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Natural History

Living on the Edge:
Prairie Rattlesnakes in Wyoming

by Matt Goode (1) and Dave Duvall (2)

(1) Wildlife Ecology Program, School of Renewable Natural Resources, The University of Arizona (2) Department of Zoology, Oklahoma State University

Some of the research I'll be telling you about is old. Most of it has been published in one form or another as those of you familiar with the literature will recognize. The presentation I'll be giving consists of short snippets of several talks that I and others have given over the years at scientific meetings and other public forums.

The prairie rattlesnake has a very wide geographic range from southern Canada to northern Mexico. Our work took place about 45 miles north of a little town on the edge of the Great Basin Desert called Rawlins, considered a big city in Wyoming with anywhere from 5,000-15,000 people depending on the current price of oil. Rawlins was our home away from home; our boom-town base camp of which we became quite fond. There are a hell of lot of rattlesnakes in this part of Wyoming. Here, they occur in high elevation prairies where they can be found denning up on steep, rocky, south-facing slopes in small mountain ranges or canyon and hill country. The snake populations we studied migrated out onto prairies during the summer.

All the rattlesnakes we studied originated from a series of large dens in the Haystack Mountains in Carbon County. Cold Creek Canyon was the site of some of our biggest study dens. The elevation here is over 7,000 feet and the snakes seem to be living on the edge of their ecological tolerance. The strategies the snakes employ in order to survive and reproduce under these harsh conditions is a theme that will keep repeating itself throughout the course of this talk. To give you an idea of the severity of climate in this part of Wyoming, we commonly received our last snowfall in June and it started up again in early September; there were fewer than 90 frost-free days per year at our field site. And what made it particularly unbearable was the wind -- blowing incessantly and often reaching speeds of 50-60 miles per hour with occasional gusts reaching 75 and even 100 mph from time to time.

How can rattlesnakes, which are ectothermic animals, manage to survive in such an inhospitable place where highs can reach 105 degrees during the day and nighttime lows, even in the hottest part of summer, can dip into the low 40's? Our research has allowed us to piece together at least a part of the answer to this question.

We found many dens over the years by searching during the spring or fall when rattlesnakes are either emerging from, or retreating into, the ground. The way to find a den is to go out and look for large concentrations of rattlesnakes in a very small area. I can remember finding a den one spring. Walking along, I suddenly heard a rattle and then another and another and another until pretty soon there are rattles coming from all around me. That's when you know you've found a den! It's both scary and exhilarating. One of our main study dens, fairly typical in appearance, consisted of a nondescript hole in the ground from which came over 600 animals! Apparently, there is a subterranean labyrinth underground where the snakes can go to survive freezing temperatures and a frost line that goes to a depth of about 4-5 feet in this part of Wyoming.

Once we find a den, we place a trap over it, which allows us to capture the snakes upon emergence. On a warm, sunny, spring morning the snakes come out to bask around the den entrance. We allow several to amass before rushing up to quickly catch the animals before they can go back down into the den. We commonly caught over 20 snakes at a time because the den opening is only as wide enough for one, maybe two snakes at a time. They get log-jammed and it is easy to capture them at that point. After filling 3-4 buckets with rattlesnakes, we take them back to camp to process. Processing consists of weighing and measuring them, marking them and gathering a host of other data. Processing is time consuming and can take several hours. We try to do it in the early morning hours when it's still cool. The snakes are a little more cooperative when they're colder. Otherwise, it would turn into quite a rodeo if you waited until mid-day and started handling prairie rattlesnakes. I've never seen a prairie rattlesnake in Wyoming in the field behave in the lethargic manner that black-tailed, tiger or western diamondback rattlesnakes often behave in Arizona. Prairie rattlesnakes are quick to rattle from a long distance, are easily provoked and just seem to be on the edgy side in general as far as rattlesnakes go.

A high-tech tool we use in processing the snakes is called the squeeze-box, which has foam rubber on the bottom and allows one to immobilize the rattlesnake without hurting it or risking a bite. Some of the snakes we captured were surgically implanted with radio telemeters: tiny, four to five gram devices that allow you to track the snakes wherever they go. Some were temperature-sensitive. These we calibrated using a water bath. Depending on the temperature of the snake, the transmitter beeps at a certain rate which can then be used to determine the body temperature of the snake. Body temperature is interesting because it can ultimately give you valuable information about energy budgets and other physiological parameters.

