The Lizard Log

The Langkilde Lab in Action

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Attracting Unwanted Attention

I’ve certainly attracted my share of unwanted attention: missing a key free throw in a basketball game when everyone in the crowd was watching, faceplanting while exiting stage right in A Midsummer Night’s Dream, or unconsciously ripping a huge burp at a fancy dinner. Fortunately, as a human, the stakes involved in these mistakes were fairly low: a little embarrassment and a good story once the shame had worn off. For many organisms, like lizards, however, attracting unwanted attention in the real world can have serious consequences…

From previous research in the Langkilde Lab, we know that invasive fire ants (Solenopsis invicta) can pose a serious danger to native fence lizards (Sceloporus undulatus), and that fence lizards from areas invaded by fire ants respond to encounters with these ants with a variety of twitches and scratches to remove ants as well as fleeing more often to escape them.

We know little, however, about the behavioral rules that govern fleeing and twitching in these lizards. Do they flee more from all predators? Do they flee from all ants? We know the benefit of twitching and fleeing (not getting stung!), but is there any cost to these behaviors? Because we’re ecologists, we looked to answer these questions by conducting a series of experiments and published them in Animal Behaviour.

So, do fence lizards from fire ant-invaded areas flee more from other predators? Because we couldn’t let actual predators attack our lizards, I borrowed a stuffed kestrel (Falco sparverius, a well-known predator of fence lizards), rigged it with a wire harness, attached some very serious and scary-looking eyes, and swung it at unprepared fence lizards to see what their reactions would be.

Taxidermied kestrels are mostly as scary as the real thing.

We found that fence lizards from sites with and without fire ants fled from simulated kestrel attacks the same proportion of the time, and with the same strength and latency (reaction time), suggesting that fence lizards exposed to fire ants don’t flee more from all predators.

We next tested our fence lizards’ reactions to fire ants (which we’ve done before) as well as two types of native ants which might annoy lizards by running on them, but lack the venom (and danger) of fire ants. In fact, these native ants are important sources of food for fence lizards under normal circumstances. In these tests, we found that fence lizards from sites with fire ants fled more from all types of ants, not just fire ants, indicating that this fleeing behavior is generalized to multiple types of ants that they encounter, including those that don’t pose a serious danger.

For the more visually oriented, this series of experiments was illustrated super-well by Tali Hammond, a behavioral ecologist, who was interested in the paper (check out that sweet Sceloporus!)

Check out more illustrated papers by Tali: @mammalLady

So what might the consequences of this generalization be for fence lizards? For one, it’s obviously not ideal to be running away from something (i.e., native ants) that isn’t a threat and should be a meal. This could result in lower food intake or time wasted running away from non-dangerous ants, though we haven’t tested for these effects. More dramatically, twitching and fleeing break crypsis, a lizard’s primary defense against its visually hunting predators, including snakes, and birds of prey like the kestrel. While fence lizards are usually quite well camouflaged, imagine how easy it might be for a predator to spot a lizard jerking around as in the video above. We looked for evidence of this cost in our lizards by quantifying the amount of injuries (broken tails, scars, missing limbs) to lizards at sites with and without fire ants.

A missing “hand” is an example of injuries many fence lizards have.

We found that lizards from sites with fire ants do indeed have more of these injuries than lizards at sites without fire ants. This result suggests that fleeing from fire ants might attract unwanted attention from other types of predators. And when we consider that these lizards also flee more from native ants, which are common in the environment, these antipredator behaviors might have a serious drawback. It is important to note that this evidence is circumstantial; we didn’t see predators preying on lizards running from ants (this would be very difficult), and there could be other explanations for this pattern, such as differences in predator communities, or perhaps differing skill levels of predators. However, this work suggests that lizard that twitch and flee in response to ants may be attacked more.

