Polar bears and brown bears have interbred opportunistically and probably on many occasions during the last 100,000 years. [Image 2 of 5 related images. See Image 3.]
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An international research team led by Beth Shapiro, the Shaffer associate professor of biology at Pennsylvania State University, and Daniel Bradley of Trinity College, London, concluded that all living polar bears trace their maternal ancestor to a brown bear that lived near present-day Britain and Ireland just prior to the last ice age. The research team analyzed 242 brown bear and polar bear mitochondrial lineages sampled throughout the last 120,000 years and across multiple geographic ranges. They found that the fixation of the mitochondrial genome likely occurred during or just before the peak of the last ice age, possibly as early as 50,000 years ago. The specific population of brown bears that shared its maternal DNA with polar bears has been extinct for roughly 9,000 years, says Shapiro.
Climate change has forced polar and brown bears to share habitats. Occasionally, the two species interbreed and produce hybrid bears. Periods of warming and cooling during the last 500,000 or more years created conditions that were favorable for hybridization between the two bear species. "Polar and brown bears likely came into contact intermittently, in particular in coastal regions where the effects of climate change may have been more pronounced," says Shapiro. "Whenever they come into contact, there seems to be little barrier to their mating."
Results of the study are expected to help guide future conservation efforts for polar bears, which are listed as threatened under the Endangered Species Act. The research was supported in part by a grant from the National Science Foundation (grant EF 04-23641).
To learn more, see the Penn State news story Ancestry of Polar Bears Traced to Ireland. (Date of Image: 2010-2011)