Nav: Home

Polar bears finding it harder to catch enough seals to meet energy demands

February 01, 2018

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.
-end-
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.

University of California - Santa Cruz

Related Climate Change Articles:

The black forest and climate change
Silver and Douglas firs could replace Norway spruce in the long run due to their greater resistance to droughts.
For some US counties, climate change will be particularly costly
A highly granular assessment of the impacts of climate change on the US economy suggests that each 1°Celsius increase in temperature will cost 1.2 percent of the country's gross domestic product, on average.
Climate change label leads to climate science acceptance
A new Cornell University study finds that labels matter when it comes to acceptance of climate science.
Was that climate change?
A new four-step 'framework' aims to test the contribution of climate change to record-setting extreme weather events.
It's more than just climate change
Accurately modeling climate change and interactive human factors -- including inequality, consumption, and population -- is essential for the effective science-based policies and measures needed to benefit and sustain current and future generations.
More Climate Change News and Climate Change Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...