So why do we see this behavior if it has these drawbacks? My personal guess is that it comes down to consequences. As a human, the consequences of my unwanted attention were fairly minor (shame). The stakes for fence lizards are a bit higher: fleeing can lead to running from your own dinner or attracting attention from predators. BUT the costs of not fleeing when attacked by fire ants are likely even higher (serious injury or death). And in many areas fire ants are much more common and likely to interact with lizards more frequently than snakes and kestrels. In other words, the lizards are likely making the best choice available to them. In the future, perhaps, they will adapt to distinguish between dangerous and native ants, allowing them to make more optimal decisions, and reduce the costs of these antipredator behaviors. More broadly speaking, I believe this research shows that we need to consider a wide variety of potential costs as well as benefits when looking at organisms adapting to changes in their environments.

The full paper in Animal Behaviour can be found here.


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Rattlesnake Research Round-up

One in a continuing series of posts highlighting the work of undergraduate researchers in the Langkilde Lab.

Michaleia Mead

My name is Michaleia Mead, and I am a senior in Wildlife and Fisheries Science, Wildlife Option. This is my first year working in the Langkilde Lab.  I spent the summer 2015 working for the lab on maternal stress of Eastern Fence Lizards.  But currently, in my research under Dr. Chris Howey, I am looking at ecological trade-offs between thermal quality and risk of predation at timber rattlesnake (Crotalus horridus) gestation sites.  A gestation site is the area that a female snake spends the duration of her pregnancy in from early spring, and throughout the summer to parturition (birth of the litter).  These sites typically receive a good deal of sunlight and allow the female to maintain an elevated, preferred, body temperature.


An adult female found basking at a gestation site later in the year.


A neonate rattlesnake found at a gestation site.  Note the size of the neonate as it is hiding under a small birch leaf.  Neonates will typically remain at the gestation site for about 1 week, basking in the sunlight, prior to following their mother to a nearby den site.

The “openness” of gestation sites range from very open to densely forested sites, enclosed by many tall trees.  For this study, we identified three sites that were very open and three sites that were more enclosed.  We compared the canopy openness, thermal quality, and risk of encountering a predator between these two types of gestation sites.  In particular, we are comparing the relationship between site openness and thermal quality as well as site openness and the probability of timber rattlesnakes encountering a predator.  Thermal quality is the amount of time that the range of temperatures a rattlesnake prefers overlaps with the temperatures that are available (thermal quality would be better at a site if it provided more time when available temperatures overlapped with preferred body temperatures).  I am testing the hypotheses: 1. Thermal quality of the open sites would be better due to a greater percentage of time that the available temperatures would exceed preferred temperatures, and 2. Risk of encountering a predator would increase as site openness increases.  

Cameras were placed at each of the six gestation sites to monitor rattlesnake behaviors and occurrence of potential predators traveling across gestation sites, despite the fact that bears sometimes take our cameras out…jerks.  We also placed biophysical models at each gestation site to measure the available body temperatures at those sites. Via these cameras, I’ve observed potential predators and also documented potential food sources for the rattlesnakes, such as chipmunks.  We’ve determined that open gestation sites are in fact warmer than more enclosed gestation sites, and the thermal quality of these sites is better.  The improved thermal quality of these open gestation sites may lead to shorter gestation times by gravid females, more successful reproductive bouts, and increased population health.  Contrary to our second hypothesis, we do not see more predators at more open gestation sites; however, we do see more predators as the overall area of the site increases.  This suggests that we may increase the openness of gestation sites in order to improve thermal quality for rattlesnakes, and, as long as we do not increase the overall open area on the ground, risk of encountering a predator should remain constant.  All of the data have not been analyzed, but they are showing signs of supporting the first hypothesis.  I am currently working on writing a manuscript for these data and should be completed sometime in late January.

A red-tailed hawk swoops in on top of a foam timber rattlesnake model (not seen). A yellow-morph timber rattlesnake model is pictured behind the hawk.

A red-tailed hawk swoops in on top of a foam timber rattlesnake model (not seen). A yellow-morph timber rattlesnake model is pictured behind the hawk.

