From Grizzly Bears to Polar Bears: A Common Ancestor and Key Differences
Written by Dr. Owen Slater
As Classic Canadian Tours wraps up another successful season of Khutzeymateen grizzly bear tours and gets prepared for our signature one-day Churchill polar bear trips, it’s great timing to discuss some of the overlap and differences between Canada’s two largest bear species.
Polar bears (Ursus maritimus) share a common ancestor with brown bears (Ursus arctos), with current scientific evidence estimating the divergence into separate species happened about 500,000 years ago. This likely occurred when an ancient population of brown bears found themselves isolated from neighbouring populations. Forced to adapt or perish, bears with advantageous traits that allowed animals to better adapt to the sea ice environment and to thrive on a high fat marine mammal diet were the most successful at reproducing and surviving.
Over many generations, surviving offspring continued to evolve, eventually leading to the present day version of polar bears that everyone is familiar with. As far as the North American grizzly bear (Ursus arctos horribilis), this is a subspecies of the brown bear that immigrated to North America from Eurasia.
During periods in history when overlap has occurred between these 2 bear species, gene flow has happened in both directions, with polar bear genes also being found in brown/grizzly bears. This includes present day bears with rare reports of interspecies breeding and production of hybrid offspring known as grolar or pizzly bears. However, because present day polar and grizzly bears differ in many ways including in ecology and behaviour, it remains very uncommon for them to come into contact in the north. This is especially true during the breeding season, which peaks in April and May and wraps up by June for polar bears but typically doesn’t start until June for grizzly bears.
Some of the physical changes that occurred as these species diverged are in colour and anatomy. The creamy-white appearance of polar bears is a product of their fur lacking pigment, being transparent and reflecting light. This provides excellent camouflaged in the Arctic landscape, allowing them to blend into the environment and greatly improves their hunting success of ringed and bearded seals. Seals rely heavily on their eyesight to detect predators so as you can imagine, the success of a typical brown coloured grizzly bear would not be very good for ambushing prey in the predominately white landscape of the north.
As the Hudson Bay ice starts to form, a polar bear with its long neck and massive paws tests the ice near the town of Churchill, Manitoba. Incidentally, if you look closely along the horizon, the Ithaka shipwreck and the Port of Churchill are in the top left and right corners respectively.
Image credit: Owen Slater
Then there’s the act of catching a fat, wet, slippery seal that are not the easiest prey to hold onto. The polar bear needs several adaptations to improve their success. This includes a longer neck than grizzly bears, which helps them extract seals from their breathing holes in the ice or grab and hold onto them after using their front feet to punch through snow drifts into seal lairs (small caves) just above the sea ice.
Other adaptations are in the curved claws of a polar bear which are shorter and sharper, providing excellent traction on the ice when they launch themselves from their ambush position towards a seal. These claws also help grip and hold onto seals and other marine mammals while the bear delivers a fatal bite. This contrasts with the longer claws on the forelimbs of grizzly bears, which are great for digging up rodents and plant roots, but would not be as functional on the sea ice.
Also on the feet, polar bears have massive paws up to 30cm wide (see photo above), with fur covering much of the walking surface to insulate them against the ice. When swimming, the large forepaws are used for efficient paddling, while the back feet are used to steer. When walking on thin ice, the large paws help distribute the weight of the bear to reduce breaking through the surface and wasting energy. Only the thick footpads are directly exposed to the elements, and these have small bumpy projections, known as papillae, that provide excellent traction to help prevent slipping. On the other hand (or paw in this case), grizzly bear feet are not nearly as wide and they have more extensive footpads with very limited fur between the pads of their feet since they spend the long harsh winters tucked away in a den, protected from the elements.
A playful female polar bear rolling onto her back provides a great example of the large paw size, the dense fur found between the toes and black footpads, as well as the shorter, but sharper black claws on the forelimbs compared to grizzly bears.
Image credit: Owen Slater
This female grizzly bear walking down a trail in the foothills region of the Alberta Rocky Mountains illustrates the much longer and in this case, lighter coloured claws on the forelimbs that extend well past the footpads and are blunter at the tips compared to polar bear. Grizzly bear claws can vary in colour from dark brown to almost white, while polar bear claws are black.
Image credit: Owen Slater
A specialized set of teeth, called carnassial teeth, which consists of the upper (maxillary) fourth premolar and the lower (mandibular) first molar are present in both bear species and act as shears to efficiently cut through the hide of prey. With polar bears being the most carnivorous of all bear species, relying almost exclusively on marine mammals to sustain themselves, their carnassial teeth are the most jagged and sharp of all bear species. In contrast, grizzly bears diets contain much more plant material, so they have evolved to have flatter carnassial and molar teeth, which provides more of a grinding surface to help break down plant fibers.
Because polar bears spend the majority of their lifetime on sea ice and rely on the marine environment to survive, they are classified as marine mammals, which is reflected in their Latin name, Ursus maritimus, meaning sea bear. Even when forced to shore when the sea ice melts in July and August in Churchill, polar bears will opportunistically attempt to hunt marine mammals. A great example of this is an incredible video by Nature on PBS, where they document the intelligence and planning of a polar bear as it patiently waits on a rock for the tide to come back in before successfully capturing a beluga whale calf that got too close. Here’s the link if you would like to view this impressive behaviour.
If you are looking to explore Churchill, MB during the seasonal polar bear migration, don’t forget to book your one-day adventure with Classic Canadian Tours!
