When polar bears hunt like sharks in the movies

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This image illustrates the diving hunting technique used by polar bears to captu
This image illustrates the diving hunting technique used by polar bears to capture moulting geese © Madeleine-Zoé Corbeil-Robitaille
Researchers describe a novel technique used by polar bears to hunt snow geese

Bylot Island, Nunavut, August 8, 2021, 1 p.m. A polar bear approaches a pond where some thirty moulting snow geese - unable to fly - have taken refuge. The bear enters the water, swims stealthily along the surface towards a goose and, once within 30 meters of it, disappears beneath the surface of the wave, then suddenly emerges... just a little too far away to capture it. He repeats this maneuver for 15 minutes and, on the fourth attempt, grabs the goose in his mouth, brings it back to shore and makes it his afternoon snack.

This technique of hunting snow geese by polar bears had never been reported before," points out student researcher David Bolduc, who witnessed the scene and who, along with other researchers from Laval University’s Department of Biology and Centre d’études nordiques, is co-authoring a study on this behavior in the journal Arctic Science. We don’t yet know how widespread it is, but no other Arctic predator uses it."

The polar bear’s diet consists mainly of seals, which it captures on pack ice or on the ice floes that break away from it. They use two strategies to hunt these skilful swimmers. The first is stalking: the bear sits close to a breathing hole dug in the ice by the seals and waits patiently for one of them to surface. The second is to swim discreetly to the ice floe where a seal is, climb abruptly and sprint to his prey.

"The polar bear’s strategy for capturing moulting geese is distinctly different from these two techniques. It is, however, similar to the approach it uses to capture seabirds near the coast. These behaviours illustrate the impressive plasticity of the hunting techniques deployed by this predator," notes David Bolduc.

Melting pack ice and the extension of the ice-free period in the Arctic Ocean pose a considerable energy challenge for polar bears, points out student researcher Matthieu Weiss-Blais, who co-authored the study. "A 45-kilogram ringed seal provides the bear with around 186,000 calories. By comparison, an average goose weighing 2.6 kilos contains just 900, or 200 times less. Unlike the black bear, the polar bear builds up its fat reserves during the long Arctic winter, when it hunts seals. When the ice disappears, it draws on its reserves to survive, and if necessary, consumes the much less rich food it finds on land. That’s why it’s so important for polar bears to make the right energy choices.

The challenge with small prey such as snow geese is that the bear must not expend more energy capturing them than they bring to its plate. The research team modeled the bear’s energy expenditure when using the dive-hunting technique to capture molting geese. Their conclusions? This technique can be energetically profitable, especially for bears that are not massive and when the pursuit is of short duration.

"Polar bears won’t survive on geese or other terrestrial resources. Without the ability to hunt seals, their future seems uncertain."

-- Matthieu Weiss-Blais "Snow geese moult for 3 to 4 weeks. This hunt can therefore provide an interesting source of energy for certain bears. However, on a population scale, the repercussions are minor for both polar bears and snow geese," says Matthieu Weiss-Blais. The context created by the melting pack ice forces polar bears to find new sources of food, and they show ingenuity in doing so. But polar bears won’t survive on geese or other terrestrial resources. Without the ability to hunt seals, their future seems uncertain."

The signatories of the study published in Arctic Science are Matthieu Weiss-Blais, David Bolduc, Frédéric Dulude-de Broin, Thierry Grandmont, Frédéric Letourneux, Mathilde Poirier, Denis Sarrazin and Pierre Legagneux, from Université Laval, and Madeleine-Zoé Corbeil-Robitaille, from Université du Québec à Rimouski.