Tommy Cerri

My name is Tommy Cerri, and I am currently a senior biology major. This is my 3rd year working in the Langkilde Lab. My current research working with Chris Howey focuses around the behavior of rattlesnakes and relationship these snakes have with predators in the community while entering into their den (referred to as ingress). The main objective of my study is to observe rattlesnakes of varying ages entering and exiting rattlesnake dens and the relationship these behaviors have with the occurrence of predator visits at those den sites. For this study, we are predicting that:

  1. Adult and juvenile rattlesnakes will arrive to the den before neonates
  2. Predator visits will increase as more snakes begin to enter the den
  3. Snake activity in front of the den will increase positively with environmental temperature and decrease when temperatures drop, and
  4. Snakes will retreat to the den as predators approach.

This research is proving to be important to the scientific community because we are beginning to see some behaviors that have never been documented before.  For example, we are finding that rattlesnakes are very active around the den for at least a couple weeks prior to hibernation.  Additionally, prior to exiting the den, rattlesnakes appear to stay under the overhang of the den and observe their environment.  We are unsure if this observation behavior is to test the thermal characteristics of the environment or to look out for predators.  In addition to this, the study may provide insight into what snake age is the preferred prey of predators within the community.  Snakes varying in age from neonate to juvenile and adult have all been seen coming in and out of multiple den sites that have been studied via different cameras that have been set up facing four different rattlesnake den sites located throughout Pennsylvania and New York.  From these cameras, we are able to see predators that visit the den as well as the behavior of rattlesnakes as they enter and exit the den during the day. While collecting data, this is what the view of the den from the camera looks like.


Can you spot the rattlesnake in this picture from one of our trail cams?


In this case, the rattlesnake is only a few feet from the mouth of its den.

In the pictures above we are actually able to see two adult rattlesnakes basking right outside two of the den sites under question. While right now the study is still in its preliminary phase of data collection, we have already observed some interesting trends between snake activity and temperature.  For example, snakes tend to be active outside of the den during both the day and night until ambient temperatures reach a specific threshold (roughly 42 degrees F).  Some of these activities appear to be basking behaviors, however, night time activities are unknown.  As of right now, we are not seeing any trends between predators and specific age-related prey choices, and we have not recorded any predation events. We hope to be finished data collection by the end of December and have a paper reporting the results completed by May.

Stay tuned, as we’ll publish a follow-up here with our official results!

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Bears Are Jerks (and Other Things I Learned Along the Way)

I would have done it differently.  Yeah, I think that is a good way to start this post.  But everything makes more sense in hindsight.

Let me supply the background:  The idea was to set up field cameras in front of den sites and observe timber rattlesnakes while they were returning back to their dens.  We would also record environmental temperatures outside of den sites via iButtons.  Once again, I would team up with fellow colleague Tom Radzio, and for this project we would also get some amazing help from undergraduate Tommy Cerri.  We would correlate timing of rattlesnake ingress and environmental temperatures.  But there is evidence that rattlesnakes don’t just dive into the den and say goodnight; they hang out in front of the den for a few days and mingle with one another, hear stories about each other’s summer vacations, and bask in the few remaining days above 10 °C.  The cameras would capture these behaviors in relation to environmental temperatures.  The cameras don’t have audio capabilities, so we are not able to capture the stories of summer vacation, but you’ll just have to take my word on this fact (#nottrue).  While relaxing in front of the den and basking in the fall sunlight, the snakes may expose themselves to potential predators.  A colleague, Chris Camacho, captured some fantastic pictures last fall showing that predators do in fact visit these den areas (check out more of Chris’ fantastic photos).

Red-tailed Hawk landing in front of a den site with a Timber Rattlesnake in front.

Red-tailed Hawk landing in front of a den site with a Timber Rattlesnake in front.

Fisher checking out den site.

Fisher checking out den site.  These are some pretty carnivorous animals!

Raccoon nosing around the entrance to the den.

Raccoon nosing around the entrance to the den.  It appears it is really looking for something.

Momma Black Bear and her cubs walking past a Timber Rattlesnake den.

Momma Black Bear and her cubs walking past a Timber Rattlesnake den.

So this year, we staked out three den sites with field cameras.  We placed two field cameras at each den site.  The one camera was on a tree about 8 meters from the den.  This camera would capture potential predators as they stopped by to visit the den.  The second camera would be much closer to the den and capture the rattlesnakes as they moved in and out of the den.  However, there was a problem with trying to put a camera so close to the den site… the problem was that there wasn’t always a tree right next to the den.  Problem solved!  I built a wooden stand that would support the camera and keep it focused on the den site.  To standardize things, we used this wooden stand for all three den sites, but kept the second camera farther away on a tree (the picture below is taken by the tree camera and you can see the den camera in the background).

Camera positioned directly in front of rattlesnake den.

Camera positioned directly in front of rattlesnake den.

This stand worked great.  And we soon began to capture a few rattlesnakes as they came back to the den site.

Timber Rattlesnake relaxing in front of den.

Timber Rattlesnake relaxing in front of den.

Rattlesnake basking in front of den entrance.

Rattlesnake basking in front of den entrance.

And then we even began to see some bears as they visited the den sites…


First Black Bear to visit one of the den sites.  This was a nice bear.  Thank you nice bear.

And then the bears became jerks.

Black Bear sitting in front of camera and bending camera over so that it can gnaw on it... jerk.

Black Bear sitting in front of camera and bending camera over so that it can gnaw on it.  The camera was attached to the wooden stand by a thick metal bolt… the bears just bent these bolts like they were flimsy plastic… jerks.

Bears even tag-teamed the camera at times...

Bears even tag-teamed the camera at times!  Not one, but TWO BEARS!!! …double jerks.

Perhaps the bears just like to mess with novel items placed in their habitat.

Bear Hug....

Bear Hug….

Bear chewing on camera...

Bear chewing on camera…

Bear sitting down and swatting the camera round and round.... jerk.

Bear sitting down and swatting the camera round and round.  REALLY! This bear just sat there for 10 minutes swatting the camera as it swiveled around and around on the bolt…. jerk.

Perhaps the camera and stand actually look like some weird creature that lost its way in the woods.

Maybe this is what the bears see?

Maybe this is what the bears see?

Regardless… we stopped seeing rattlesnakes enter the den….


Our “Den Site” View for the majority of time…. Maybe the snakes will go up in the trees…

We did get some great pictures of the backside of bears though….

Bear Butt... Jerk

Bear Butt… Jerk

So what did we learn?  We learned that you should never place novel items in the woods with bears.  We learned that if you do this, bears will make sure to mess with your equipment, chew on your cameras, and rip apart your wooden stands… We also learned that bears are really really strong!  We learned that bears are really fuzzy…

Fuzzy Bear Legs. ... jerk.

Fuzzy Bear Legs. … jerk.

I learned that I would have done things differently.  If I were to do it all again (which I probably will), I would move all of the cameras to a nearby tree instead of a wooden stand.  After four weeks of bears being jerks, this is exactly what I ended up doing.

Okay, so bears are jerks.  But we did see something interesting here.  We placed the cameras out by the dens well before rattlesnakes began ingress.  For one and a half weeks we didn’t see any rattlesnakes or bears.  Then rattlesnakes began to come back to the dens, and it wasn’t until this time that we began to see bears visiting these same areas.  So there does appear to be a correlation between rattlesnake timing of ingress and bear activity outside of dens.  But are we seeing other potential predators?  Well we don’t know yet.  We have been too preoccupied cursing bears to review all of the videos.  The bears did not mess with the tree cameras and perhaps we will see other potential predators visiting the den sites.  We are excited to finish analyzing these video data and update everyone on what we find (look for Tommy Cerri’s blog post in the future).