The radio telemeters were implanted into the peritoneum, or gut cavity, of snakes. We radio tagged and tracked over 150 snakes total and very few were lost due our surgical procedure. When we first started radio tagging snakes in 1982, the radio telemeters were much larger than they are today, which caused some problems. Back in 1982, before we implanted the transmitters, we didn't have a clue as to what the snakes were going to do once we released them. It's very important to realize that when you begin a long term research project, you'll spend a huge amount of time just getting out there and becoming familiar with the animals and building a sort of natural history database. This database of observations is critical to the formation of hypotheses and the direction of future research endeavors.

That is what we did to begin the project, and we immediately began to see very interesting strategies and behavior patterns emerge. These patterns are what grabbed our attention and inspired us to look at them in more detail, oftentimes with a more evolutionary slant or more pinpointed on some other interest. We were largely motivated by the simple desire to learn what it might be like to be a prairie rattlesnake, and our studies quickly became more and more involved and the questions we asked, more complex.

So what do the rattlesnakes do when they emerge from the den in the spring? First, they typically leave the den site very soon after emerging. Mating does not occur at the den like you find with Crotalus atrox here in Arizona. The snakes are not ready to mate yet because their gonads are not active during hibernation. They require extended periods of higher temperatures before oogenesis and spermatogenesis can occur. They seem to be occupied with finding something to eat. After all, they've been more than a meter deep in the ground for at least seven months. Peak ingress takes place in mid-September, and peak egress is in mid-May. In a nutshell, the snakes typically go on incredibly long-distance, straight-line movements during what we have termed a vernal, or spring, migration period. There is some variation in this pattern, but for the most part they have lengthy, fixed-angle movements. Interestingly, males move significantly farther than females. This was very intriguing to us from the beginning and we set out to try and explain these sex differences. This led us into some interesting research that started with basic natural history observations and has recently culminated with some theoretical models published by Dave Duvall and Steve Arnold. For the purposes of this talk, I'm not going to get into any heavy-duty theory. I want to stick more to the natural history stuff -- I want to tell the story of the rattlesnakes.

After leaving the dens, snakes typically move anywhere from 100 to even 1000 meters per day. As an example, a snake might move 720 meters at 115 degrees one day, and 420 meters at 112 degrees the next. The next day it may only move 65 meters at 119 degrees. When you plot it out on a map it is striking to see how straight the overall movement has been, amazing because they move on straight lines. In fact, I went out with a compass and tried to move lines as straight as the rattlesnakes were moving, but I could scarcely manage it, especially in areas with rolling topography.

The high plains of southern Wyoming are sparsely vegetated, with greasewood and Wyoming big sagebrush being the dominant plants. They occur in large clumps and are randomly and patchily distributed across the prairies and hillsides. Rattlesnakes key in especially on greasewood patches because, as it turns out, that is where mice are located. We gathered data on deer mice and the presence of active burrow systems along the routes taken by our rattlesnakes. We knew the rattlesnakes were eating deer mice because Mike King, one of Dave Duvall's students, analyzed fecal matter taken from our study animals. Prairie rattlesnakes are very opportunistic feeders, eating lots of other things as well which I will tell you about later.

After discovering this amazing and reliable life history strategy of moving on a straight-line vernal migration path, we decided to see if we could determine the functional significance of the movement patterns. Our first observations of rattlesnakes migrating were informative. It is amazing to watch them because they seem to be in some kind of zone, almost oblivious to their surroundings. You can actually follow them fairly closely while they cruise across the prairie, reaching speeds of about 0.5 meters per second. You can tell they are migrating, or engaged in a directed episode of movement, because they don't rattle. It is easily distinguishable from other escape locomotion. They seem to be in this sort of "migratory zone." They don't necessarily move from point A to point B in an exact straight line; they may have wandered around to a certain extent. But in many cases we witnessed them moving in very straight lines as if they were on a mission to get to where they were going. We had one snake that moved about fifteen kilometers one way, and then turned around in a looping motion, coming back to the exact same den which was visible only as a nondescript hole in the ground. While this is the general pattern for males and most non-pregnant females, some snakes remain relatively close to the den, foraging up and down the canyon below the den site.

