© Mike Lockhart
3/27/2018 5:21:16 PM
Polar Bears Burn More Energy Than Previously Thought
Energy balance is at the core of everything we do, and imbalances have consequences. We add energy in the form of gas to our vehicles, or we get stranded on the side of the road. We add energy in the form of food to ourselves, but too much or too little can lead to health issues. Polar bears and other wildlife must balance their energy, too, consuming enough calories to meet their needs.
An animal’s energy needs can be talked about in terms of metabolism or metabolic rate, which simply indicates how much energy they burn over time. The metabolic rate of every animal, including humans, can be separated into two types: (a) resting metabolic rate—the energy cost of basic functions, like blood circulation and breathing, and (b) the field metabolic rate—which is the resting metabolic rate plus the energy cost of other activities, like walking and swimming. How much energy an animal uses helps determine whether it can survive and reproduce, so these rates are important to consider in conservation.
How much energy a polar bear really needs has always been a big unknown. Better understanding this can help improve predictions of how polar bears will react to the loss of their sea ice habitat from human-caused climate change.
As our climate has warmed, sea ice has declined in area and thickness across the Arctic, negatively impacting animals that depend on it for survival. For polar bears, losing sea ice translates to losing opportunities to hunt seals and having to expend more energy to find the same amount of food. But is this truly unbalanced? Recent research says, yes.
How much fuel do polar bears need?
Energy requirements are difficult to measure, especially for large, far-traveling mammals like the polar bear. However, in a new paper, Dr. Anthony Pagano and a team of scientists used novel methods to get an accurate estimate of the polar bear’s energy use. First, researchers assessed the polar bear’s resting metabolic rate by measuring the oxygen consumption of a zoo polar bear who was trained to lie down quietly in a chamber. Next, the researchers quantified the field metabolic rate of nine wild bears on the sea ice off the coast of Alaska.
To do this, the scientist gave the polar bears water with isotopically-labeled hydrogen and oxygen; ten days later, they recaptured the bears and took a blood sample. By measuring how much labeled oxygen and hydrogen remained in the blood, the researchers were able to calculate the bears’ field metabolic rate. The researchers also used video camera collars on those bears to measure their activity levels and foraging success.
The energy needs of the wild bears were, on average, 1.6 times higher than earlier estimates. Also, on average, the polar bears were active for 34% of their time, more like large terrestrial carnivores (39%) than other, more omnivorous, bear species (50 to 60%). These findings underscore the fact that polar bears rely on energy-rich marine mammal prey (i.e., seal blubber) accessed via sea ice, and that they would not be able to find enough calories in most terrestrial habitats. Polar bears are more carnivorous and have different energy requirements than their land-based brown bear cousins.
Results also showed that polar bears do not reduce their metabolism when food is scarce. Four of the nine bears lost 10 percent or more of their body mass during the study, and at least one of the bears lost substantial muscle and fat (the primary form of energy storage). The bears used a lot of energy even though food was limited.
This research also revealed just how much increased energy bears use when they speed up their rate of travel. Because thinning sea ice now drifts across the Arctic at a faster rate, these data allow the calculation of how many more calories bears must burn to keep pace with the ice, and how such costs can vary by subpopulation at a more local level. Ice loss, if unabated, will eventually cause the extinction of polar bears in the wild, but continued research is needed to understand the specific and shorter-term climate-related pressures that polar bears face.
Important questions remain about the energy balance of polar bears. This study was done during the spring, the peak hunting season for polar bears. However, during winter their activity declines and polar bear body condition is at its lowest; it is unknown whether their metabolism slows at this time. In addition, this study looked at bears without cubs, so the cost of rearing young (including nursing) is yet to be quantified.
Overall, better understanding the energy needs of polar bears will help us better protect this species. By reducing our carbon emissions, we can help keep nature balanced for polar bears, and for people.
We believe it is also important to recognize the earliest work done on this subject, as what we are learning now is built upon the hard work of many who came before. In the 1970’s in Churchill, Manitoba, the late Dr. Nils Øritsland and his graduate students led ground-breaking research by developing the first polar bear treadmill to study aspects of polar bear energy use. This group found that the polar bear’s oxygen consumption rate was more than two times that of most mammals, concluding this may be due to heavy legs and paws. Though it happened over 40 years ago, this boundary-pushing work paved the way for better understanding the energetic needs of polar bears at a time when we knew relatively little about the species, and well before we knew that climate change would threaten their future.
Øritsland NA, Jonkel C, Ronald K. 1976. A respiration chamber for exercising polar bears. Norwegian Journal of Zoology 24: 65-67.
Pagano AM et al. High-energy, high-fat lifestyle challenges an Arctic apex predator, the polar bear. Science. Vol. 359, February 2, 2018, p. 568. doi: 10.1126/science.aan8677.
Whiteman JP. Out of balance in the Arctic. Science. Vol. 359, February 2, 2018, p. 514. doi: 10.1126/science.aan6723.