Science says what? is a monthly column written by current Great Lakes contributor Sharon Oosthoek that explores what science can tell us about what’s happening below and above the waves of our beloved Great Lakes and their watershed.
The last few decades have been good for southern flying squirrels in the upper Great Lakes.
Like other species around the world, these treetop rodents have responded to warming temperatures by moving northward. In their case, gliding under cover of darkness from tree to tree using flaps of skin between their front and back legs. Taking advantage of air resistance, they can glide about three times their initial height while using their tail as a rudder.
Today, southern flying squirrels are routinely found in Ontario’s Algonquin Park, approximately 62 miles (100 km) from their historic northern limit and firmly in the territory of a distinct species of squirrel, the northern flying squirrel.
Jeff Bowman, a population ecologist with the Ontario Ministry of Natural Resources and Forestry and a professor at Trent University in Peterborough, was the first to notice their northward creep and continues to track their progress. His research reveals some intriguing implications.
Back in 2003, he discovered that where the two species overlap, some of their babies look a little bit like southerners and a little bit like northerners.
While both have almost comical-looking bulging eyes and can flatten their bodies like fur pancakes for aerodynamic gliding, southern chipmunks are smaller and have pure white fur on their bellies. Greater northerns have bicolor gray and white bellies.
But Bowman was finding some southern-sized squirrels with mottled gray-white belly fur.
Not surprisingly, he also found both species sharing tree hollows, where the squirrels huddle together for warmth on cold winter nights. And make babies.
DNA analysis would later confirm that the odd-looking squirrels were actually hybrids, and Bowman’s discovery would prove to be the first documented example of interbreeding after the species’ range expanded due to modern climate change.
Jeff Bowman measuring a squirrel. (Photo: James Proffitt)
Hybrids 101
To understand what is at stake, first a brief introduction about һybrids: Crossbreeding wild animals is not new, but human-caused changes, such as global warming, the development and introduction of alloһtoniһ creatures, are bringing previously separate species together.
Although there are no basic studies to show that there are more hybrids than nature intended, anecdotal evidence is mounting.
In the Pacific Northwest, interbreeding between spotted and barred owls threatens a tiny population of spotted owls whose old-growth forest habitat has been squeezed by logging. Across western North America, cutthroat trout populations have declined as they interbreed with various introduced trout species. And in central and eastern North America, the red wolf/coyote cross is a long-standing example of hybridization resulting from human development.
Interbreeding can have several consequences, none of which are well understood. That could increase genetic diversity, helping species cope with rapid ecosystem changes—perhaps Mother Nature’s response to human-induced upheavals.
But if the hybrids are better adapted to the changed habitat than either of their parents, this could lead to the genetic dilution of their parent species, even beyond recognition. In that case, the hybrids could become the dominant species, or what is known as a “swarm”.
Bowman is now pretty confident that this doesn’t happen with squirrels. His research shows that hybrids have been stable for the last 20 years in just under five percent of the population.
Although they can breed with each other and with their parent species, they don’t seem to do so much and this is probably because they are not well adapted to the habitat. Northerners tolerate the cold well, while southerners are good at fighting diseases from warmer regions. Perhaps their hybrid babies are capable of neither.
Whatever the problem, they don’t seem to live long enough to reproduce above the five percent threshold. They can essentially be a genetic dead end.
But it’s hard to know in advance whether a new mix of hybrid genes will hurt or help. One example of a genetic gamble that didn’t work out so well: Grizzly-polar bear hybrids in a German zoo excelled at hunting seals, but lacked the strong swimming abilities of their polar ancestors.
Bowman and his team recently sequenced the genomes of hybrid squirrels to find out what genetic changes might be responsible for their inability to increase their population, but they still have no results.
Graduate student, Rebecca Persad, searches for truffles (typical food for northern squirrels). (Photo: James Proffitt)
Crisis averted?
In the meantime, he’s looking closely at how all three types of squirrel habits might affect boreal forests. Bowman’s graduate student, Rebecca Persad, for example, recently discovered that their restaurant preferences have significant implications.
Northerners tend to eat fungi – mushrooms and truffles – spreading fungal spores and nitrogen-fixing bacteria as they defecate in the forest. This is important because boreal forests depend on both spores and nitrogen to create connections between roots that allow trees to share water and nutrients in the soil.
But southern fliers mostly eat seeds, having evolved in deciduous forests that produce seeds. If they take over from their northern cousins in coniferous forests and don’t become mushroom eaters, it could threaten the entire ecosystem.
Fortunately, southerners don’t seem to be picky eaters, and Persad’s early research suggests that they—and their hybrid babies—may be changing their diets to include fungi.
That could be good news for the boreal forests. For now, anyway.
As we humans continue to break down barriers between species, that may mean more hybrids, along with more questions about their impact on new habitats.
Get more Great Lakes news now:
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Featured Image: Southern Flying Squirrel. (Photo: James Proffitt)