Are they white—or brown?

It doesn’t take a scientist to tell a polar bear and a brown bear apart. Or does it? Genetic studies have revealed that all living polar bears have Irish brown bear genetic markings in their DNA. An international team of scientists has traced the species’ collision back to a night of romance between a male polar bear and a female brown bear from Ireland between 20,000 and 50,000 years ago. Researchers believe that, today, climate change has forced grizzlies and polar bears, for example, to move into each others’ habitat, giving them several chances to engage in “opportunistic mating.” In the past, scientists say because the glacial ice sheet extended south into Ireland during colder times, polar bears could also meander into brown bear territory. These shared quarters resulted in hybrid offspring, according to the findings, which were published in the journal Current Biology. “When they come into contact, there seems to be little barrier to them mating,” said Pennsylvania State University’s Beth Shapiro, who led the study.