A polar bear in the tall grasses during summer in the Arctic

Photo: BJ Kirschhoffer / Polar Bears International

Can Polar Bears Go Terrestrial? (The Short Answer: No.)

By Dr. Thea Bechshoft

5 MINS

 

27 Apr 2022

Caribou, muskox, birds and bird eggs, beached whales, seals, fish, berries, and other plant foods … humans and other animals have been successfully living off the land during the ice-free summers in the High Arctic for millennia. Why can’t polar bears just do the same?

The answer: evolution! Over the past 500,000 years or more, polar bears have become hyper-specialized to living on the Arctic sea ice and preying on seals. Their adaptations to this lifestyle are evident in their behavior, physiology, and ecology. Studies show that even the size and structural integrity of the polar bear’s skull and teeth are adapted to surviving on soft blubber and flesh. This extreme specialization is the beauty of the bears as well as their bane. It makes them the undisputed top predator in the Arctic marine ecosystem but also extremely vulnerable to changes in their environment. 

Curious by nature

Every now and then a story will pop up in the media about polar bears that were spotted eating prey other than seals and how this shows their potential to adapt to a life without sea ice. Unfortunately, this is wishful thinking. While polar bears are extremely specialized, they are also very curious, intelligent, and opportunistic animals. If something looks new and potentially edible, they are very likely to want to take a closer look and an experimental nibble. 

This holds true for most polar bears, especially for those that are summering on land while they wait for the sea ice to re-form or those that find themselves in areas with few seals. Polar bears in such situations may eat berries, kelp, bird hatchlings, meadow voles, and the like if there are no seals to be found. However, none of these food items are a viable option for keeping polar bears alive long-term; only blubber from seals (and the occasional whale carcass) has a high enough energy content to do this. A round polar bear is a happy polar bear, and it requires a lot of energy to keep a polar bear plump!

A large bearded seal on sea ice platform

Photo: Ian Stirling / Polar Bears International

A large bearded seal rests on a sea ice platform.

Limiting factors

In addition to terrestrial foods generally being leaner and thus less calorie-dense than seals, there are two other reasons why these alternative food items cannot work as a long-term prey substitute for polar bears. 

First, the terrestrial food sources are limited. For example, eggs from ground-nesting geese or eiders are a reasonably energy-rich food. However, bird eggs are very small compared to a seal. This means that the bears must consume a much higher number of eggs to get the same amount of energy they would from eating a single seal. Studies have found that a single bear’s visit can be devastating to a bird colony, and that in some areas bear predation of nesting sites has increased seven-fold since the 1980s. In other words, foraging on eggs may have the potential to be a good strategy for a few bears for a while, but if most of the eggs produced by the birds in a colony are eaten every year, it won’t take long before that colony simply doesn’t exist. 

The comparatively barren areas in the High Arctic where the polar bears are likely to end up when there is no sea ice do not provide anything close to the amount of food the bears would need. This scarcity of food is also why barren-ground grizzlies (the northernmost brown bears) are the smallest of their kind. Additionally, the areas on land that are suitable for terrestrial bears are already filled by these grizzlies. Even if polar bears were able to evolve at lightning speed and change their diet completely, there would be definite competition between the two bear species, as they would be trying to fill the same ecological niche. 

Second, polar bears are specialists in hunting seals from a sea ice platform, not in hunting on land. Going back to the nest-raiding discussed above, a recent paper found that as fewer and fewer eggs were available (because the rest had already been eaten), polar bears were so inefficient at distinguishing full nests from empty ones that their search for eggs led to an overall declining net loss of energy. 

Polar bears are also at a disadvantage when it comes to capturing moving prey. Although a polar bear can sprint the last 20 meters or so towards a seal lounging on the sea ice, running for much longer than that causes them to overheat, even in cold temperatures. A caribou or similar prey would likely have to be either very sick or very inattentive to its surroundings in order for a polar bear to catch it. I am by no means suggesting that a hungry polar bear wouldn’t give it a try—in fact, a friend of mine once witnessed a female polar bear chase a reindeer calf in Svalbard. Quite predictably though, the chase ended with Calf 1, Bear 0.

A polar bear waits by a seal hole

Photo: Madison Stevens / Polar Bears International

A polar bear waits beside a seal hole on the frozen ocean. Polar bears rely on a platform of sea ice to reach their sea prey.

A life tied to sea ice

In summary, although terrestrial foods could potentially benefit a few individuals out of a local population, polar bears that are seen snacking on land-based food sources during the summer are not adapting long-term, they are simply doing their best to survive short-term while waiting for the ice and the seals to come back. Blubber is the only food that’s rich enough in energy to keep the bears alive and well long-term. The fatter the bear, the better its chances of surviving the summer and being healthy and ready to hunt again (and for some females, to give birth to their cubs) once the sea ice returns in the late fall. Without sea ice, there will be no polar bears. Neither the bears nor their seal prey can tolerate the continued loss of their primary habitat.

Dr. Thea Bechshoft is a staff scientist with Polar Bears International based in Aarhus, Denmark. She has studied polar bears in Greenland, Norway, and Canada and is the author of the popular Polar Bear Questions series on Facebook and Instagram.