Monday, June 20, 2011

Are Rattlesnakes Rattling Less Because of Hogs?

Remember the Snakes at Your Service Blog Carnival? Well, our network of bloggers has expanded and organized a bit more (you can visit our website, like us on Facebook, and follow us on Twitter) and this week we started our second carnival: Herps Adapt! This event is all about the unique adaptations of amphibians and reptiles so I've decided to feature this post about whether rattlesnakes are adapting to feral hogs. You can follow me on Twitter and/or subscribe to this blog by e-mail.

The short answer:

There's no compelling reason to think so.

Full column:

In the past few months, I’ve received the following e-mail (or some similar version) several times. Generally, the scene is set in Texas (Coleman), but recently the location was switched to Georgia (Ohoopee River, Vidalia, or Lyons).

“We have killed 57 rattlesnakes on two separate ranches this year. 24 @South bend & 33 @ Murray , since mid May. Not one has buzzed! We provoked one fair sized boy with a stick and he coiled & struck at the stick a couple of times before he buzzed up and rattled. The purpose of this explanation is that I have been hearing the same from fellow ranchers and hunters in regards to the lack of warning with rattlesnakes. 

    
I had lunch with a friend today and he offered a theory about the fact that these bugs aren't rattling anymore. He raised pigs for years and reported that when he would hear a rattlesnake buzzing in the sow pen, the sows would bee line to it and fight over the snake. For the uninformed, pigs love to eat rattlesnakes. Therefore, the theory is they are ceasing to rattle to avoid detection, since there are plenty of pigs roaming the countryside. “

  Our first warning that we should be skeptical is the photograph that frequently gets attached to this e-mail. The featured snake is an eastern diamond-backed rattlesnake from Cooperville, Georgia (covered here). The closest this species gets to Texas is eastern Louisiana, and they are extremely rare there.

  But what about the text? Is it reasonable to suggest that rattlesnakes aren’t rattling much anymore because the noisy individuals are being eaten by pigs?

   Pigs are a relatively recent phenomenon in North America. Many were brought over from Europe as people from that part of the world colonized this continent. Many more pigs have escaped from farms and hunting preserves in the past few hundred years. Today, hogs can be found virtually everywhere across the United States. Once in the wild, animals that escaped from farms rapidly become feral and nearly unrecognizable from the barnyard creatures we know and love.  Hog populations can quickly form and grow as pigs reproduce and take advantage of their surroundings.

  And take advantage they do. As the habitats of North America did not develop with hogs, they are often unable to adjust to them. In the course of foraging for food, hogs are thought a destructive force, particularly as they root around in sensitive wetlands. Among many detrimental effects, the severe decline of some salamanders in the southeast has been attributed to the presence of hogs. So, it’s not out of the question to suggest that pigs (again, a relatively recent phenomenon), are causing changes to rattlesnake populations. But, let’s investigate this further.

  The first question is whether pigs eat rattlesnakes at all. Aren’t rattlesnakes equipped with enough potentially deadly venom to discourage pig predation? Apparently not. Although pigs are known to eat many amphibians and reptiles, there are few reliable observations of pigs eating rattlesnakes. But, I did find some accounts in a couple ancient rattlesnake tomes (ancient, but perhaps remaining the definitive source of information on these animals: Rattlesnakes: Their Habits, Life Histories, and Influence on Mankind, Second edition (2 volume set)). So, we know that although being bitten by a rattlesnake is surely an unpleasant experience, the prospect is not enough to dissuade a pig from eating one of these animals on occasion. Pigs primarily eat vegetation, so it is unlikely that rattlesnakes represent a major component of their diet, but anyway...

  Let’s move on to the next part of the e-mail, specifically the suggestion that rattlesnakes are not rattling anymore so as to avoid detection by pigs. This is where we seriously begin to strain credibility.

  A major assumption of this statement is that rattlesnakes used to rattle a lot and they don’t now, regardless of the cause. No study has ever investigated this potential phenomenon, and it is a big stretch to say it’s happening at all. It’s easy to find people offering personal observations one way or another. So...I’ll add mine! 

  Although I don’t have personal, historic experience with rattlesnakes, I have plenty of experience in recent years in an isolated area of southwestern Georgia, before there were many feral pigs. On this site, there were lots of eastern diamond-backed rattlesnakes (as well as timber rattlesnakes), and they rarely rattled unless they were disturbed. I have been within a few feet of rattlesnakes on several occasions (that I knew of, surely there were many that I did not even detect) and they were unlikely to make any noise or even move a muscle. We can’t attribute this behavior to pigs, because they were virtually absent.

  Why didn’t these animals rattle whenever I came close by? Because rattlesnakes don’t want to advertise their presence, they have nothing to gain by attracting attention. If a potential predator walks by a highly camouflaged rattlesnake without ever noticing it, well, that is quite alright with the rattlesnake. Only when the potential predator has discovered the rattlesnake will the snake benefit by giving a warning rattle.  That is another reason I’m skeptical of the story in the e-mail. A hidden rattlesnake in the woods is unlikely to rattle when there is a herd of pigs nearby (presumably causing the pigs to “bee line” towards it).

  Some readers will likely be quick to point out that there are exceptions to this rule, and sometimes rattlesnakes rattle when they’re not being disturbed. I agree, but I’m talking in general terms.

