The Squirrel-Conifer-Fungi Connection

The evolutionary divergence of the northern flying squirrel (Glaucomys sabrinus) from the southern flying squirrel (G. volans) is an excellent example of speciation resulting from environmental change.  Genetic studies suggest both of these American species of flying squirrels diverged from Eurasian flying squirrels between 4-6 million years ago.  Eurasian flying squirrels are much more diverse and include 44 species, most of which live in southeast Asia–evidence this part of the world is where they originally evolved.  During the late Miocene about 5 million years ago, a forested landbridge connected Asia with America, explaining how the ancestor of both American species of flying squirrels colonized this continent.  Genetic evidence suggests the 2 American species of flying squirrels diverged from each other early during the Pleistocene between 1-2 million years ago when Ice Ages began to become more severe.  Boreal spruce forests expanded during Ice Ages, growing as far south as middle Georgia and Alabama.  In the middle south spruce forests grew in higher elevations while deciduous oak forests still occurred in adjacent lower elevation.  Oak forests are rich in mast such as acorns and nuts, but spruce forests offer less food for squirrels–seeds from spruce cones are only available for 2 months of the year.  However, underground fungi, also known as truffles, are available year round in spruce forests.  For most species of squirrels, fungi is a minor component of their diet, but truffles and other fungi make up 85% of the northern flying squirrel’s diet whereas southern flying squirrels eat more acorns, nuts, berries, and animal matter.  The ancestors of the northern flying squirrel were those individuals from the parent population best able to subsist on a diet of mostly fungi.  These individuals were able to colonize spruce forests, while the rest of the parent population remained in oak forests.  Eventually, this habitat partition resulted in a divergence between the 2 American species.

Northern flying squirrels eat mostly fungi which is a minor component in most squirrel’s diet.  The ability to subsist on a diet of mostly fungi enabled this species to colonize spruce forests.  Eventually, they evolved into a different species than southern flying squirrels because of this capability.

Elaphomyces or truffle–favorite food of the northern flying squirrel.

 

 

Red spruce (Picea rubens).  Red spruce, truffles, and northern flying squirrels are beneficial and interdependent to each other.

Fossils of both species of flying squirrels have been found at Ladds and Kingston Saltpeter Cave in Bartow County, Georgia.  This is evidence that patches of spruce forest grew near patches of oak forest in this region during some climatic stages of the Pleistocene.  Northern flying squirrels are confined to the former; southern flying squirrels require the latter.

There is an interesting ecological interdependence between northern flying squirrels, red spruce, and several species of fungi.  Truffles grow intertwined with the red spruce roots, and they exchange nutrients.  The squirrels eat the truffles and spread their spores throughout the forest in their droppings.  A healthy spruce forest requires an abudance of truffles.  Many red spruce forests have been logged, and without the squirrel’s help, trees such as oak, maple, beech, and cherry are replacing them.  In West Virginia the U.S. Forest Service has successfully re-established red spruce forests.  Foresters discovered that red spruce seedling grow best in ground ripped apart by bulldozers and strewn with woody debris.  Some of these young spruce forests are on land reclaimed from strip mining. 

 

Fox squirrel.  This species may play a role in distributing fungi in longleaf pine savannah. 

Rhizopogon nigrescens–a fungi common to longleaf pine savannahs and likely an item in the diet of the fox squirrel.

Virgin stand of longleaf pine (Pinus palustris) in east Texas (circa 1908).

Although fox squirrels (Scirius niger) have a much more varied diet than northern flying squirrels, they occasionally eat fungi and may play a role in the health of longleaf pine savannahs.  Certain kinds of fungi that grow in the soil of savannahs also exchange nutrients with longleaf pine trees, and fox squirrels spread these spores in their dung as well.  Fox squirrels and longleaf pine savannahs were formerly common in the south, particularly on the coastal plain, but today both are rare.  The changes man has wrought have really sickened the natural communites of the world.

Reference:

Arbogast, Brian

“A Brief History of the New World Flying Squirrel: Phylogeny, Biogeography, and Conservation Genetics”

Journal of Mammalogy 88 (4) 2008

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Another Excerpt from Frances Harper’s Mammals of the Okefinokee Swamp–the Southern Flying Squirrel (Glaucomys volans)

I’ve been periodically posting excerpts from a rare book published in 1927 entitled Mammals of the Okefinokee Swamp by Frances Harper.  This time I’m posting Harper’s collection of accounts about the southern flying squirrel (Glaucomys volans).

Southern flying squirrel

http://www.youtube.com/watch?v=tNUom4Dmwp8

Youtube video of a southern flying squirrel gliding from tree to tree.  They can glide for up to 30 yards.

Supposedly, the southern flying squirrel is common and widespread throughout the state of Georgia, but I am skeptical.  They prefer old oak forests with lots of  snags and woodpecker holes.  I’ve never seen one in the wild and my cat never brought me a specimen, so I really doubt there are any in the woodlot behind my house or in any of the woods in my neighborhood.  Thirty years ago, a  college buddy of mine  did find and tame a specimen he found living in a birdhouse in his backyard, but that is the only time I’ve ever seen a flying squirrel in person.  Most flying squirrels commandeer woodpecker holes (sometimes eating the eggs and nestlings of the evicted birds in the process), although they do build their own nests on occasion.  The old oak forests of the Georgia piedmont have been replaced by young 2nd growth forests with far fewer snags and woodpecker nests than in former days.  Moreover, as far as I can determine from an internet search, no study on flying squirrel abundance in Georgia has been conducted…ever.  Flying squirrels are probably still common in the north Georgia mountains where unlogged oak forests still occur (See:https://markgelbart.wordpress.com/2013/06/25/old-growth-oak-forests-in-north-georgia/ ).  And they are a problematic predator of red-cockaded woodpeckers in south Georgia.

Fossils of southern flying squirrels have been found in several Pleistocene-aged sites in Georgia, including Kingston Saltpeter Cave and Yarbrough Cave in Bartow County, and the Isle of Hope site in coastal Georgia. The advanced evolutionary trait of gliding is probably an ancient characteristic of this species.

Below is Frances Harper’s collection of accounts of the southern flying squirrel which he refers to as the Florida flying squirrel.

Florida Flying squirrel–Glaucomys volans querceti

“The Flying Squirrel is known to most of the residents, and by its regular name.  It has been recorded or reported from the following localities in the swamp: Floyd’s, Minne Lake, Billy’s, Honey, and Chesser’s Islands, Clayhole Island, and Mixon’s Hammocks and Billy’s Bay.  It is said to be more or less common in various localities on the eastern side of the St. Mary’s River; north of Macclenny, Florida; along the Satilla River near Hoboken; and near Milltown, Lanier County…

…In the choice of its home within the Okefinokee this species does not exhibit a narrow taste, being found in such widely varying habitats as hammocks, pine barrens, and cypress bays.  It is perhaps attracted more particularly to the hammocks by reason of the acorns which it finds there on the live oak and other oaks.  Without the swamp it is found in unwelcome abundance in pecan groves.  It is entirely nocturnal, as far as my observations go.

In early January, 1917, at our camping place in the hammock on Floyd’s Island, several Flying Squirrels were heard moving about in the great live oaks overhead, and giving their slight, sharp, sibilant, little cries.  They were known to feed on some shelled corn stored in a large wooden box, and two specimens were trapped there.  In June, 1921, Jackson Lee reported hearing this species in the same camp.

On several nights in September, 1922, I heard the squeaky tseet, tseet, tseet of Flying Squirrels in the oaks about our camp in the hammock on Chesser’s Island.  One evening acorns began dropping outside my tent, and a couple of times one of the little creatures seemed to be scampering over the tent fly.  It was very successful, however, in eluding the rays of  a flashlight which I tried more than one to turn upon it. 

Ben Chesser once found a Flying Squirrel in a nest of  Spanish Moss which it had built in a quart cup about 6 feet above the ground by a spring in the piney woods on this island.

On June 19, 1922, David Lee cut down a dead slash pine (Pinus elliottii) in the pine barrens close to the hammock on Billy’s Island.  As the tree fell, a Flying Squirrel jumped out to another tree, then made for still another, but fell short and was caught.  It was kept in captivity for about six weeks, meanwhile feeding upon pecans, watermelon seeds, and huckleberries (the last with perhaps special avidity).  It refused peanuts.

Harry Chesser spoke of seeing several in the pine barrens on Billy’s Island.  Two sailed out of a living pine which he was cutting in the spring of 1922.  W.F. Keaton reported one or two during the previous spring in an old dead pine on Honey Island.

During the summers of 1921 and 1922 several were reported in holes in girdled cypresses, and one in a ‘green’ or living cypress.

On August 6, 1921, between 8 and 9 p.m. , a Flying Squirrel jumped on the roof of our tent, which was pitched on an oak ridge along the St. Mary’s River north of Macclenny, Florida.  At about the same time we began to take note of a shrill, sibilant, almost incessant calling on the part of two or three creatures of some sort, apparently in the trees overhead.  At the time I was inclined to consider them insects rather than Flying Squirrels, although, as David Lee remarked on a later occasion, the note of the latter is so much like that of some insects that it is difficult to tell them apart.  Meanwhile, several rat traps, baited with peanuts, were set on the trunks of near-by oaks, and presently, one of them contained a fine specimen of a Flying Squirrel.  Several nights later one was heard about our camp in a pine grove about 5 miles south of Traders Hill.

Some prejudice has been aroused against this species on account of its depredations on pecans in various localities near the swamp, where the pecan-growing industry has been considerably developed in recent years.  Its nocturnal habits enable it to pilfer to an extent not possible for a diurnal animal, and in places it evidentaly becomes a rather serious nusiance.  For example, various members of James Johnson’s family, living near Thompson’s Landing on the St. Mary’s, stated that a cat of theirs had caught 37 Flying Squirrels about their place during the pecan season of 1921.  The cat would eat each squirrel behind a certain door, and leave the tail there, thus enabling the members of the household to keep a tally.  They themselves made no effort to kill the animals saying, ‘It ain’t no use.’  Further complaints were heard concerning depredations on pecans near Cornhouse Creek, Charlton County, near Hoboken, Pierce County, and near Milltown, Lanier County.