There is another interesting bit of information to digest as well: Bears have never been documented as a predator of rattlesnakes.  But we have seen bears swiftly attacking rattlesnake models in the field (see previous blog post).  We have also seen bears visiting other gestation sites and den sites. Would it really be too far-stretched of an idea for bears to attack and eat a rattlesnake?  But there is the possibility that bears just like to mess with novel things that they find in the woods.  There is also the possibility that whatever environmental cue drives rattlesnakes to return to their dens for the winter, also instigates bears to begin foraging for food (other than rattlesnakes) along the hillsides of Pennsylvania.  Regardless, bears are jerks.

Bear and Camera Cartoon

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Wrapping Up A Great Field Season

As the days are getting colder, snakes are slowly making their way back to their dens.  My technicians are still tracking their progress, but little-by-little each of our 15 radio-tagged snakes are getting closer to the location where they will spend their winter sleep.  This summer was a huge success as we collected ecological data for our rattlesnakes in our pre-burned habitat.  Much of this success was made possible due to a great team of technicians: Alyssa Hoekstra, Andrew Brown, Zack Maisch, Alex Dyson, and Mark Herr (lab undergrad extraordinaire)!

Our snakes led us to some great data this past summer.  Males did not disappoint us, and they typically had us hiking over large tracts of land. Sometimes, males were able to travel over 1-km in 24 hrs, up and over large mountains.  Much of this traveling was to find receptive females, and we observed many mating encounters as well as male-male combats!

Male and Female Timber

Large male (black phase) wrapped around a female (yellow phase). Typically males will follow females and attempt to entice her to mate by rubbing his head along her body. This female obviously didn’t want to play those games and just remained coiled. (Photo by A. Dyson)

Females stayed a little closer to the den sites and study area. The females main concern was foraging for food, but sadly none of our females at the main study site were gravid this year. Given the high abundance of chipmunks and mice throughout central PA this summer, I would not be surprised if many female rattlesnakes were gravid next summer.  It is believed that good reproductive years for timber rattlesnakes typically follow good food years.  Although we were not measuring small mammal abundance last year, I suspect it was lower than this year. Following the conclusion of our project, we may be able to shed some light on this relationship between rattlesnakes and their prey.  Additionally, prescribed fire can enhance small mammal abundances, which may lead to increased reproductive rattlesnake fitness!

Rattlesnake Eating Chipmunk

Rattlesnake consuming a chipmunk… Alvin!!!! (Photo by Z. Maisch)

In addition to following the snakes around, we also took some time to characterize the pre-burn study sites and the available resources for timber rattlesnakes.  We measured small mammal abundances, operative temperatures, available vegetation, and acorn mast production.  We will compare these available habitat characteristics to next year’s to see how these variables change based on year-effects and the prescribed burn.

chipmunks in tomahawk

Two chipmunks captured in a Tomahawk trap. Each small mammal receives an ear tag so we can identify it at a later data when it is captured again. This mark-recapture technique allows us to measure small mammal abundance throughout the study area. (Photo by A. Dyson)

In addition to the prescribed fire project, we also embarked on two new projects with the help of Tom Radzio!  Tom is a colleague from Drexel University, and he is using cameras to observe tortoise behavior outside of burrows in the southeastern United States for his dissertation.  Tom was gracious enough to loan us a few cameras so that we could embark on these great, new side-projects.

For the first side project, we are currently looking at the ecological trade-offs between thermal resource acquisition and predation at gestation sites of various sizes.  We collected some great data regarding potential predators, including black bears, bobcats, raccoons, and hawks.  Whereas we have already pulled all of our cameras from the field, we are still collecting data from these videos.  Currently, I have a great team of undergraduates assisting me with this process, including: Mark Herr, Michaleia Mead, and Tommy Cerri.  We hope to have all of these data collected and analyzed by the end of November!  We are predicting that we will see a higher amount of predator activity at more open, larger gestation sites, but we will also find higher quality thermal habitat at these same sites as compared to smaller, more enclosed gestation sites.  We are looking forward to the results!

