Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”
Naomi from Concord asks: “Yesterday, while rowing on the Merrimack I saw two squirrels swimming, in the middle of the river. That same day, a friend of mine called from Dover. She too was out rowing, thought she had hit a log and looked down and nope… it was a squirrel! A swimming squirrel. I came into work. I told someone about these strange swimming squirrels and they said yeah, that just happened to a friend of mine the other day. They were paddling in the middle of a lake and this squirrel came swimming towards them, jumped on the raft, ran across the raft, jumped off the other side and ran away. What is these swimming squirrels? Never saw a squirrel swim before this year. Dying to know! Is it a sign of the apocalypse?”
Well Naomi, yours is not the only squirrel question we’ve got in our inbox, but the other two are “why is there so much squirrel roadkill?” (Looking at you Ransom from Lee and Paul from UNH) which the inimitable Britta Greene answered on our airwaves precisely one week ago.
The answer? Because last year was a “mast year” for our local oak trees, meaning there were more acorns than usual in the forest. More acorns last year means more squirrels this year. Simple.
Fish and Game types pay very close attention to these food sources and will often point out the effects they have on wildlife. Low mast years make it easier for hunters to kill bears because the animals are ranging more widely in search of food, but also shorten the bear hunting season because they go to den early. Similarly, low mast years make deer easier to find, since they’ll be looking for shoots to nibble and venture out into grassy areas more often.
But evolutionarily, mast years are all about squirrels. We’ll get back to that in a second, but first let’s answer Naomi’s question, which I choose to summarize as “are swimming squirrels a sign of the apocalypse?”
The answer is YES.
Or at least, yes, it’s a sign of a squirrel apocalypse. And they used to be much worse.
When I started reaching out to squirrel researchers to answer this question, I was immediately forwarded an essay from 1920 describing massive squirrel emigrations that very frequently feature squirrels swimming across large bodies of water.
In particular, the piece includes two naturalists describing swimming squirrels in 1856 that is pretty jaw-dropping. For starters, there were so many that as they went they were “laying waste the corn and wheat-fields of the farmer,” and the pressure to move was such that “though swimming with difficulty, manage to cross broad rivers, like the Niagara and the Ohio, though many are drowned in the attempt.”
These migrations were observed intermittently, every few years – 1847, 1852, 1857 – but seem to have petered out at the end of the 1800s as the forests became increasingly fragmented by development.
But they still move in smaller numbers. One researcher I spoke with, John Koprowski at the University of Arizona, told me that squirrels have been seen swimming between the islands of the Florida keys.
So yes, squirrels swim. And they do so because they are hungry.
Last year there was plenty of food, which meant many squirrels had two litters instead of the usual one, and coming into this spring, there were many more in the woods than usual.
I can verify that our garden was absolutely plundered this spring, which was my first indication of trouble. As the summer progressed, and this bumper crop of squirrels worked its way through the bumper crop of acorns, they are willing to take any risk to find a spot with enough food to support them.
This includes crossing water and crossing roads. The migration opens the squirrels up to more danger — more chances to be seen by predators, clocked by a car, or drown in a river — which rapidly brings the population down to levels that the available food can sustain.
So, it's a Squirrelmageddon out there.
Now while there are many folks who likely find the quantity of dead squirrels out there distressing (and perhaps this is a teachable moment about how to decrease wildlife mortality on our roads), but here’s a bit of comfort: this has been going on for as long as there have been squirrel trees and oak trees.
And the reason for that is that this is a cycle that is part of a battle between trees that produce nuts and the creatures that eat those nuts. Certain trees have evolved to follow a boom-bust cycle of seed production. The reason for this is something called the predator satiation hypothesis.
“In a high mast year, it favors the dispersal of the seeds, and some then survive and that favors the tree, but the animals do very well,” explains Michael Steele of Wilkes University in Pennsylvania. “And then in a low-mast year it culls the seed-predator population and it crashes.”
In other words, when there are suddenly a lot of acorns available, the squirrels are overwhelmed and can’t eat them all. And when there aren’t many acorns the next year the squirrels die back in preparation for the next cycle.
While dead squirrels along the side of the roads point to people as a big source of squirrel mortality, many of those squirrels weren’t going to make it anyway, and if we weren’t here, they would have starved to death or been eaten by predators.
Swimming squirrels and squirrel roadkill are two sides of the same coin, and the coin in question is an evolutionary dance that has been going on since long before we were in the picture.
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