Why do wild polar bears eat seaweed?

Each year in fall, hundreds of polar bears congregate along the Hudson Bay and James Bay coastlines, waiting for winter sea ice to form in order to hunt the fat-rich seals they depend on. While they wait, polar bears have been observed scavenging other food sources such as berries, goose eggs and even seaweed that washes up in large piles along the shore. Although it is frequently observed, the reason for the consumption of seaweed by this obligate carnivore (animals which rely on meat to survive, and in polar bears, fat specifically) is not yet understood.

In 2019, the Polar Bear Habitat and researchers from the Dalhousie University in Nova Scotia collaborated in what is believed to be the very first study aimed at understanding why polar bears eat seaweed. By looking at potential effects of seaweed consumption on the gut microbiome (microorganisms including, bacteria, fungi and viruses that live in the digestive track of all animals, including humans) of polar bears, we hoped to shed light on how polar bear health may be impacted by prolonged ice-free seasons. To do this, our bears were fed a portion of seaweed everyday during two separate 28-day feeding trials and samples of their fecal matter were collected for analysis over the following month.

Henry eating seaweed

Unlike their regular diet, seaweed wasn't something we could just order and have delivered to our door. Lead project authors Dr. Stephanie Collins and Jing Lu travelled to Churchill Manitoba, a small community of around 900 people on the the Hudson Bay coast, and collected samples of seaweed that had washed up on the shore. These were then sent to us in pre-portioned bags, ready to be incorporated into the bears' regular diet.

As with all studies we participate in, the bears were given the choice to participate or not, which in this case meant whether to eat the seaweed or not. Henry, Inukshuk and Eddy happily ate the seaweed whole, slurping it up like spaghetti, especially when it was coated in seal oil. Ganuk, not unexpectedly, was not a fan of eating this new salty vegetable whole, rather he preferred to eat his when it was ground up into small pieces and hidden in a ground up mixture of fish, moose and seal oil. Collecting fecal samples in the subsequent weeks involved finding a fresh sample in their overnight dens, using a long-handled swab to collect a small amount of fecal matter, placing it into a small tube, shaking it to mix with the stabilizing solution, and then freezing it before sending it to the University for analysis.

Fecal sample collection

The results from the analysis of our bears’ gut microbiome were compared with that of wild, onshore polar bears. As expected, our bears had a more diverse gut microbiome from their wild counterparts, likely due to the variable array of foods available to them and the environment in which they live. Interestingly, this comparison is also similar to previous studies which have shown that onshore polar bears (those that live in the more southern ranges and spend the summer on land) had a more diverse gut microbiome than offshore polar bears (those that live in the more northern ranges that stay on ice year-round). One explanation could be the abundance of dietary options that can be found onshore and in captivity, compared with limited options found on the sea ice.

As for the effect of seaweed on the bears’ gut microbiome, unexpectedly, the study’s findings show minimal effects. Researchers speculate this could be because our polar bears are already exposed to various types of vegetation in their daily diet. Or perhaps this could be related to their evolutionary capacity to digest seaweed, as previous evidence indicates that seaweed is a regular component of wild polar bears diets. Although it was anticipated that the 28 day feeding trials would be long enough to show changes, due to the small amounts that were fed, future studies may benefit from a longer feeding duration. The vastly different microbiomes observed between captive and wild polar bears gives insight into what the future for wild polar bears may look like in the decades to come, should climate change continue to accelerate at the current rate.

Although not the results we were expecting to find, that’s just how science works sometimes! Information such as this is crucial in guiding future scientific study and influencing those policy makers responsible for climate change mitigation and preservation of the natural world.

The opportunity to participate in studies like this are always an exciting time for us. Depending on the topic being investigated, the bears may be exposed to a new food source, be trained for a new voluntary behaviour, or it may be completely non-invasive where the bears don’t even know that anything is happening. No matter the topic, we value the opportunity to contribute to the ever-growing scientific knowledge base of polar bears and understanding how best to care for them in human care, and in the wild.

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