Bobcat at Study Site

Bobcat walking through a large, open gestation site. Whereas the bobcat did not show any interest in the foam, rattlesnake models that we placed at the site, the mere presence of the predator suggests that an encounter is possible.


A black bear attacking a foam model at an enclosed, smaller gestation site. The bear first approached two foam, rattlesnake models and swatted off their heads. It then bit two more models (one shown here) before taking off out of the gestation site.

For the next side project, Tom Radzio and I are looking at when rattlesnakes decide to go back into their dens, how this correlates with environmental temperatures, and if predators are attracted to these den sites during this time of rattlesnake ingress.  We are currently collecting data at these den sites and we have a great undergraduate, Tommy Cerri, who is assisting us with analyzing these videos.

Rattlesnake basking in front of a den site. If you look closely you can see some small grey iButtons recording environmental temperatures.

Rattlesnake basking in front of a den site. If you look closely you can see some small grey iButtons recording environmental temperatures.

This fall will be filled with a lot of data analyses, writing, and hopefully a few published results. Stay tuned as we finish up a few of these projects.  I will make sure to update everyone on the results to each of the finished products.

Field work in action! Me capturing a small timber rattlesnake. (Photo by T. Langkilde)

Field work in action! Me capturing a small timber rattlesnake. (Photo by T. Langkilde)

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Don’t Prey on Me: Part 2

Note: This is a follow up to my first blog on this project. For the background and an explanation of the study, see Part 1

By Mark Herr

The field season is heating up for my summer Timber Rattlesnake project! As described in the first post, we’re using foam models of rattlesnakes to measure how the risk of predation varies between the different summer basking sites used by gravid female rattlesnakes. This project is a sub-project of the main study being conducted by Chris Howey looking into the effects of prescribed fire on rattlesnake ecology in general, and for most of the summer up until now I’ve been assisting with that study while we’re waiting for the gravid female snakes to arrive at their summer gestation sites. Well, it seems that they’ve (finally!) arrived, and so this week Chris, Alex (another one of the field techs on the rattlesnake project), Tom Radzio, and I have been deploying models and initiating the first field phase of the study!

Here I am with a beautiful yellow phase female rattlesnake that we found down the mountain below one of our known gestation sites. If she’s gravid then hopefully she’ll head up to the top to bask!

Here I am with a beautiful yellow phase female rattlesnake that we found down the mountain below one of our known gestation sites. If she’s gravid then hopefully she’ll head up to the top to bask!

One of our black phase models deployed against the substrate. We take a photo of each model after we place it out in the field so that we can reference them when examining them for signs of predation at the end of the deployment.

One of our black phase models deployed against the substrate. We take a photo of each model after we place it out in the field so that we can reference them when examining them for signs of predation at the end of the deployment.

Tom Radzio is a graduate student who’s currently pursuing his PhD at Drexel University. He’s an old friend of Chris’, and was generous enough to (awesomely!) provide us with field time-lapse cameras that we can use to record the happenings on the gestation sites while we aren’t there! Tom has used these cameras in his research on Gopher Tortoises in Georgia, and they should be incredibly valuable for us in this project, as they’ll let us truly see what’s happened to the foam snake models while they’re deployed. We were previously planning on trying to decipher any potential predation attempts on the models by examining the imprints left in the foam (as has been done in other studies) but now we’ll be able to look at the footage and see for ourselves exactly what happened!

Tom and Chris (in the tree!) setting up one of the cameras at a gestation site.

Tom and Chris (in the tree!) setting up one of the cameras at a gestation site.

