Evolution, squished squirrels and cars
May 19, 2015 8:24 AM   Subscribe

This came up in conversation, and now I'm wondering: has a little over a century of cars squishing squirrels made an evolutionary difference in their behavior? Or to put it another way: does that century of cars hitting squirrels that can zigzag better than their cousins that run in straight lines mean that automobile drivers are forcing an evolutionary selection in favor of the straight-runners?

The friend and I who were discussing this agreed that it's a given that squirrels (like many other prey animals, such as rabbits or deer) use suddenly dashing in different directions, rather than constantly running in straight lines, as a method of avoiding capture by predators, on the theory that the predators find sudden twists & turns harder to keep up with. Okay, that's worked fine vs. those predators, but it doesn't appear to work as well when avoiding cars: zigzag darting makes it hard for a driver to avoid hitting them, no matter how much a driver might prefer to do so. So: the squirrels that are better at darting-to-avoid-predators end up as roadkill, leaving the straight-line-runners to reproduce and theoretically pass on their straight-running tendencies.

And if the last century has not yet made such a difference, how long (if ever) would it take?
posted by easily confused to Pets & Animals (11 answers total) 5 users marked this as a favorite
A relevant question would be whether automobiles represent a significant killer of squirrels compared to other means of death. If not -- and I rather think this is the case -- then car deaths would not be a driver of evolution in squirrels.
posted by slkinsey at 8:34 AM on May 19, 2015 [9 favorites]

Best answer: 100 years is nothing in evolutionary time with organisms that cycle through generations on ~ an annual basis. If the selective pressure is intense you might be able to measure a difference, but it'd be pretty small. In a general sense, you're looking at thousands to tens-of-thousands of years for that needle to move in a big way. And that's if the contrasting selective pressure (i.e., the predator avoidance strategy) goes away or is minute in comparison to the avoiding cars strategy.

On the specific theory--It's an interesting one, however a couple of things I'd point out:

First, I'm not entirely sure I buy the argument that zigzag darting is a worse strategy than running in a straight line. There are plenty of times the only reason I haven't hit a squirrel with my car is that it zig-zagged.

However, if it was a better strategy to beeline it across the road, you'd also need to prove that roadkill deaths are a significant enough selective pressure to have an effect on the population. More specifically, a significant effect population-wide on the ability for squirrels to pass on that behavior to the next generation. My gut says it's not actually that big of a deal. Too many squirrels that don't have to contend with cars at all.
posted by mcstayinskool at 8:37 AM on May 19, 2015 [6 favorites]

I think even among squirrels who do have to contend with cars, cars aren't the main source of death. Looks, drivers actively try to avoid killing squirrels. Hawks, owls, foxes, are trying to kill squirrels. I think an evolutionary strategy that protects against things trying to kill you will generally be better than one that protects against things trying not to kill you.
posted by If only I had a penguin... at 8:49 AM on May 19, 2015

Best answer: Evolution can actually happen rather quickly in populations that are under intense selective pressure; I suggest anyone who is interested in learning about rapid shifts in animal morphology and/or behavior read the book The Beak of the Finch by Jonathan Weiner, about Peter and Rosemary Grant's work in the Galapagos islands. It's an amazing read.

I don't know of any studies that have been done on the effect of traffic on squirrel populations but I do know there is evidence that birds are already evolving in response to the presence of cars. I imagine certain urban squirrel populations are already evolving adaptations, too.
posted by BlueJae at 8:57 AM on May 19, 2015 [2 favorites]

Best answer: I don't know about squirrels, but 100 years is certainly plenty of time for moths to evolve.
Although grey squirrels are hardy mammals, many different things can affect how long they survive. Some of these things include the harshness of the winter months and sustenance availability -- their extremely diverse meal staples include beechnuts, acorns, bugs, grapes, bird eggs, birds, grass and amphibians. Grey squirrels sometimes even display cannibalistic tendencies, feeding on members of their own species. Predators, too, can affect grey squirrel life spans for the worse. Some of the critters' biggest predator threats are coyotes, red foxes, weasels, grey wolves, hawks and bobcats. Lastly, parasitic infections can negatively affect their survival, as they can sometimes trigger extensive loss of fur -- a serious wintertime hazard.
via http://animals.pawnation.com/average-life-span-grey-squirrel-4857.html

Looks like there's at least a million animals roadkilled in the USA daily. I'd call this enough to potentially have an effect on evolution.

If we're talking about cars and squished squirrels, another question you could ask is whether cars are having an effect on the crepuscularness of squirrels, since the roads are usually the most full with cars during dawn and evening, when people are travelling to/from work.
posted by aniola at 9:04 AM on May 19, 2015

Best answer: It has been proven that squirrels comunicate with one another the best way to get nut: once one squirrel knows how to get nuts from a particular bird feeder, the local group of squirrels will end up knowing.
The assumption can be made that they pass on how to best cross one particular intersection.
But that is not an evolutionary trait.
posted by thegirlwiththehat at 9:07 AM on May 19, 2015

41 million squirrels killed by cars each year in the USA, according to an estimate from a dead link on the Wikipedia page for roadkill. Out of how many total squirrels?
posted by aniola at 9:11 AM on May 19, 2015

Usually I see the following behaviors from squirrels as I approach their path in the road with my bicycle. First, they freeze or wait, presumably to determine my velocity and trajectory and level of interest in having them for dinner. I feel like usually they wait on the side of the road rather than in my path, but my path is narrow compared to that of a car. Then, if they think they can make it, they make a mad dash across my path for the other side of the street.
posted by aniola at 9:15 AM on May 19, 2015 [2 favorites]

I wonder if many squirrels die to cars. It seems to me I see a great deal more raccoons and cats.. probably because both are active at night, when it's harder for cars to see them.. so driver avoidance has to play a decent factor..

Then there's the squirrel I see on my route to work that uses the power lines and poles to cross the intersection.. I always kind of admired the smarts of that squirrel to figure out the least deadly path was over the speeding hunks of metal then through them.
posted by royalsong at 9:32 AM on May 19, 2015

Best answer: Best adaptation: being able to distinguish among threats (cars, cats, hawks) and reacting accordingly.
posted by amtho at 11:43 AM on May 19, 2015

Evolution can actually happen rather quickly in populations that are under intense selective pressure

Anecdatally, I've lived in a heavily treed neighborhood with lots of very old oaks, and a huge squirrel population. This particular population has a relatively high ratio of melanistic squirrels - maybe as high as 10%? In 22 years, I have seen only one dead black squirrel in the road. The gray ones are an almost perfect match for the particular paving used in Arlington, VA; and during the Spring when there are new, inexperienced kits, and the Fall when they ramp up their winter prep, there are smushed squirrels almost every day. More anecdata, the black ones always seem to be the biggest and fattest. There's one that lives in the trees overarching my place who chewed through two layers of cardboard to eat a half of a pound of See's chocolate buttercreams that my Mom sent me. How that little fucker did not die instantly of diabetic shock is beyond me. I expected to find his furry little ass dead on my patio the next day. Nope. Still living the life, tormenting the cats from the other side of the glass.
posted by ereshkigal45 at 12:43 PM on May 19, 2015 [1 favorite]

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