5 strange facts about polar bears

Polar bears face a number of threats to their survival: As their sea ice habitat dwindles in the face of climate change, and polar bear cubs perish during arduous swims in the open ocean, lawyers continue to wrangle over whether the bears should be listed as a threatened or an endangered species. (The two designations carry different levels of legal protection.) While these challenges mount, scientists are finding out more surprising details about the animals. Here, a look at five strange facts about polar bears:

1. They're Irish: All polar bears alive today have something in common with President Obama: Irish ancestry. Each polar bear claims a common ancestor, a mama bear who lived in Ireland some 20,000 to 50,000 year ago. By extracting the DNA from ancient bear bones, researchers were able to find that one female was the source of all modern polar bears' genetic material.

2. They once mated with brown bears: Though brown bears such as grizzlies are an entirely different species, with distinct behaviors and body types, DNA research proves that polar bears and brown bears mated in the distant past. "We know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years," says researcher Beth Shapiro, as quoted by U.S. News. In fact, the Irish mama bear was a brown bear.

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3. They're starting to mate with brown bears again: The last ice age forced brown bears into polar bear habitat, and as the two species overlapped, they began breeding with each other. Now that climate change and melting sea ice are forcing polar bears onto land to find food, they're moving into grizzly bear habitat, where the bears are producing hybrid cubs called "pizzlies" or "grolars."

4. They're setting up homes on land: Not only are polar bears moving south for food, they're also moving from sea ice onto land to find dens for rearing their cubs. According to a 20-year study by the U.S. Geological Survey, the percentage of polar bears who make dens on ice fell from 62 percent in 1985 to 37 percent in 2005.

5. They could be saved by goose eggs: As sea ice breaks up earlier in the year, polar bears have a hard time hunting on the water for seal pups, a favorite meal. So the carnivores have started eating eggs from snow goose rookeries. This doesn't threaten the survival of the snow goose: The egg-laying period for the geese overlaps just a little with the hunting season for polar bears, so the birds face only a small impact on their population numbers.

Sources: Bloomberg, LiveScience, The New York Times, NPR, Science, Treehugger, U.S. News