Our data show that females move less than males, and their movement patterns are not as straight as males. We were very intrigued by these movement patterns and wanted to find out why they occurred. People often ask how they move in straight lines, which we may have been able to answer by conducting experiments where we snip their optic nerve or place goggles on them to see if they were using the sun to migrate or some sort of landscape navigation technique. While that's all very interesting, our research group was more interested in the 'why' questions than the 'how' questions. We were looking at the ultimate cause, as opposed to the proximate mechanism, of these distinctive movement patterns. Other people have looked at the mechanisms, and they appear to be using sun-compass and celestial cues for orientation. Landmark and chemical cues probably also play a role.

In an attempt to explain why the snakes were moving the way they were, we developed several alternative hypotheses based on why these snakes might move in the first place. An obvious one is the need to find food, and our small-mammal trapping gave us some preliminary data to support this notion. Other reasons for moving included finding mates. This is an appealing explanation because prairie rattlesnakes in Wyoming, and elsewhere, don't mate at the dens upon emergence in the spring. The dens don't appear to have a social function as you might expect when you see such a large aggregation of animals. We think that the reason you get huge aggregations is because hibernacula or den sites are a limited feature of the habitat. There are only so many places where snakes can go deep enough underground to get below the frost line. Rattlesnakes that don't go deep enough don't survive.

Perhaps they are moving to shed, but that doesn't seem to be much of an alternative hypothesis because there are suitable shed sites close to the den. Thermoregulation might be a reason, but again, basking sites are not limiting. There are areas which are better for thermoregulation, but it is not like timber rattlesnakes in the eastern U.S. where snakes move into rocky clearings in the forest to obtain sunlight. There are literally no trees in this area of Wyoming, except in riparian corridors.

Another reason to move might be to avoid inbreeding, or to outbreed optimally. While interesting, we have not been able to systematically rule out this alternative explanation, but we do have dens located at different distances from each other. Interestingly, we find very little mixing of rattlesnakes among dens, even though some are within the range that rattlesnakes typically move. This may be because the manner in which rattlesnakes return to the dens is mediated through chemical pheromones which the snakes can detect olfactorily. In this way, they may place themselves on a trajectory back to their native den and not to some other den in the vicinity.

In looking at the feeding hypothesis, we discovered that rattlesnakes were ceasing their migrations in the same places we were catching mice, and they were passing through areas where we were not catching mice. This led us to believe that perhaps the rattlesnakes were looking for mice. It was just one of those brilliant revelations that we scientists make from time to time. It's one thing to say the snakes are stopping for mice, and quite another thing to demonstrate that feeding is the functional significance of these straight-line movements. So, we concocted some field experiments ("controlled" field experiments are not often attempted, but can be powerful if done properly) and coupled them with some lab work to try to get at, once and for all, the reason why the rattlesnakes were making these unusual movements. We felt strongly that they were dedicated to solving some problem of survival in a harsh environment.

In our field experiments, we placed mice in cages and covered them with a little hut-like structure to provide shade. We placed little bags containing sawdust from inside the cage around the cage to allow rattlesnakes to smell them. In some cases, cages contained a live female deer mouse with a litter that had been captured on site, and the bags contained sawdust soiled with deer mouse odors.

We also wanted to know what makes the snakes stop for a mouse: is it the mouse's odors or must it be the presence of a live deer mouse? So some cages and bags contained only soiled sawdust and no live mice.

Our control consisted of cages without mice, and the bags contained clean sawdust.

Every time our snakes stopped, we quietly moved in and placed our conditions near the snake. Then we came back later to see what happened. In summary, what we found out, at the risk of boring you with some data, is that prairie rattlesnakes stopped longer at places where we augmented the "prey base" with our caged deer mice. They were not as interested in the odors as they were in the deer mice themselves. The presence of active deer mice made the snakes stop. In greasewood patches with deer mice, the snakes would often be found waiting in ambush at the entrances to deer mouse burrows or actively hunting for deer mice in the burrow systems. These early experiments are what we occupied ourselves with out in the middle of nowhere in the Wyoming wilderness.