We’ll be leaving the cameras up for the rest of the summer, and hopefully we’ll be able to record not only what happens to the foam predation models, but also anything interesting that the actual snakes using these sites are doing during the day. We might be able to use this data to figure out when the snakes are emerging at the different sites and when they go back underground for the night. Questions like that will help us to explore the other side of this project: how do the thermal qualities of the sites differ from one another? Do snakes need to emerge at different times, or stay out longer, at some sites because they are thermally inferior? Hopefully the cameras will help with resolving some of those issues.

The final setup! You can see the two lower arrows pointing at two models, one black and one yellow. The top arrow shows one of the cameras that’ll record the site for later analysis.

The final setup! You can see the two lower arrows pointing at two models, one black and one yellow. The top arrow shows one of the cameras that’ll record the site for later analysis.

Of course, our primary tool for examining those questions will be the copper thermal models that we’ll be placing out at the gestation sites very soon -as early as this next week! They’ll go out after we retrieve the foam models from the sites at the end of this first deployment (which’ll be on Monday – we’re keeping the foam models out for one week at a time).

I’m excited that this project is finally in full swing, and hopefully we get some interesting results over the course of the summer! Also exciting is the fact that I received an SSAR Roger Conant Grant In Herpetology for this project! I had applied this winter during the semester and was anxiously waiting to hear back, and now we’ll be able to use this grant to assist with some of the costs associated with the cameras and models. I’m so happy to have received the grant, and I couldn’t have done it without Chris and Tracy who helped to devise the project!

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In the News!

A couple quick updates from the lab!


Brad‘s research on sexual dimorphism of scorpions was recently featured on livescience:   Want to read the actual paper? It’s available on PLoS One (doi:10.1371/journal.pone.0097648).


Bark Scorpion – Photo Credit: Matthew Rowe


Sean, a former postdoc in the lab, also recently published a 5-part series about salamanders over at Living Alongside Wildlife. Start the saga with Part 1 (and continue with the rest: Part 2Part 3Part 4Part 5). Check it out!

Male Southern Two-lined Salamander (L) and Male Brownback Salamander (R)

Male Southern Two-lined Salamander (L) and Male Brownback Salamander (R) – Photo Credit: Sean Graham



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Why Did the Fence Lizard Cross the Road?

To get away from fire ants of course! Fence lizards, like most organisms, generally prefer to avoid being messily devoured by predators, and, to elude predators, animals employ a variety of defensive strategies. Animals may hide, flee, avoid detection (by camouflage or other means), be poisonous, pretend to be poisonous, have armor, and even preemptively attack a predator. For example, the Eastern Hognose Snake (Heterodon platyrhinos) employs a truly astounding suite of defensive strategies, including: puffing itself up to look bigger and more like a venomous snake (hence their nickname, the “spreading adder”), striking with a closed mouth, vomiting, and defecating to make itself as gross/distasteful as possible, vibrating its tail to mimic a rattlesnake, and playing dead by turning over on its back and sticking out its tongue.

A hognose snake feigns death (thanatosis) as a defensive strategy. Photo credit: Douglas Mills.

A hognose snake feigns death (thanatosis) as a defensive strategy.
Photo credit: Douglas Mills.

Fence lizards have defensive strategies too. Research in our lab has shown that one of the main ways that fence lizards adapt to fire ants is by fleeing from ants more frequently. Fence lizards from areas without fire ants will only flee from fire ant attacks about 50% of the time; the other half of the time, they freeze and remain motionless, and the lizards would likely die if we did not remove the ants and allow them to recover. However, when we study fence lizards from areas with fire ants, we notice that up to 95% of lizards flee when attacked by fire ants, a much higher percentage. This strategy is very effective, since fire ants can only prey on adult fence lizards when they can recruit to attack in large numbers.

