Squirrel speed

How fast can a squirrel run? I’m talking here about a red squirrel (the type we have in Jersey) and I’m assuming that the creature has been startled and is dashing for cover. Think for a moment: what sort of experiment could you arrange to answer this question?

At its simplest, this is a time-and-distance calculation. If we could time how long it takes a red squirrel to run a certain distance then we could determine the speed by calculating distance divided by time. The tricky part is the “if” in the previous sentence… How can we get the squirrel to run a measured distance (in a straight line, without deviation) at the exact moment when we are ready to do the timing?

If you are thinking that this sounds like an awful lot of effort to find out something that is unlikely to matter in the grand scheme of things, then let me come clean and admit that I did some of this work during lockdown when there wasn’t much else to be doing. And I’ll also admit that I like the idea of using the simple distance-and-time equation to find out the speed of various moving objects (see also my previous post on this topic, here).

Having got some squirrels used to going to a certain tree for food, and ensuring that the most likely route was along a narrow wall, I simply measured a section of the wall and set-up a camera on a tripod ready to capture a sequence of pictures. I was using a Nikon D700 and 100 mm Micro-Nikkor lens, with the camera set to its fastest setting.

I set the camera running then gave a loud cough to capture the sequence of pictures shown below. (Don’t worry, I’ve kept feeding the squirrels since then and I’d like to think that this one uncomfortable experience, in the interest of science, has now faded from their memory!)

In order to get a distance marker, I arranged two file-holders on the wall, as shown in the picture below, exactly 2 m apart (measured to the nearest millimetre).

To a first glance, it is easy to see that the squirrel moved slightly less than the distance between the two markers in the time taken to record two successive frames (after the starting image). The camera had a nominal capture rate of five frames per second (5 fps) so that means the time for this sequence was 0.4 s. If we take an approximate distance of 1.8 m then we get an average speed of about 4.5 m/s (1.8 / 0.4).

Of course, we can do better than to approximate – and we’ve assumed that the camera was running at exactly 5 fps, which we need to check.

All of that can wait until the next post – but you’re welcome to look at the pictures closely and decide exactly how far the squirrel really moved. (Hint: Which part of the squirrel would be best to use for these measurements?)