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Polar Bear: Evolved

April 8, 2010

ResearchBlogging.org
A recent study reports that grizzly bears are encroaching on polar bear habitat in northern Manitoba. That could be a bad thing—or it could just be a thing.

Polar bears and grizzly bears (also known as brown bears) have met before. In fact, they’re cousins. The brown bear came first. At some point (hundreds of thousands of years ago) brown bears began poking around on the ice and discovered a plethora of delicious nutritious seals. They tried hunting the seals, but they weren’t always successful. The seals could easily spot the dark-coated brown bears against the white ice, giving them plenty of time to escape. The lighter-coated* brown bears, however, were better suited for stealth hunting on the ice. For them, seal-hunting was awesome. Gradually, the light-coated seal-hunters began to split from their omnivorous relatives. And, according to DNA evidence from a recently unearthed polar bear jawbone, the polar bear branched off from the ABC brown bears** and became a distinct species between 110,000 and 130,000 years ago.

Polar bear (Ursus maritimus) in Wager Bay (Ukkusiksalik National Park, Nunavut, Canada). Photo: Wikimedia Commons, Ansgar Walk

Of course, the two species have met more recently and, at least in one instance, they seem to have gotten along quite well. A liaison between an apparently compatible female polar bear and male grizzly bear produced a grolar bear (a.k.a. a “pizzly bear” or a “polizzy”) that had its father’s shallow face and humped back and its mother’s creamy white fur. The animal—the first confirmed grizzly bear-polar bear hybrid in the wild—was shot in 2006.

Wild grolar bears may be rare, but intermingling between captive grizzlies and polar bears has resulted in a few hybrid bears. In 2004, a female brown bear and a male polar bear produced two grolar bear cubs. The captive grolar bears had a polar bear’s long neck and visible tail with the humped back of a grizzly. They were larger than a grizzly bear, but smaller than a polar bear and their feet were partially covered with fur (unlike their furry footed polar bear father and bare footed grizzly bear mother).

Physically, the grolar bears were an even blend of both parents, but behaviorally, they were all polar bear. The bears—who had been moved to another zoo shortly after their birth to prevent them from learning behaviors from their parents—even lay down polar bear-style with their legs splayed behind them. When they played, the grolar bears displayed behaviors unique to polar bears. They used their front paws to stomp on objects just as polar bears stomp on ice and used their teeth to shake bags back and forth the way polar bears shake prey.

Just because brown bears and polar bears are capable of making grolar bears doesn’t mean they will. Sometimes their little rendezvous end quite differently. Male polar bears are huge (about 1,200 pounds), but male grizzlies (600 pounds) are larger than female polar bears (400 pounds). This size differential can lead to trouble. Male grizzlies have been known to eat female polar bears and their cubs, which are born blind. Polar bears, in turn, may prey on hibernating grizzlies. Although the two bear species have different food preferences, grizzlies are fast enough to take down prey like caribou and polar bears are big enough to steal a grizzly’s kill.

The two species could make life miserable for each other, make babies together or totally ignore one another. With receding sea ice and minimally-restricted polar bear hunting, polar bears have plenty to deal with. They might just get the hell outta there and head north to follow the ice.

*There’s natural variation in brown bears’ coats from blonde to chocolaty brown.

** The ABC brown bears are a distinct population living on Admiralty, Baranof and Chichagof islands in southeast Alaska. These bears are more closely related to polar bears than to other brown bears.

Lindqvist, C., Schuster, S., Sun, Y., Talbot, S., Qi, J., Ratan, A., Tomsho, L., Kasson, L., Zeyl, E., Aars, J., Miller, W., Ingolfsson, O., Bachmann, L., & Wiig, O. (2010). Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear Proceedings of the National Academy of Sciences, 107 (11), 5053-5057 DOI: 10.1073/pnas.0914266107

Rockwell, Robert, Linda Gormezano, and Daryll Hedman (2008). Grizzly Bears, Ursus arctos, in Wapusk National Park, Northeastern Manitoba Canadian Field Naturalist, 323-326

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2 Comments leave one →
  1. April 10, 2010 3:22 am

    Great read! I figured grizzlies and polar bears were related, but I never thought of the possibility of interbreeding, or what the warming temperatures could mean for overlapping habitats… thx for sharing!

  2. Elizabeth permalink
    April 10, 2010 6:34 pm

    Very informative!!!! Since I was a child I have been fascinated by Polar Bears, but not so much Brown Bears. I never knew Polar Bears evolved from Brown Bears, so I learned something new today!

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