However, in our lab meetings we began to wonder: why do fence lizards from areas without fire ants freeze so much of the time? It seems most likely that lizards from areas without fire ants freeze when attacked because this is an effective, adapted defensive strategy against native predators. In other words, just as fire ants have put pressure on fence lizard populations to adapt (in this case, by running away more often), native predators have put pressure on fence lizards to freeze (about half the time, at least). While not running away from a predator when given the chance may seem like a bad strategy, this approach makes more sense when we consider what the native predators of fence lizards are. They include many types of predatory birds, such as hawks and falcons, as well as several snake species, including black racers (Coluber constrictor) and coachwhips (Masticophis flagellum) among others. These predators are primarily visual hunters and also very speedy. A fence lizard in danger from one of these predators would have little chance of outrunning it, unless a refuge were near at hand (or claw); however, as fence lizards have excellently camouflaged patterning, freezing could be a very effective strategy, as the bird or snake might, like the erroneous T-rex in Jurassic Park, have a difficult time seeing the fence lizard if it remained motionless.

After these discussions, I, along with two undergraduate researchers, Shannen McGinley and Elexa Baron, decided to test whether fence lizards from different areas (those with fire ants and those without), would react differently to a simulated predator attack. To conduct these tests, we first needed a predator; after some quick research, I discovered that a common avian predator of fence lizards is the American Kestrel, a smaller falcon that preys on many species of lizards. The Penn State University Bird Collection happened to have a preserved kestrel salvaged from a car strike that they were kind enough to lend us. After naming our kestrel “Cadfan”, I proceeded to get him ready for his close-up: I braced his wings into a half spread position, as if he were swooping in for a strike; built him a wire harness, to allow us to hang and swing him from the ceiling in a simulated attack; and gave him a pair of really intense eyes (as we know from personal experience that fence lizards will respond to large eyes and being stared at).

Cadfan and me chilling in the lab. Note his intense and intimidating gaze.

Cadfan and me chilling in the lab. Note his intense and intimidating gaze.

Next, we needed to design an arena in which to conduct behavioral trials. We set up a small arena in the middle of an animal room that contained a tub identical to those our lab lizards live in, and we lined it with sand and put in a half log, a type of lizard shelter. To allow us to simulate a kestrel attack, we hung Cadfan from the ceiling of the animal room using fishing line. We also built a blind for Cadfan and for ourselves, to prevent the lizard from seeing either the kestrel or the humans. As part of the construction, we built a webcam into our blind to allow us to observe and record all the trials for later analysis.

Left: Cadfan hanging over the arena showing how close he can swoop to the lizard perch. Right: The backside of our blind, with datasheets, laptop, and webcam set up to allow data collection.

Left: Cadfan hanging over the arena showing how close he can swoop to the lizard perch. Right: The backside of our blind, with datasheets, laptop, and webcam set up to allow data collection.

After finally getting the room set up to our exacting specifications, we were ready to run trials with our lizards. Conducting a trial required two people. One person stayed behind the blind and held Cadfan up and out of view of the lizard. The other person fetched a random lizard from its housing and walked it quickly to the trial room, where we took it’s body temperature and set it on the log perch in the middle of the arena. That second person, the lizard-getter, then walked behind the blind, starting recording with the webcam, and gave a big thumbs-up signal to the bird-holder. The bird-holder simultaneously released Cadfan, and also turned on a small lightbulb that could be seen in the webcam but not by the lizard (this allowed accurate timing of the trial). We let Cadfan make four swoops over the head of the lizard and then returned the lizard to its housing, while recording if the lizard reacted to the simulated attack and how long it took to react. We also classified how strong any reaction was; some lizards just ducked their heads or shifted to the side, much as you or I might if a large insect flew at our heads. Other lizards reacted much more strongly and dove into the sand or underneath the log shelter. See below for a video showing our setup and a couple of trials!

We’re still analyzing the data from these trials now, but, to the naked eye, we haven’t seen a large difference in responses to the bird predator based on where the fence lizards come from as we’ve seen in response to fire ant attacks. This could mean that lizards might be able to distinguish between different types of predation threats or that something else is going on. We’ll post again when we publish a paper on our trials, so keep an eye out for those results here (and any attacking kestrels that might be nearby)!