Longer Summers And The Fate Of The Polar Bear

As summers get longer, Polar bears are forced to spend more time on land and less time hunting on their best-suited food resource: ice seals.

All ecosystems on Earth are interconnected; they all depend on each other. And in turn, ecosystems depend on ‘non-biotic’ factors such as temperature, precipitation, fire regimes, or floods. Break any of these factors, and all systems will rapidly collapse, affecting animals and human economies.

On the other hand, we tend to only think about the consequences of climate change on human populations. But what would be the immediate consequences of a rapid climate change on other species? Let’s look at one of the most charismatic ones. What about the Polar bear (Ursus maritimus)?

Dr. Anthony M. Pagano and his collaborators recently published a peer-reviewed paper addressing how Polar bears respond to ongoing changing climatic conditions in Manitoba, Canada. In western Hudson Bay, increasingly warmer conditions have increased the ice-free period by three weeks from 1979 to 2015.

You may think three extra weeks of ice-free conditions may not adversely affect the polar bears’ survival. However, if we examine their ecology, we soon understand its devastating implications for the species. The research authors state, “Polar bears acquire most of their energy resources during a brief period in the late spring and early summer when [ice] seals are giving birth to and weaning their pups. Climate warming is increasing the duration that some areas of the Arctic are ice-free, which in turn forces polar bears in these regions to move to land.”

When Polar bears live on land, they tend to fast or forage on other foods that are less nutritious or fit to their energetic needs. I was lucky to study mammal teeth during my PhD dissertation and postdocs. What got my attention about polar bears was how specialized to a carnivorous diet their teeth are compared to other bears (i.e., grizzly or black bears). Indeed, Polar bear teeth are sharper, with pointer canines and fewer grinding surfaces to process vegetation properly.

To understand how a longer ice-free, and therefore seal-free, period would affect Polar bear survival, the authors tracked the activity and foraging behavior of 20 Polar bears on land for 19–23 days from August to September (2019–2022). These included 8 solitary adult females, 5 adult males, 4 subadult females, and 3 subadult males. They used GPS-enabled video camera collars that measured energy expenditure, diet, behavior, activity, movement rate, blood chemistry, and body composition. Ultimately, the authors wanted to understand how energy expenditure and dietary change vary when the animals live solely on land.

Dr. Pagano and his collaborators were surprised with the results. They hypothesized that during these times on land, Polar bears would limit their activities to minimize spending energy. Instead, they found that each individual followed a different adaptation strategy. While some increased their activity levels to increase their likelihood of finding food, others stayed within a small area and lowered their metabolism to a degree only seen in hibernating bears.

Lower metabolisms were interpreted as a fasting response to reduced food accessibility, as this was only seen in the bears that didn’t eat during this period. The authors were also surprised to observe that the bears that weighed the least and, therefore, were in the poorest conditions at the beginning of the study, were also the ones that opted for fasting. On the other hand, pregnant females with the highest body fat concentrations were the ones who moved the least during this period. Having been pregnant twice myself, I totally sympathize with their strategy.

Another interesting finding was that some of these bears engaged in long-distance swimming (>50 km) during this period. Since their collars had video cameras attached, the scientists could identify that these bears could locate beluga carcasses on their swims. Fascinatingly, they used the carcass as a raft but most didn’t feed on them. Another bear found a dead seal they tried to drag to shore unsuccessfully. But overall, these energy-expensive swimming adventures didn’t bring these bears many gains regarding food and nutrition. They seemed to be ill-fitted to feed on floating carcasses.

Overall, the authors observed that whether the bears opted for reduced activity and fasting or increased activity and foraging, their body mass loss didn’t differ significantly. However, most of the bears opted for the increased activity strategy, which could have important implications for terrestrial ecosystems in Hudson Bay.

The Hudson Bay ecosystem has evolved with little influence from the Polar bears, who tend to find their food on ice during the seals’ breeding season. But with longer summers, these apex predators will spend more time on land, competing for food resources with other carnivorans such as grizzly bears and wolves and potentially altering other animal populations through predation. For example, snow geese used to nest in the late spring, when polar bears used to forage on ice seals. But with summers coming up earlier due to climate change and Polar bears spending time on land earlier in the year, they have started to feed more on snow geese’ and other migratory birds’ eggs. This has already had devastating effects on these bird populations, which have been steadily decreasing.

However, even if bears feed on other land resources, this doesn’t prevent them from losing important body fat that they may need for the winter ahead. These food resources weren’t as nutritious for their ecologies as ice seals are. For instance, about 70% of the Polar bear’s diet while on ice is fat. Only one of the Polar bears gained weight during the 3 weeks of the study: the one that was lucky to locate a large land mammal to feed on.

The authors also reported that the most active polar bears lost body fat and lean mass (muscle), which is not common in hibernating bears. They offer a long explanation for these changes in metabolism that those familiar with the science of “ketosis” may find especially interesting, so I highly encourage you to read the discussion section of the original article if you want to learn more.

The authors acknowledge that a bigger study would be needed to fully comprehend the full range of adaptations Polar bears will need to face due to changing climatic conditions and shortening summers. For example, they didn’t investigate body mass changes in lactating mothers, which face the most energetically expensive component of the reproductive cycle in mammals and can more than double energy demands, especially in smaller mammals.

Phys.org interviewed Dr. Andrew Derocher, a biology researcher from the University of Alberta who didn’t participate in the study but stated:

“This paper clearly shows that polar bears cannot adapt to the pace of change in the Arctic and that the bears are already using everything they have to stay alive.’’

But the most alarming result in the study is that, with longer summers, bears could face starvation before the next breeding season for ice seals kicks in, preventing them from surviving for long enough. 95% of the bears lost body fat during the study, emphasizing that “none of the energetic strategies [adopted by the studied individuals] were more beneficial for surviving the on-land period.”

Back in the past, during interglacial periods when ice cover may have also been lower, bears fed on large mammal carcasses that drifted to shore to compensate for the high-fat diet they needed to survive. However, with the reduced whale population in the present time, these opportunities are becoming rarer for the bears. Thus, the authors finalize their paper with a clear message: with longer summers come longer periods spent on land for Polar bears, exponentially increasing their risk of starvation.

In a previous article, I discussed how global warming and climate change may have consequences beyond warmer climates, even leading to a deeper freeze. However, while possible, most models point to ongoing warming due to carbon emissions from human activity. The devastating chain of events from climate change is far-reaching and will push all species out of their comfort zone — including us.