The Lizard Log

The Langkilde Lab in Action

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Basking Site Use by Timber Rattlesnake Morphotypes – By Shawn Snyder

My name is Shawn Snyder and I am currently a senior majoring in Wildlife and Fisheries Science.  This is my first and only year working in the Langkilde Lab.  During the summer of 2016, I worked under Dr. Chris Howey as a Research Technician studying the effects of prescribed fire on timber rattlesnake populations.  This position provided me the opportunity to radio-track timber rattlesnakes, record habitat data on tracked snakes, catch new snakes (extremely fun), learn how to safely tube a venomous snake (even more fun), and conduct vegetation surveys.  Also, this position provided me the opportunity to formulate my own scientific question to test! Together, Chris and I thought up a small side-project that I could conduct throughout the summer, which provided me the fantastic experience of going through the scientific process, collecting my own data, analyzing those data, and now writing a manuscript so that I can share those results with the scientific world.

When we first started collecting data for my side-project I was a little apprehensive.  Once the data was collected and analyzed I realized that this project was going to take time and a large amount of effort to complete.  As the process of analyzing the data and then coming up with a plan for the manuscript began to take shape, I started to feel challenged and nervous by this new task. But weekly meetings with Chris to discuss the process of writing a manuscript have helped immensely.  This is my first manuscript and yes it is challenging, but it will all be worth it once we have a finished product. I have ambitions to continue on to a Graduate program after I graduate and this manuscript will help me build my C.V. to apply to Grad schools.


Two yellow morphs bask alongside three black morph timber rattlesnakes at a gestation site. Although we did not use gestating (i.e., pregnant) females as part of this project, this shows you the posture of a basking snake and the difference in color morphs.

My research is investigating if the two distinct morphotypes of timber rattlesnakes (a dark, black morph and a lighter, yellow morph; see above picture) use basking habitat with differing amounts of canopy openness and solar radiation. Previous research suggests that the dark morph evolved in response to thermal limitations in the northern parts of its range.  Darker snakes have more melanin in their skin, which allows them to absorb more solar radiation and maintain a higher body temperature than yellow morphs.  Yellow morphs having this thermal disadvantage, in theory would have to choose basking sites that receive more solar radiation to compensate for this limitation if they wanted to maintain a similar body temperature to the black morphs.  Specifically, I am testing the hypothesis that yellow morphs use basking habitat that has more canopy openness and receives more direct solar radiation (i.e., sun) than basking habitat used by black morphs.



A black morph male timber rattlesnake is seen courting a basking yellow morph female.  Once again, the difference in color morphs is striking and has led many to ask what selective pressures are maintaining this polymorphism.

To test this hypothesis, I measured canopy openness over basking yellow and black morphs. I used the timber rattlesnakes that are being radio-tracked for Dr. Howey’s main study as my sample population and placed a flag where a snake was found exhibiting basking behaviors (see picture below  for example).  We took a picture facing skyward directly over the snake using a camera with a fisheye lens.  This lens takes a picture of 180 degrees and captures an image of all of the canopy over the snake (see picture).  We can then analyze these hemispherical photographs using a computer program called Gap Light Analyzer to measure the percent canopy openness and the amount of direct solar radiation transmittance (i.e., rays of sunlight) for each basking site.  Direct solar radiation is when the sunlight reaches the forest floor with no obstructions from the canopy; as opposed to indirect solar radiation which may be radiation that is being reflected off of clouds, trees, or the ground itself.  Our study site is characterized as having a mature Oak/Maple forest with an abundance of closed canopy throughout the area.  Both morphotypes use this “closed canopy” forest throughout the summer as foraging grounds, and when they need to bask they must seek out areas where some sunlight is making its way through the canopy.  This is where my question becomes very important comparing the habitat used by each morph.



A flag is placed next to a basking yellow morph.  An exact description of the habitat is recorded so that I can come back at a later time (when the snake is not there) and take a photo of the canopy directly over where the snake had been.


