(CN) – Environmental changes created by climate change cause polar bears to expend more energy to catch less prey, according to a harrowing new study that underscores their plight.
The report, published Thursday in the journal Science, uncovers the physiological factors that have led to observed declines in polar bear populations.
“We’ve been documenting declines in polar bear survival rates, body condition and population numbers over the past decade,” said first author Anthony Pagano, a doctoral candidate at the University of California, Santa Cruz.
“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.”
To analyze these circumstances, Pagano and his team monitored the hunting success, behavior and metabolic rates – the amount of energy used by an animal per unit of time – of nine female bears without cubs as they hunted for prey in the Beaufort Sea in the spring.
The researchers attached high-tech collars to the bears to record locations, video and activity levels over a span of 8 to 11 days. Metabolic tracers were used to determine how much energy the bears expended.
The bears’ metabolic rates averaged more than 50 percent higher than previous studies had estimated. Five of the bears lost body mass, as they were not catching enough fat-rich marine mammals to meet their energy requirements.
“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,” said Pagano.
Climate change is having a dramatic impact on Arctic sea ice, forcing polar bears to travel greater distances and making it more difficult for them to catch prey. In the Beaufort Sea north of Alaska, sea ice begins to recede from the continental shelf in July and most of the bears travel north on the ice as it withdraws.
As the Arctic heats up and more sea ice melts, polar bears must move farther than in past years. This forces them to expend more energy during the summer, when they are fasting until the ice returns in the fall.
In other areas, like the Hudson Bay in northeastern Canada, most polar bears travel onto land when the sea ice retreats. There, Arctic warming causes sea ice to break up earlier in the summer and return later in the fall, which forces the 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 research efforts have attempted to estimate polar bears’ energy expenditures and metabolic rates based on some inferences about their physiology and behavior. For example, scientists have thought that since the bears are primarily “sit-and-wait” hunters, they would minimize their energy use while hunting. Researchers have also guessed that the bears could reduce their metabolic rate to conserve energy if they failed to catch seals, according to Pagano.
“We found that polar bears actually have much higher energy demands than predicted. They need to be catching a lot of seals,” Pagano said.
During spring, polar bears mostly prey on recently weaned ringed seals, which are easier to catch than adult seals. By fall, however, young seals are older and wiser, and the bears cannot catch as many of them.
“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.
Researchers at the U.S. Geological Survey have been studying polar bears around the Beaufort Sea since the 1980s. The agency’s most recent population estimate suggests the bears have declined by about 40 percent over the past decade.
However, it has been difficult for scientists to analyze the behavior and fundamental biology of the bears in this harsh and remote environment, according to Pagano.
“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.