We had found out where and why the snakes were stopping, but we still hadn't figured out why they were traveling in straight lines to get there. Furthermore, if the greasewood patches where they were stopping were not on their path, then they would pass by them. We knew that active rodent colonies were patchily distributed throughout the prairie, and we suspected that this might have something to do with the way the animals were moving. Eventually, computer simulations, enabling us to manipulate prey patchiness, demonstrated that a straight-line path will contact more patches than a random movement will. The rattlesnakes appear to be moving in straight lines because it increases the frequency at which they will encounter a patch consisting of an active population of deer mice. It's a nice example of optimality in foraging behavior that took quite a while to piece together in an empirical way using field experimental conditions.

To shift gears here a little, I want to talk about the pregnant females at our site because they are a completely different but equally interesting story. First of all, they don't go out on long migrations. They tend to stay in an area at the bottom of the canyon within a few hundred meters of the den site. Here they remain immobile for the most part, thermoregulating while gestating their young. The terms gestation and pregnant are proper in the case of prairie rattlesnakes because they are viviparous or live-bearers. Egg-layers are referred to as gravid (but you can also say gravid for rattlesnakes because they do have eggs which are retained inside the body). The area where the pregnant females aggregate is called the birthing rookery, and it seems to have important thermal properties that aid the females in pulling off a successful litter. The rookery is at the base of a large south-facing slope where there are many large, fairly flat rocks. At any one time we would have from 6-20 pregnant females in one confined area.

This was a very interesting discovery, but it took us awhile to figure out what it meant, and we still know very few details of how these rattlesnakes spend their time. We are not even sure if they eat, because we've never found a pregnant female eating. We don't know if they store sperm from matings that occur the previous fall. We don't know why they would aggregate, perhaps drawing the attention of predators. There are far more unanswered questions than answered ones.

After giving birth, the females typically remain with their babies until the babies shed and disperse. Oftentimes the babies would disperse before the mother. Perhaps some of you have heard Harry Greene and others talk about this phenomenon as potential evidence of maternal care in snakes. It is very interesting to me, because we often don't think of snakes, other than the famous brooding pythons, as being capable of such social acts. Brent Graves, one of Dave Duvall's graduate students, looked at pregnant female defensive behavior both before and after they gave birth and found that they tended to be more defensive after they gave birth, suggesting that they may be defending their young. I have witnessed a post-parturient female come out from under a shelter and rattle and coil while the babies scrambled in underneath a rock. She may have simply been protecting herself, but the neonates certainly seemed to take advantage of her presence.

I would like to talk about some other interesting observations we made while out there living with prairie rattlesnakes. Heavy-bodied pitvipers, like rattlesnakes, are usually thought of as sit-and-wait predators, too fat and lazy to go out and find a meal, preferring to wait until something tasty crawls within reach. This is certainly true to some extent, and our prairie rattlesnakes were frequently found with their heads at the entrances to mouse burrows waiting for some poor unsuspecting Peromyscus to come home and walk into their trap. Interestingly though, prairies are not the lethargic sit-and-wait predators you might think. We found our radio-telemetered animals actively foraging on numerous occasions, often apparently hunting underground while we followed their signal around from above. Many times after such an episode we would see them with a food bolus the next day.

They also capable of taking amazingly large food items. We found one large prairie rattlesnake eating a full-grown cottontail rabbit. In one case we found a baby rattlesnake that had eaten a huge prey item. When we handled the snake it disgorged a mouse that weighed only a fraction less than the snake. They are also opportunistic feeders, taking birds such as a ground-dwelling horned lark and carrion such as a mouse that is already somewhat decomposed.

