Posts Tagged ‘black bear color phases’

Black Bear (Ursus americanus) Diversity during the Pleistocene

December 19, 2016

Fossil evidence suggests North American black bears evolved from an Holarctic population of bears about 3 million years ago.  The founding preceding species is known as Ursus abstrusus in North America and Ursus minimus in Eurasia but they were likely the same animal.  Eurasian black bears ( Ursus thibetanus ) diverged from North American black bears during some climate phase when the ancestral populations became geographically isolated.  Before this divergence moderate climate allowed forested conditions to exist across the Bering land bridge.  But deteriorating climate transformed the land bridge to tundra when it wasn’t submerged under the Bering Sea.

Genetic evidence indicates western populations of North American black bears diverged from their eastern counterparts about 1.8 million years ago.  This corresponds with the beginning of the Pleistocene.  Although weak Ice Ages occurred during the preceding Pliocene, they became much more severe at the onset of the Pleistocene.  Glaciers covered most of Canada and the upper elevations of the Rocky Mountains, blocking gene flow between eastern and western populations of black bears.  Some mixing occurred (and is presently occurring) during interglacials, but because glacial climate phases are 5-10 times longer than most interglacials, isolation between eastern and western populations has  been the norm.

An ancient isolated population lives along the coast and islands of British Columbia.  Glaciers covered most of British Columbia during the Last Glacial Maximum but a strip of land along the coast, including the now submerged continental shelf, hosted a temperate rain forest with a population of black bears.  These forests were probably snow-covered for much of the year, an environmental condition that may have selected for white bears.  The white color may also help improve success hunting for salmon.  The lighter color is harder for the fish to see during the day. Today, this region is home to the highest incidence of white-coated black bears, variously known as Kermode or spirit bears.  On mainland British Columbia 1 in 40 black bears have white coats, while on some of the British Columbian islands 1 in 8 have white coasts.  (They are not albino.)  White bears in most black bear populations are extremely rare in other regions, though when the species was more common there were occasional individuals with white coats.  A white-coated bear was killed in a 1760 ring hunt (See: https://markgelbart.wordpress.com/2014/07/27/the-pennsylvania-mammal-holocaust-of-1760-a-rare-record-of-an-old-fashioned-varmint-drive/ )  Black bears may also be cinnamon, blonde, or even blue.

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The Kermode or Spirit bear–a white color phase of the black bear most common along the coast of British Columbia.  It’s an Ice Age relic population.

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Blue color phase cubs of a black bear mother.

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Cinnamon color phase mother with black and blonde (?) phase cubs.

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Blonde black bear.

During the Pleistocene before humans reduced black bear populations, many grew as large as grizzlies, and they had a much greater genetic diversity.  However, they competed with giant short-faced bears ( Arctodus simus ) and Florida spectacled bears ( Tremarctos floridanus ).  In open environments I think the former may have excluded black bears much in the same way grizzlies kept black bears from ranging into California valleys.  Spectacled bears co-existed with black bears in the same forested habitats for over a million years, and the environment in many places was rich enough to support both species.  Black bears were more adaptable than both of these now extinct species.  I hypothesize that unlike giant short-faced bears, they learned to fear man.  The ability to hibernate during cold weather also made them more widespread and successful than spectacled bears which were probably limited to regions with warmer climate.

Reference:

Marshall, H.D. and K. Ritbad

“Genetic Divergence and Differentiation of Kermode Bear Populations”

Molecular Ecology 2002

Wooding, Stephen and R. K. Ward

“Phylogeography and Pleistocene Evolution in North American Black Bear”

Molecular Biology and Evolution  1999