A recent study of 45 years of observational data about polar bears shows the animals hoard the bodies of kills, burying them in the snow or dirt so they can return to them at a later date.
The behavior, called caching, is relatively common in other bear species, especially brown bears, from which polar bears evolved around 500,000 years ago.
The behavior is exceptionally rare among polar bears, but Ian Stirling of the University of Alberta and the Scientific Advisory Council for Polar Bears decided to investigate after receiving an intriguing photo from a friend.
A team of scientists reviewed observational data of polar bears taken between 1973 and 2018, and found just 19 instances of a bear burying its kill after being unable to eat it in one sitting, a rare behavior observed in just 0.5 of bears
‘My curiosity about short-term food caching was re-activated by receiving a photo from a friend of a male polar bear lying on the ice with a mostly snow-covered harp seal,’ Stirling told Polar Bears International.
‘I had seen this kind of behavior only once in over 40 years of research on polar bears.’
For his study, Stirling reviewed observational data of polar bears taken between 1973 and 2018 in Svalbard, Greenland, and the Canadian Arctic, finding just 19 instances of kill caching from just 0.5 percent of the bears observed.
Polar bears live on a diet of mainly seal meat and can eat between 10 and 20 percent of their own body mass in one sitting.
The average adult polar bear weighs around 1,200 pounds, while adult harp seals weigh between 265 and 300 pounds.
The data reviewed consisted of video, photographs, and logs of bear behavior taken in the Canadian Arctic, Greenland, and Svalbard, a Norwegian archipelago in the Arctic
In rare instances when a polar bear was unable to finish eating a seal t had recently killed, it would sometimes bury it in the ground and guard the site from other predators, a common behavior among other bear species but one that’s rarely been documented among polar bears
This disparity means a polar bear couldn’t, or wouldn’t, eat an entire seal in one sitting only in rare circumstances.
When those rare circumstances do occur, polar bears will burrow into the snow or dirt to hide their kill, then cover it with snow or more dirt to make it difficult for any other bears who may happen by to pick out by sight.
Some bears were observed lying over their buried kills, unwilling to let them out of eyesight but also unable to eat them fully.
Polar bears can eat between 10 and 20 percent of their 1,200-pound body weight in one sitting, and seals, which comprise their main food source, are rarely too big to eat
Researchers suggest the behavior could be so rare because bears live in comparatively underpopulated areas, with fewer scavenger species and other bears to compete with for food sources
‘Basically, the fat and meat from smaller seals, such as pup or yearling ringed seals are largely devoured immediately, leaving little to hoard,’ Stirling said.
‘In contrast, the carcasses of adult ringed seals, harp seals, and bearded seals may be covered with snow, to reduce the chance of kleptoparasitism [theft] by another bear or other scavengers visually detecting a dark spot on the ice, while the hoarding bear lies nearby.’
One possible explanation for why polar bears cache kills so much less frequently than brown bears is that they live in less densely populated regions.
Brown bears typically live in closer proximity to other bears, as well as significant populations of scavenger species like crows.
WHY DO POLAR BEARS NEED ICE TO SURVIVE?
Loss of ice due to climate change has a direct impact on the ability of polar bears to feed and survive.
The bears need platforms of ice to reach their prey of ringed and bearded seals. Some sea ice lies over more productive hunting areas than others.
Like other predators at the top of the food chain, polar bears have a low reproductive rate. One or two cubs are born in midwinter and stay with their mother for two years.
Consequently, females breed only every three years. The bears don’t reproduce until they are five or six years old.
From late fall until spring, mothers with new cubs den in snowdrifts on land or on pack ice. They emerge from their dens, with the new cubs, in the spring to hunt seals from floating sea ice.
Simply put, if there isn’t enough sea ice, seals can’t haul out on the ice, and polar bears can’t continue to hunt.
End of summer measurements of sea ice in the Arctic in September revealed that the region has hit the eighth lowest extent in modern record keeping.
Satellite data showed the Arctic reached its yearly lowest extent on September 13, at 1.79 million square miles (4.64 million square kilometres).
While the Arctic hits its summertime minimum around this time every year, the experts say the extent has been decreasing rapidly as a result of climate change, seeing dramatic declines since the late 1970s.