Another aspect of the research which I became particularly interested in dealt with prairie rattlesnake anti-predator behavior. They have a complex suite of defensive behaviors that most of us are quite familiar with. People often think of rattlesnakes as being dangerous and aggressive, but they are really only trying to defend themselves, often bluffing impressively. When backed into a corner they do have the means to retaliate if necessary, but they often choose a more passive form of defense such as hiding or escaping. I was very intrigued by this defensive behavior from the start and I wanted to look at it from a couple of different angles. First, I had to establish that they were in fact being preyed upon -- a necessary first step, especially for an animal that people often think of as doing the preying and not the other way around! There are a lot of animals out at our field site that are known to eat rattlesnakes, including badgers, skunks, coyotes, golden eagles and a variety of other raptors. We found a dead rattlesnake in a golden eagle nest, and rattlesnake scales in coyote feces (their scales are easy to identify because they are keeled, like bullsnakes, which do not occur at our field site).

From previous work we knew that rattlesnakes often coiled threateningly, but rarely did they strike. Dave Duvall and Mike King threatened over 120 free-ranging rattlesnakes and less than 2% actually struck. The snakes were threatened by shoving the sole of the boot into their faces, but the snakes were never actually physically contacted.

This also enabled us to precisely quantify the sequence of their defensive behavior, which we found to escalate from passive to active defense in a predictable manner. Basically, the sequence typically begins with hiding or crypsis. They are very good at doing this. Most of you have probably seen how well rattlesnakes can blend into their environment. I'm convinced that you walk by a lot more than you ever see. I don't know if that's very comforting or not but I think it's probably true. After they have been detected they almost always go into an escape mode, trying frantically to get away. If escape doesn't work they will perform a behavior we have termed cocking, where the snake's lower body is locomoting away from the threat while the upper body is poised and ready to deliver a strike if need be. To me it's just further evidence that they are very reluctant to escalate to higher levels of defense, and strike. The snake seems to be torn between trying to get away and fighting back. Then they get into the notorious coiled posture. If you provoke them enough, a small percentage of the animals will do a neat behavior called head hiding where they sort of give up, and actually hide their head under their body coils. We found this in about 3% of the animals we tested. We've seen it in other situations outside of the times when we where doing our trials. And, they often make the rattle stick up a bit; maybe it's some sort of diversionary display that directs the predator's attention away from the more vital head to the more expendable tail. In the final analysis, we found that rattlesnakes are very reluctant to strike.

I wanted to look at what happens in the case where you actually physically contact a rattlesnake. I hypothesized that if you were to grab a rattlesnake it would be more likely to bite. My first design was a Rawlings hockey goaltender's glove, but the animal protocol people didn't go for it, so I came up with a modified pair of snake tongs, which were foam padded to avoid injury to the snake. I found 72 free-ranging rattlesnakes which I threatened in a rigid, stereotypic manner. Sixty-seven of the snakes promptly struck the tongs, so the sequence of their defensive behavior changed dramatically. It appears that rattlesnakes can somehow assess the degree of threat in a predatory encounter or the context in which it finds itself and respond accordingly. Now is this an earth-shaking result? No, but it does add to our knowledge of these fascinating animals.

One thing I noticed with the prairie rattlesnakes I studied was that their defensive behavior seemed to change depending upon how hot or cold they were. We expect behavioral performance to change with temperature in ectotherms, but I wanted to explore the temperature-dependency of the shifts in defensive behaviors that I observed. Work on agamid lizards in the Negev Desert of Israel showed that when they are cold they become mean as hell, curling their lips to expose razor-sharp teeth. They will give you a nasty bite, given the chance. But when they're warm, they simply run away, sprinting for cover. I did not see such a clear temperature-dependent switch in the rattlesnakes but it did seem similar.

My study design required me to threaten animals in the field and catch them immediately afterwards to get a body temperature using a quick-registering cloacal thermometer. To make a long story short, I found no differences in defensive behavior of males and non-pregnant females at different temperatures, but I found a significant difference in pregnant females. As their body temperatures increased their latency to strike increased. In other words, the colder they are the quicker they bite. Why is that? Well, I don't know for sure, but I have all kinds of arm-wavy explanations and a few not-so-wild speculations.

I think it has to do with their strategy, as discussed earlier, of staying put in the birthing rookeries, and with their obvious and important reproductive condition. They are in a specialized microhabitat that provides them with the type of thermal environment they absolutely need in order to reproduce successfully. They need to stay above ground where they can bask more, reaping the benefits of warm body temperatures on the development of their embryos. Rather than having to retreat underground where temperatures are not conducive to the development of ova, I think they take the risks involved with dealing with predators so they can stay above ground and get warmer quicker. The non-pregnant females and males do not have this constraint to deal with. That's just one potential explanation, or speculation, or whatever you want to call it.

