How many seals does a polar bear need to eat to maintain a healthy weight?

Key points

  • Until now, no-one knew exactly how much energy polar bears needed to consume to maintain a healthy body weight
  • Study captures the bears' behaviour, activity levels, hunting strategies and metabolic rate in the wild
  • Findings have important implications as sea ice declines and polar bears move into potentially less productive areas
A lot more than many of them are catching in prime spring hunting season, according to a new study.
US scientists captured and fitted collars equipped with GPS and "bear cam" to nine cub-less female bears.
They then tracked their hunting strategies and how far they moved as they crossed the Beaufort Sea ice fields in April of 2014, 2015 and 2016.
"It's quite fascinating to see how the bears are using the habitat and how the habitat is influencing their behaviour, activity patterns and their ability to catch seals,'' said lead author Anthony Pagano, a wildlife biologist at the US Geological Service.
The scientists also analysed blood and urine samples taken at the beginning and end of each bear's 8–11 day journey across the ice fields to ascertain the animal's metabolic rate.
The study, published in the journal Science, found that polar bears have a higher metabolism than previously thought.
Even though the Arctic should be flush with seal pups in the month of April, the researchers found many of the bears were spending more energy chasing down prey than they consumed.
While four bears put on weight, five bears lost up to 10 per cent of their body weight — between 18 and 20 kg — in 10 days.
And the bears' challenge to survive is only likely to increase as sea ice coverage continues to decrease.
"In the Beaufort Sea we are seeing that the ice is retreating much further to the north than it had historically," Mr Pagano said.
"The bears are moving with the ice and moving into these deeper water areas where it's thought they are having much less opportunity to catch seals."

That's a lot of seals

Polar bears rely on energy-rich foods such as ring seals to bulk up in spring and summer to survive the lean times when they're fasting.
It had been thought that polar bears could conserve energy by using a "sit and wait" technique, where they hang around a hole until an unsuspecting seal comes along and they pounce on it.
Most of the time the bears tracked in the study did use the "sit and wait" technique.
But the scientists found the bear's metabolic rate was 1.6 times greater than previously thought — akin to that of other carnivores.
The scientists estimated that an active female polar bear needed around 51 megajoules of energy a day.
To achieve this, they calculated the bear would need to eat either one adult ringed seal, three subadult seals or 19 newborn ringed seal pups every 10-12 days to remain in energy balance.

Why this number matters

Pinning down that figure is an important step in understanding what is happening to polar bears now and in the future with climate change, said polar ecologist John P Whiteman of the University of New Mexico.
It's estimated that Arctic sea ice coverage is declining by about 14 per cent every decade, pushing the bears into new hunting territories that may or may not be productive.
"The chain of logic that everyone is concerned about is that the melting of sea ice reduces the amount of time that bears get to spend on sea ice in the productive regions where seals are concentrated," said Professor Whiteman, who wrote a commentary in Science.
"As they reduce that amount of time it follows that they catch fewer seals total and they are taking in a lower amount of energy. "And as they take in a lower amount of energy over time their body condition declines and then they get to the point where their reproduction and their survival becomes hindered."
Until now, he said, we haven't had any hard numbers about how many seals the polar bears needed to eat.
"That [number] tells us a lot about modelling into the future what's going to happen with the bears."

So what happens when times are tough?

Mr Pagano said the next step was to study what happened to polar bears throughout the year, particularly when the ice breaks up and the bears move further north with the ice.
"We want to get a better understanding for what that means for their energy demands, how many seals they need to be catching as they move these greater distances," he said.
Professor Whiteman said it was important to understand what happens when the bears are fasting.
"There can be multiple fasting periods in the polar bear cycle and those fasting periods are growing longer as a result of climate change."
Previous studies estimate the mortality of male polar bears would increase from 6 per cent to 48 per cent if fasting periods increased from 120 days to 180 days.
"When you're talking about some places extending their melt season to double digits by the end of the decade we're not that far out from that kind of lengthening of the melt period in some parts of the Arctic," he said.