Posts Tagged ‘melanism’

Genetic Study Suggests Fox Squirrels (Sciurus niger) and Gray Squirrels (S. carolinensis) May Interbreed

September 14, 2019

Naturalists have observed male fox squirrels chasing female gray squirrels.  Male Squirrels chase female squirrels during mating season, and if they catch the opposite sex, it demonstrates their fitness for procreation.  Fox squirrels have never been observed actually mating with gray squirrels, and as far as I can determine, nobody has ever reported an hybrid between the 2 species.  However, a new study suggests they may interbreed or have interbred in the past.

Fox squirrel a coat of many colors

Black fox squirrel.

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Melanistic fox squirrel I photographed outside Spring Island, South Carolina.  This was before I had the benefit of a telephoto lens.  Click to enlarge.

It is estimated there could be as many as 25,000 black squirrels in the east of England

Melanistic gray squirrel.

The mutation for having a black coat (melanism) arose twice in fox squirrels from 2 different populations–an example of convergent evolution.  Western fox squirrels have 1 allele associated with melanism, while eastern fox squirrels have a different allele associated with melanism.  (An allele is defined as 1 of 2 or more alternative forms of a gene that arose by mutation and are found on the same place in the chromosome.)  Scientists believe the alleles for melanism in fox squirrels arose for different reasons.  The black coat on western fox squirrels helped them stay warm and more active in a region with colder climate.  The black coat on southern fox squirrels helps camouflage them because it makes them harder for hawks to see in a shady canopy.  The allele for melanism in western fox squirrels is exactly the same as found in melanistic gray squirrels.  Statistical models suggest the most likely explanation is interbreeding between fox and gray squirrels, though they can’t rule out 2 other explanations.

The allele for melanism may have originated in the common evolutionary ancestor of both species.  Alternatively, the allele for melanism may have arisen first in gray squirrels and was passed on to fox squirrels.  But the most likely explanation is it arose in fox squirrels and was passed on to gray squirrels during interbreeding.

Melanistic gray squirrels are more common in the northern region of their range because the darker coat keeps them warmer when they are outside during winter.  The authors of this study believe the black coat helped gray squirrels colonize newly deglaciated territory following the end of the last Ice Age when the climate was still quite cold.  They didn’t estimate how long ago fox squirrels passed on the melanistic mutation to gray squirrels.  It seems likely this may have occurred a long time ago when both species had recently split from their common ancestor.

Gray squirrels were introduced to England and have now almost completely displaced native Eurasian red squirrels ( Sciurus vulgaris).  Melanistic gray squirrels are replacing the original population of gray squirrels that outcompeted red squirrels in England.

It’s easy to tell the difference between gray and fox squirrels.  The latter are generally twice the size of the former.  A juvenile fox squirrel may be about the same size as an adult gray squirrel, but it is still easy to identify the correct species.  Gray squirrels generally have a white belly, while fox squirrels are solid-colored on the torso and belly.  Fox squirrels often have masks; gray squirrels almost never do.

Reference:

McRobie, H.; N. Moncrief, and N. Mundy

“Multiple Origins of Melanism in Two Species of North American Tree Squirrels (Sciurus)”

BMC Evolutionary Biology   2019