I've gone on long enough, so I would like to close by exhorting the virtues of rattlesnakes. They are fascinating animals and there are many things about their biology that are completely unknown. We studied prairie rattlesnakes for 12 years and felt like we were only touching the tip of the iceberg. Questions and Answers

Q. What do the snakes do in June when it snows?
A. They find a place where they don't freeze, I guess. They find some shelter. I don't doubt that some of the animals succumb to the weather. But they have to deal with freezing temperatures pretty much throughout their active season anyway.

Q. Are they active pretty much just during the day?
A. For the most part yes. They don't seem to do much at night. Around here we always want to go out at night to find our rattlesnakes, but you don't go road riding at night for rattlesnakes in Wyoming because it's too cold. Most of the things we did were during the day, although we have found snakes actively migrating at night, on a warm night. How they're doing that I'm not sure.

Q. Who owns that land? Are there people picking them off?
A. That land is the I-Lazy B Ranch and it is owned by a guy named Bob Miller. Actually, I think it's owned by a corporation, but Bob Miller is the guy who runs it. It's a 357,000-acre private ranch and he doesn't let people out there. So, we have unharassed rattlesnakes. However, we had one snake, the one that moved fifteen kilometers, that made it all the way out to a road where some people found him, and we followed the telemeter to the back of their pick-up.

Q. When the snakes are moving in straight lines are they following the same path year after year?
A. That's a good question. One of the guys that came to study, Doug Brown, looked at exactly that question. We weren't able to do that till later in the study, till the early 90's, because we didn't get radio telemetry equipment for over two years. He had small sample sizes, unfortunately. However, he published that in the Chicago Herp Bulletin, I believe. To make a long story short, it's not conclusive but some of the snakes did go back to the same places they went to the year before. Not exactly on the same line, but they visited some of the same rocks, so kind of followed the same pattern.

Q. What determines the direction they go when leaving the den site, when they haven't been experienced?
A. I don't know the answer. I can speculate though. They do go all different directions from the den. A huge majority go out to the south, southwest and southeast, out onto this flat prairie area. Most likely because it is a better place to get rodents. We didn't see any differential survival rates in animals that went in different directions. It could be that gravity helps take them downhill, and downhill leads out onto the plains so maybe they're responding to that as a mechanism. I don't know for sure, but they did move in different directions. Interesting thing about their first time out was how they found their way back to the den again. If the animals got somewhere in the vicinity of the den I think at that point it could be easily mediated by pheromones.

Q. You mentioned something about a snake that has eaten a large meal would have to void the meal before hibernating if it was too late in the season. Did you actually witness this or any evidence of it?
A. No , I think that's perpetuating another 'old wives tail'.

Q. Where were these snakes breeding since the females didn't travel as far as the males? Did the males track back to where the females were?
A. They have what Dave Duvall has termed a 'wanderlust polygeny' mating system. Here you have snakes that are reproducing, if they're lucky, biannually, and probably not even that often. What that means is they aren't able to have babies every year. So they have to really build up their stores before they can go again, and that may take two to three years. Thus, if sex ratios are even in the first place, which they are in neonates, then in any given year you have females being a scarce resource. And that probably explains differences in males and females and the way you see them move. Those males are probably not only out looking for food but are also out looking for receptive females. The females don't mate until after they've shed. It's interesting because the source of the pheromone, which indicates to the males that females are receptive, is at the surface of the skin, post-shed. Sometimes you'll find a female with two or three satellite males hanging out around her waiting for her to shed and to see who will get the prize. Which is probably good for combat and male-male agonism but we don't see that there. Another strategy which you'd think would be effective would be to wait for the females as they come back to the den and mate with them atthat time. But the females aren't receptive when they return to the den. They're mating for the most part in a very narrow time window. You can peg it pretty much from July 1 to about July 15. We even had some years where everyone mated between July 5 and July 12. You can imagine the timing of things would be such if you live an environment like that.