  In any case though, for the sake of furthering the discussion, let’s assume that pigs are eating rattlesnakes that rattle. Is it likely that rattlesnakes are ceasing this behavior to avoid detection? No, it’s not likely at all. This statement assumes that individual rattlesnakes are learning that there is a big disadvantage to attracting a pig’s attention (a similar assumption is required to accept the myth that baby rattlesnakes are more dangerous than adults). For a rattlesnake to learn there is a disadvantage to rattling, wouldn’t it have been necessary for that rattlesnake to have some unpleasant (but not fatal) experience with a pig? Not only that, it would have been necessary for an individual snake to learn, over several occasions, that rattling leads to pigs finding them, and they don’t like that, so they should stop rattling. 

  Presumably, these pigs wouldn’t be very efficient at killing the snake, and if the snake is surviving numerous encounters with pigs, then maybe they’re not such a concern after all.  Laboratory experiments make this hypothesis even harder to accept. Some researchers have demonstrated that by repeatedly disturbing a rattlesnake over a short time period, it becomes less likely the rattlesnake will rattle. The snakes get used to the disturbance. But, the next day, the snakes start rattling like nothing had ever happened, they don’t remember or learn what they had gotten used to the day before.

  A separate question is whether pigs are leading to populations of rattlesnakes changing their behavior. For example, if the tendency for a rattlesnake to rattle had a genetic component, then if pigs are eating rattlesnakes that tend to rattle they are influencing the gene pool. Remaining rattlesnakes would be those that are genetically predisposed to staying quiet, these animals would be more likely to pass on their genes, resulting in even more quiet rattlesnakes. This would basically mean that rattlesnakes are evolving. This seems potentially feasible, but it should be noted there is no evidence that rattling behavior has anything to do with a snake’s genes, or that pig predation is intense enough to affect populations to such an extent.

  In conclusion, although pigs may eat rattlesnakes, let's say it's unlikely they're influencing rattlesnake behavior because 1) we don't even know for sure that rattlesnake behavior is changing for any reason, 2) rattlesnakes generally rattle to deter predators, not attract them, 3) rattlesnakes don't learn to stop rattling, even if the unlikely scenario would emerge that would facilitate learning, and 4) there is limited evidence to suggest there is a genetic component to rattling, or even if pigs are exerting a powerful enough effect to alter rattlesnake gene pools.

  Feral pigs are a destructive force, having invaded many sensitive habitats, yet the effects of these invasions are not yet fully-understood.  However, there will likely be many additional studies on the subject, as it is unlikely pigs are going anywhere anytime soon....


Interested in reading about a fellow Auburn Tiger's experience hunting wild pigs across the country? Check out this book: Year of the Pig



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Much of what I write is based on my experience in the field, however I also rely on the research of others, citations of some relevant scientific articles are below.

Means, D., & Travis, J. (2007). Declines in Ravine-inhabiting Dusky Salamanders of the Southeastern US Coastal Plain Southeastern Naturalist, 6 (1), 83-96 DOI: 10.1656/1528-7092(2007)6[83:DIRDSO]2.0.CO;2


Jolley, D., Ditchkoff, S., Sparklin, B., Hanson, L., Mitchell, M., & Grand, J. (2010). Estimate of herpetofauna depredation by a population of wild pigs Journal of Mammalogy, 91 (2), 519-524 DOI: 10.1644/09-MAMM-A-129.1

Place, A., & Abramson, C. (2008). Habituation of the Rattle Response in Western Diamondback Rattlesnakes, Crotalus atrox Copeia, 2008 (4), 835-843 DOI: 10.1643/CE-06-246

6 comments:

Ian Nance said...

It's be rumor for twenty years in FL that rattlesnake populations have taken a dive due to hogs...and indigo snakes. I'm not all that sure about them killing full grown adult ones, but young ones and nests is certainly feasible. Coyotes probably putting pressure on them too. And humans!

David Steen said...

Ian,

I think it's safe to say that there are many less rattlesnakes today than there were a hundred years (or less) ago, due solely to the fact that the species has only a fraction of their original habitat left. When you add additional, new threats like hogs and coyotes (and, of course, humans), rattlesnakes have many problems to contend with. I'm not a proponent of the indigo snake theory. Although indigos eat rattlesnakes, they've been doing so for many, many years, so I doubt we can attribute recent declines to them. In addition, indigo snakes are now rare themselves.

Dave

Ian Nance said...

Yeah, that came off weird. I was meaning to say hogs eating indigos. Alot of the indigos problem is lack/declining of scrub areas - but hog trample through what's left of that too. Long time since I've seen an indigo

David Steen said...

That makes more sense! Indigos are known to use wetlands heavily, depending on the season. This would probably make them vulnerable to hogs.
I've never seen a wild indigo.

Dave

what said...

Very intriguing article. I've heard for several years that eastern diamondbacks do not rattle because this makes them easier to locate by humans. Humans (like hogs in your article) easily locate and kill and the snakes that rattle; therefor the quiet snake lives and passes on the "quiet" gene. Whether on not this is true, I always encourage my friends not to kill rattlesnakes.

David Steen said...

What,

Thanks, I'm glad you found this information useful. I have also heard this rumor in reference to humans. The first step in evaluating your hypothesis is to find out if there is a genetic component to rattling behavior. If there was (and there could be), then your hypothesis is certainly plausible.

So many rattlesnakes are killed by humans, whether they're rattling or not (on the road, for example), that I am skeptical that human-caused mortality is selective enough to lead to this type of change in rattlesnake populations. But, I imagine we'll be able to test this someday...