears evolved quickly to a fully carnivorous diet, eating seals and other animals.
"This was a test case in understanding the effect of differences in diet on copy number variations," Gibbons says. "If we didn't see it here, we might not see it anywhere. It was pretty widespread and pretty convincing."
Gibbons points to two of the interesting findings. Of the genes annotated as olfactory receptors, 88 percent had lower copy numbers in polar bears, compared with brown bears and black bears. He explains, "First, there is less to smell in the Arctic. The polar bears mainly have to hone in on two things—seals and mates. They aren't looking for berries, grasses, herbs, roots and bulbs, like the brown bear."
Polar bears also were found to possess fewer copies of the gene AMY1B than brown bears. AMY1B encodes salivary amylase, the enzyme that jump-starts the digestion of starch when animals chew plant-based food. "Human populations with a high-starch diet have more copies of this gene in their genome than human populations with a lower-starch diet," Gibbons says. "We found the same thing with bears. If you think about their diets, it makes sense."
The new research concludes that analyzing copy number variants is an important tool when investigating evolutionary changes driven by natural selection.
"Evolution acts on different types of genetic variants to do the same thing," Gibbons says. "Now that we have the technology to detect CNVs, the consensus is that this type of mutation should be examined, along with the traditional methods for detecting parts of the genome that are shaped by natural selection."
Gibbons plans to build on the polar bear genomic research with an investigation of another species: Homo sapiens. "Our next step is to look at two different human populations to see if we see similar differences in copy number variants."

Journal Reference:
David C. Rinker el al., "Polar bear evolution is marked by rapid changes in gene copy number in response to dietary shift," PNAS(2019).

Polar bear (Ursus maritimus) and brown bear (Ursus arctos) are recently diverged species that inhabit vastly differing habitats. Thus, analysis of the polar bear and brown bear genomes represents a unique opportunity to investigate the evolutionary mechanisms and genetic underpinnings of rapid ecological adaptation in mammals. Copy number (CN) differences in genomic regions between closely related species can underlie adaptive phenotypes and this form of genetic variation has not been explored in the context of polar bear evolution. Here, we analyzed the CN profiles of 17 polar bears, 9 brown bears, and 2 black bears (Ursus americanus). We identified an average of 318 genes per individual that showed evidence of CN variation (CNV). Nearly 200 genes displayed species-specific CN differences between polar bear and brown bear species. Principal component analysis of gene CN provides strong evidence that CNV evolved rapidly in the polar bear lineage and mainly resulted in CN loss. Olfactory receptors composed 47% of CN differentiated genes, with the majority of these genes being at lower CN in the polar bear. Additionally, we found significantly fewer copies of several genes involved in fatty acid metabolism as well as AMY1B, the salivary amylase-encoding gene in the polar bear. These results suggest that natural selection shaped patterns of CNV in response to the transition from an omnivorous to primarily carnivorous diet during polar bear evolution. Our analyses of CNV shed light on the genomic underpinnings of ecological adaptation during polar bear evolution.
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