Survival of the fattest: Why the climate crisis is making it hard for polar bears to get enough calories
7 July 2020
To survive, polar bears need two things: seals to eat, and a platform of sea ice from which to hunt them. Pregnant bears, in particular, must get very fat from hunting seals before they hibernate, as they may not eat for eight months: they rely on stored fat for the energy they need to produce and nurse cubs, meet their own needs and travel back to the ice in spring. But as Nick Lunn explains, warming Arctic temperatures and declining sea ice habitats are making it challenging for polar bears to stay fit by staying fat.
It's summer in northeastern Manitoba, Canada and as the Hudson Bay sea ice melts, polar bears are forced ashore. For the next four months, they will use stored fat reserves to meet their energy needs while they wait for ice to reform. But not all bears will go back to the sea when the ice returns: pregnant females will remain in their dens on land for another four months. When they finally emerge in spring, they will not have eaten seals for eight months, and will be famished. How fat they were when they first went ashore is critical, not only for their own survival, but for the health and survival of their cubs.
Polar bears are a keystone species, meaning they provide us with insights into the overall health of biodiversity in the Arctic marine ecosystem. They are distributed throughout the ice-covered waters of the circumpolar Arctic in 19 relatively discrete subpopulations. Although polar bears still occupy much of their historic range, the Arctic has been warming more rapidly than the global average, and the loss of sea ice is accelerating. This has raised long-term conservation concerns for Arctic marine mammals, including polar bears.
Less ice means less fat—and fewer cubs
Ongoing research on the Western Hudson Bay subpopulation of polar bears began in 1980 and has provided researchers with an unparalleled opportunity to examine and speculate about the effects of past, present and future environmental conditions on the bears. This subpopulation lives near the southern limit of the species’ range. Like all polar bears, they need ice to hunt seals, which make up the bulk of their diets.
But increasing temperatures in this region have resulted in earlier sea ice break-ups and later freeze-ups, forcing the bears to spend progressively longer periods on land. The onshore period is now 34 days longer than it was in the early 1980s.
Longer ice-free periods have been linked to declines in the body condition, survival, reproduction and abundance of Western Hudson Bay polar bears.
Female polar bears are critical to the sustainability of polar bear subpopulations. We know that larger, heavier females tend to have larger litters and produce heavier cubs with higher survival rates. We also know that when female bears are not in good condition, they may not produce any cubs at all. Unfortunately, the average weight of solitary and presumed pregnant adult females declined by 15 per cent between 1980 and 2019—from 266 kg to only 226 kg. While we don’t know the exact weight below which female polar bears will not produce cubs, over the course of our long-term study, the lightest female known to have done so weighed 189 kg.
The abundance of Western Hudson Bay polar bears has declined by 30 per cent—from an estimated 1,200 animals in 1987 to just 842 in 2016. The production of litters declined by 39.7 per cent from 2001 to 2004 and from 2017 to 2019. The persistence of polar bears as a species depends on their ability to reproduce. Continued reductions are likely to lead to further declines in the abundance of the Western Hudson Bay subpopulation.
Polar bear habitat needs protection
Polar bears are not only an iconic Arctic species, but an important natural and cultural resource for Indigenous People. Their conservation is guided by the 1973 Agreement on the Conservation of Polar Bears.
In 2015, five nations—Canada, Greenland, Norway, the United States and the Russian Federation—adopted a co-operative circumpolar action plan to strengthen polar bear conservation. One of its key objectives is to communicate to the public, policy makers and legislators around the world the importance of mitigating greenhouse gas emissions to protect essential sea ice habitat for polar bears and ensure their continued presence in healthy numbers.
Although the Arctic is far removed from everyday life for most of us, and many will never experience its beauty first-hand or see a polar bear in the wild, there is no escaping the fact that the environmental changes occurring there are key drivers of change across the entire planet. If we want polar bears to continue to exist and thrive, then we must work together to protect sea ice habitat so polar bears can stay fit by staying fat.
NICK LUNN is a Canadian government research scientist. His primary interest is in polar marine ecology, with particular emphasis on polar bears and marine mammals.
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