Two examples of hemispherical photographs taken over two different basking timber rattlesnakes.  Both canopies actually have similar canopy openness, but the canopy on the left receives far more direct solar radiation based on the placement of those canopy openings.

So far, my results show that the two morphs use habitat that have similar percent canopy openness, however, there was a difference in the amount of UV transmittance between the basking sites used by the two morphs.  Canopy openness doesn’t necessarily designate a “warmer” site because the sun path may not go directly over the gaps in the canopy of that site, thus, the site wouldn’t receive large amounts of direct solar radiation.  Black morphs use basking sites that received lower amounts of direct sunlight.  They may be able to do this because the greater amount of melanin in their skin provides a greater ability to absorb whatever direct or indirect solar radiation is available more effectively. Yellow morphs use basking sites that received more direct solar radiation.  They could be forced to use these sites to compensate for their disadvantage in their thermal ability.  I am currently working on writing a manuscript for these data and hope to have it completed by the end of 2016.  Stay tuned for more on this manuscripts progress!


Here is a picture of Shawn (holding a Hellbender!!) while on a break from collecting some amazing data.


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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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A Rattlesnake Summer

This past summer I began putting the pieces together for my 4 year project investigating the effects of prescribed fire on Timber Rattlesnakes.  The objectives of this project will be to determine how Timber Rattlesnakes are directly impacted by the fire and to determine how rattlesnakes are affected by changes within their environment which are caused by the fire.  This past summer’s objectives, however, were to basically get my feet wet, get to know the areas that may serve as potential study sites, get to know the focal species, and attempt to understand how to work with state agencies (SPOILER ALERT => this last objective would prove to be the most difficult).  It took awhile to get permits and all other paper work in line for the project, but all-in-all the summer was a great success because I identified 2 potential study sites (and possibly 2 more), I learned a great deal about these wonderful little creatures, and with the help of 2 great technicians (Rex Everett and Mark Herr) we came up with a couple great side projects.

Timber Marked

Male Timber Rattlesnake post-data collection. Note the painted rattle which allows us to identify it as a previous capture if we happen to stumble upon him later in the day. For all rattlesnakes we recorded basic morphometrics, sex, reproductive status, and marked the individual with a PIT tag which will allow us to identify it if captured much further down the road.

To date, we have identified two great study sites (and possibly more).  The first is located in Forbes State Forest and will be a rather large burn (a little over 600 acres!).  We’ve identified at least three potential gestation sites in this area, and although we were not able to spend a great deal of time down in Forbes, we know through talking with state foresters that there is a healthy population of Timber Rattlesnakes in the immediate area.  The actual area was nicknamed the “Snake Pit” by the district forester.  Aside from being a beautiful landscape, it is also the highest elevation within the state!  The “Snake Pit” is scheduled to burn in 2016 via an aerial ignition burn (dropping fire from helicopters!), which will provide me with a full pre-burn year to collect data in 2015 (Fantastic!).  In addition to Forbes, we also have a smaller prescribed burn planned for Rothrock State Forest.  We have visited this area quite frequently throughout the summer and we’ve captured a handful of Timber Rattlesnakes within the area as well (including one individual that we’ve radio-tracked to a den site).  The odd thing about this area is that all of the rattlesnakes we’ve captured thus far… have all been males of various ages.  Where are the females?!?  The presence of 1 year old males, however, suggests that the females are hanging out somewhere near by but we have not found where….  But, it leads us to question why we did not find any gravid females hanging out on the fantastic rocky slopes within the area?  Was this just a bad year for gravid females? Could the females be down in the valleys hunting chipmunks and gathering the energy needed for future reproductive bouts?

