Wednesday, September 7, 2011

A 25-pound rattlesnake in South Carolina? Unlikely.

The reportedly 5.5 foot long, 25 pound rattlesnake
This morning I was made aware of a large Eastern Diamondback Rattlesnake that had been captured on Hilton Head Island in South Carolina. This in itself isn't too surprising, the species can range up to North Carolina and are often found on barrier islands. Although, there isn't much habitat left for snakes on these islands because they are popular tourist areas.

In any case, the articles describing the snake's capture note that the animal was five and a half feet long. This is a very large individual but well within reason. However, the weight of the snake is particularly puzzling: 25 pounds. This is well beyond the mass of any rattlesnake I have ever heard of, regardless of length.

To determine if it is conceivable for a 25 pound rattlesnake to exist, I looked through some snake data I had from Alabama and Georgia. Over the many years of trapping represented in my data, not a single rattlesnake reached 5.5 feet, so it wasn't possible for me to directly compare. However, there was one large rattlesnake that was 5.3 feet long. This snake weighed only 6.5 pounds. Using this information, I tried to crudely estimate how much I would expect a 5.5 feet long rattlesnake to weigh. A 5.5 foot snake is only 1.03x larger than a 5.3 foot snake, but I decided to be extra-cautious and estimated that it would weigh 50% more. As an analogy, this is like saying if a man that is 5 foot 9 inches tall weighed 180 pounds then a six foot tall man (someone about 1.04x taller), would weigh 270 pounds (180 pounds plus 90 pounds, which is 50%). Just three inches taller but 90 pounds heavier.  I think we can all agree this is probably a generous estimate.

Using these rough and crude numbers, I estimated that a 5.5 foot long rattlesnake would weigh 9.75 pounds (again, this is probably way, way more than it would really weigh, I just wanted to know if there was any chance of finding a 25 pound rattlesnake). Well, we are still about 15 pounds under. Not even close. Okay, but you might argue that a snake's weight can be influenced by a recent meal. I agree. So let's try to factor that in.

The largest prey item a big rattlesnake could eat is probably about the size of a rabbit. But, a big rabbit only weighs about four pounds. So, our theoretical snake, which was already morbidly obese at 9.75 pounds, would have to eat about four rabbits at once to reach 25 pounds. This just doesn't seem realistic and I am going to conclude that the reported weight is bogus. All of these estimates of course, assume that the length of the snake wasn't exaggerated. If the snake was actually smaller it would be even more of a stretch to say it weighed so much (off the record, the snake in the picture does look like it would be smaller than 5.5 feet long, but I can't say with certainty how long it was).

A large rattlesnake from southwestern Georgia
But, I only had a small sample size of snakes to examine and I wondered if it was possible that others may have data for larger snakes. So, I looked through a recent paper that reported the weights of the biggest Eastern Diamondback Rattlesnakes that were brought into Alabama and Georgia rattlesnake roundups each year for the last few decades. In these roundups, prizes are awarded for the largest snake, so there is a big incentive for people to bring in the biggest and baddest snakes they can find. Since 1959, the biggest rattlesnake was only about 15 pounds (this must have been a massive and impressive animal) and on average, the snake that won the prize for being the biggest in a given year was less than about 11 pounds.

I am going to read between the lines and hope that this South Carolina rattlesnake story has some good news. It sounds as if accurate measurements weren't made for the snake possibly because it was relocated alive elsewhere and nobody wanted to take the chance of measuring a large and ornery rattlesnake. In that case, let's cut some slack to all those involved. Perhaps the snake is growing heavier as we speak.

Added 9/9: For reference, here's an Eastern Diamondback Rattlesnake that is about 5'6" long (perhaps slightly larger) captured as a component of a research project in southwestern Georgia. The snake weighed less than nine pounds


D. B. Means (2009). EFFECTS OF RATTLESNAKE ROUNDUPS ON THE EASTERN DIAMONDBACK RATTLESNAKE (CROTALUS ADAMANTEUS) Herpetological Conservation and Biology, 4 (2)

No comments: