It’s a bright, chilly morning on the Canadian tundra. You crawl out of your den, eyelids heavy from sleep, into the light of day. The Sun hardly sets this time of year, its harsh light makes you squint as you emerge onto the flat expanse of squishy, wet, half-frozen earth. It’s summer in the Arctic. You are a polar bear. And you are hungry.
Lucky for you, you spent the wintertime hunting seals out on the ice. This is how most polar bears spend their winters, scouring the frozen sea for their next chance at a delicious and filling meal. Seals are high in both protein and fat, which means polar bears gain lots of calories, or much needed energy, from eating them. They also incorporate the fat into their own bodies, storing the energy for later and keeping them warm between meals. Throughout the winter, bears gain so much fat that they hardly have to eat at all when summer comes!
But what happens when winter gets shorter? Climate change has made it harder for polar bears to hunt seals. Warmer weather in the Arctic means the sea ice they hunt on melts earlier in the year, forcing the bears onto land and away from food sooner than normal. With less time to hunt, they can’t build up their fat reserves before the long summer months set in and risk going hungry.
So, how do you satisfy your appetite when seals aren’t around? Easy! Find something else to eat. Polar bears and many other animals do something called prey switching, which means when their favorite food is hard to come by, they’ll switch to food that is easier to find, even if they don’t like it as much or if it’s not as good for them. Scientists have recently noticed polar bears eating eggs from the nests of common eiders, a kind of arctic duck that nests in large groups on the tundra1. They wanted to know how the bears benefited—and how they might not—from foraging for eggs when seals were not available. To do so, they took to the skies.
On a small island in the Canadian Arctic where a large group of eiders nested, the researchers used remotely controlled drones to observe record polar bear behavior from above. This allowed them to observe the bears without disturbing them and from a safe distance away. To measure benefits and costs, they measured the number of clutches (all the eggs in a nest) the bears ate, the amount of time they spent searching for eggs, and the amount of time it took them to eat the eggs. Eating more eggs benefits the bears because eggs provide the energy they need to live. The time bears spend searching for and handling their food, however, is a cost because the bear must use energy to do these things. Foraging eggs is good for the bears as long as the benefits of eating eggs are bigger than the costs of looking for and handling them.
Early in the season, the bears ate many eggs very quickly and gained lots of energy. These bears received many benefits and paid few costs for foraging because eggs were plentiful and easy to find. The bears arriving later in the season weren’t so lucky; the bears who arrived before them had eaten most of the eggs! These later bears ate less and spent more time searching for nests that hadn’t already been visited by another hungry bear. This is called diminishing returns: the benefits of foraging for eggs go down as more eggs are eaten at the same time that the costs go up. This means that foraging is good for bears arriving early, but not so good for bears arriving later.
Bears arriving to eider colonies early in their breeding season will likely be able to eat enough eggs to make it through the summer, but those arriving later face a higher risk of going hungry before the ice returns and they can hunt seals. How can we make sure that all the polar bears get enough to eat? The solution lies in protecting their Arctic environment and stopping climate change. Protecting seals, for example, will mean more and better food for polar bears. Stopping sea ice from melting is another way to make sure bears have enough time to build up fat before summer. It will take effort from all of us to make these things happen. In the meantime, it is important to study how bears behave in these conditions so we can better understand how to protect them.
- Jagielski, P. M., Dey, C. J., Gilchrist, H. G., Richardson, E. S., & Semeniuk, C. A. D. 2021. Polar bear foraging on common eider eggs: estimating the energetic consequences of a climate-mediated behavioural shift. Animal Behaviour, 171, pp 63–75. https://doi.org/10.1016/j.anbehav.2020.11.009