Gestation Site Paired

On the left, rising out of the thick ocean of Mt. Laural is a beautiful rock slab bathed in sunlight. This is fantastic gestation site. The large cap rock atop of the rock slab provides cover for the gravid females and refuge from high temperatures during the mid-afternoon and low temperatures at night. On the right, another type of gestation site is the large rock fields strewn out among Pennsylvania’s mountain sides. These gestation sites may be 100 m long and very open. Although this provides good thermal habitat for longer periods each day, would snakes be detected by predators more in this largely open area?

Throughout the summer we located close to 50 Timber Rattlesnakes.  Although we did not find gravid rattlesnakes at some potential study sites, we found many gravid rattlesnakes at other sites.  Sometimes these gravid rattlesnakes were in small areas that were about 10 m X 10 m (top left picture).  These smaller areas had all the essential habitat characteristics for a gestation site: an open canopy, rocks to lay out on, and rocks to hide underneath.  Other times, gravid rattlesnakes were found in large rock fields (top right picture).  Obviously the thermal environment would differ between these gestation sites.  Sun light would hit the more open rock fields early in the morning and last till much later in the day as compared to the smaller gestation sites surrounded by towering trees.  This may translate into the gravid Timber Rattlesnakes being able to maintain a preferred, optimal body temperature for a longer period of time each day within the more open rock fields.  Maintaining this optimal body temperature for a longer period of time each day may translate into more energy available for developing offspring and shorter gestation periods.  However, these more open areas may provide an advantage to visual predators such as hawks.  So would it be more beneficial for a gravid Timber Rattlesnake to gestate within a large open rock field despite the potential increased risk of encountering a predator?  To answer this question, a Penn State undergrad, Mark Herr, and I are putting together a side project for next summer where we will measure the thermal properties and predation intensity of these different types of gestation sites.  Additionally, we will radio-track gravid females at each gestation site in order to determine body temperatures maintained by individuals, duration of time spent at gestation sites, and date when females birthed young.  This should be a fantastic study! This past summer we began collecting preliminary data at a few gestation sites (using biophysical models) and found that operative temperatures at smaller gestation sites averaged 6 degrees C less than operative temperatures at large rock fields.  Additionally, this was a cooler summer than years past and only the gravid females at large rock fields birthed young at those gestation sites.  Gravid rattlesnakes from smaller gestation sites remained gravid as they traveled back to their den sites.  Most of these individuals appeared to return to their dens gravid, which likely means bad things for those potential young (i.e., mortality).

Mark and Neonate

On the left, Penn State undergrad, Mark Herr, collects habitat data at a smaller gestation site. Note the biophysical model in the foreground next to the backpack. These models will measure the operative temperatures for Timber Rattlesnakes, which are basically the potential body temperature of an individual if it was not thermoregulating (i.e., if it were conforming to the environment). On the right, a neonate Timber Rattlesnake hiding under a birch leaf (… that is a small leaf). Only at large, open gestation sites that maintained warmer operative temperature did we find neonates this summer.


Gravid females from smaller gestation sites returned to their dens still gravid.  This likely spells disaster for the potential young.  Can you find the Timber Rattlesnake at this den?

Next spring we are ready to hit the ground running as snakes begin to emerge from their den sites.  In addition to the project’s main objectives, we are also prepping for a few side projects.  In addition to the project described above, I am also trying to understand why Timber Rattlesnakes choose one potential gestation site over another.  I will also be surveying and monitoring vernal pools within burn sites to determine how amphibians are affected by prescribed burning.  Stay tuned as we continue this project.  In addition to Mark, I also have one of the best Timber Rattlesnake expert in the state assisting me with this project: Rex Everett.  Together Rex, Mark, and myself are determined to make this project a success.  Additionally, we will be taking on at least two more technicians for next summer and any other undergrad interested in reptile ecology, thermal biology, and conservation.  For more information feel free to contact me at

Me and Timber1

The fearless leader collecting data on a Timber Rattlesnake found crossing the road at Rothrock State Forest.

Rex and Ratsnake

Rex Everett, snake expert, with a large Rat Snake found adjacent to the planned burn area at Rothrock State Forest