How long is that snake?

This is one of the most frequently asked questions that a herpetologist hears. No doubt about it. Although I am still waiting for someone to ask “How tall is that snake?” Answer: “I’d say about 2 inches tall.” But, like height in giraffes, length is snakes is a perfectly relevant and interesting phenomenon. Let’s cover some basics first: 1) it is impossible to accurately determine the length of a snake that is coiled up, moving quickly, or resting in vegetation or rocks; and 2) it also is extremely difficult to accurately judge the length of a snake that is stretched out straight right in front of you. Our brains are just not accomplished at such a task. However, we could train ourselves to do so.

A legendary and herpetologist named William Martin made field observations on timber rattlesnakes in the Appalachian Mountains for decades. He is brilliant, keenly observant, and obsessive—in other words, the perfect field biologist. During the winter months, when his snakes were all hibernating, he decided to train his brain to accurately assess the length of snakes in any natural circumstance where he might find them, but without disturbing them. So, he spent every winter cutting various “rattlesnake-sized” pieces of rope and throwing them over his shoulder into rock piles, shrubs, leaf litter and such. He measured the ropes, of course, and did not give up until he could consistently get very good estimates of their lengths in any situation. So, what are the rest of us normal people to do?

Now, let’s cover a second round of basics: 1) as with gamefishes and alligators, people invariably exaggerate the size of snakes they have seen, so you can just count on getting bad data second-hand; 2) there are some very simple photographic tricks you can use to make a snake (or largemouth bass) appear very much larger than reality in an image, so you can discount most of what you see on social media; and 3) skins removed from dead snakes can be stretched far beyond the size of the snake, so you can forget about reports of 40-foot anaconda skins. Moving more into the realm of science, we have some other, quite pesky, basics with which to contend: 1) it is very difficult to stretch a snake out straight along a ruler, and the snakes really do not enjoy the experience, so we avoid that approach; 2) flexible measuring tapes, even cloth ones from fabric stores, are also insidiously difficult to use reliably, because the darn snake rarely will sit still (imagine trying to measure a high-pressure garden hose that has cut loose and is waving around your patio). These methods become even more useless if the snake is venomous, very large, or very strong and angry, and other proposed methods have not really solve the problems either. Anesthesia, such as for a veterinary procedure, would seem to be the perfect opportunity, no? Problems: 1) it is not realistic to anesthetize, say, 350 snakes a week at a remote field site on a research project; and 2) length measurements taken from anesthetized snakes are not correct! This last point is the real stumper, and leads to the core issue of this intractable herpetological conundrum. It appears that an individual snake does not actually have a consistent length. Whoa—what?! So, it’s as if I am 6’2” sitting at my desk one day, but 6’3” if I’m napping on a park bench. What do I put on my drivers’ license? Snakes have hundreds and hundreds of vertebrae, with a small, slightly compressible cartilaginous disc between each pair. As the waves of muscular contractions move rearward along a snake’s body, these discs are serially compressed or not. Because there are hundreds of these discs, the length of the snake changes slightly, but actually, every moment. An anesthetized snake has no muscles contracted, so it has a completely artificial length. Dead snakes have the same issue, and one can tug gently on such a snake and feel the backbone “pop” hundreds of time (like cracking your knuckles) and easily add nearly an inch to the length of a medium-sized snake. A living snake that is resisting your efforts to measure it with a ruler or tape will have many muscles tensed, giving you the impression of a slightly shorter snake.

What to do? Enter digital technology, to the rescue! As part of our recent collaborative research with Georgia Tech, we needed best-possible measures of lengths of the sidewinders we were studying. They are venomous, on top of all the other issues raised above. Our post-doctoral researcher, Dr. Henry Astley, had the idea to use a dual-image system of a living, unrestrained, and relaxed snake in a simple bucket and, knowing the diameter of the bucket, was able to use digital software to “draw a line” along the snake’s backbone and calculate its length. In the spirt of William Martin, we tested this with ropes of known (and unchanging!) length and had numerous different people take turns digitally measuring snakes and ropes and determined that we could actually get repeatable results that are correct for the ropes, and therefore for the snakes as well. In other words, this system achieved the ultimate goal in scientific data, yielding results that are both accurate and precise. We recently published our method and, I have to say, we are pretty darned proud of ourselves.

Publication: Astley, H. C., V. E. Astley, D. Brothers, and J. R. Mendelson III. 2017. Digital analysis of photographs for snake-length measurement. Herpetological Review 48:39–43.
Joe Mendelson, PhD
Director of Research