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.

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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.

 

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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.

 

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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.

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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!

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

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Efficacy of Daylighting… Part I

Pregnant female rattlesnakes prefer to maintain an elevated body temperature (~32 °C), which allows for a more optimal development of embryos.  In Pennsylvania forests, however, these warm temperatures are not very abundant.  So, in order to achieve these elevated body temperatures, pregnant females seek out rare, open habitat (known as gestation sites) that receive a lot of sunlight.  Sometimes, females may travel up to a mile from their den sites just to gain access to openings within the forest canopy.

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Yellow morph female timber rattlesnake sits coiled under a small rock overhang.

However, not all open gestation sites are created equally.  Last year, Mark Herr, Michaleia Mead, and I uncovered a trade-off at timber rattlesnake gestation sites of various sizes.  Gestation sites that were very open provided pregnant females with more sunlight and warmer body temperatures for a longer duration of the day.  But, these same sites also came with an increased risk of predation!

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At a timber rattlesnake gestation site, 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.

More predators visited these more open sites, predators like bobcats, fishers, and hawks.  But, smaller, more enclosed gestation sites were so thermally poor, we observed females returning to their den at the end of the field season still pregnant!  The use of thermally unfavorable gestation sites may provide the rattlesnake with a sanctuary from potential predators, but there are still terrible repercussions for choosing to use these sites.  We have found that snakes using thermally unfavorable sites tend to give birth at a later date.  The timing that an animal gives birth is very important.  If rattlesnakes give birth to their offspring too late in the year, the small neonate offspring will have little, to no time to complete their first shed and then obtain a small meal before entering the den for hibernation.  It is believed that survival for neonates unable to do these two things is close to zero.  Further, some pregnant rattlesnakes that use thermally unfavorable gestation sites are known to abort their entire litter toward the end of the summer if she decides that they are developing too slowly.

So why would pregnant females continue to use small, more enclosed, thermally poor gestation sites?  Possibly because there was a decreased risk of predation?  Possibly, however, because of strong site fidelity?  Possibly these sites were, at one time, thermally favorable, but over the years vegetation has encroached upon these open areas and shaded out the once warm, sunny rocks.  Due to the rattlesnake’s fidelic response to locating a favorable gestation site, they now find themselves sitting among the shade throughout much of the day.

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Timber rattlesnake taking advantage of a small break in the canopy and some sunlight reaching the forest floor.

 

We’ve addressed the problem, now how do we fix it?

In order to manage timber rattlesnake populations better, forest and wildlife managers have begun to open up gestation sites, un-shading these areas from vegetation, in a process called “daylighting”.  However, recall that more open sites have an increased risk of predation.  So can we open up these sites just enough to let the sun in, but keep the hawks out?  In an attempt to suppress the increased risk of predation that we observed at more open sites, I have begun to direct daylighting techniques to target specific trees that would increase the amount of solar radiation a site would see, without greatly increasing the risk of predation.  To do this, I will use hemispherical photography (see picture below), observe the path of the sun throughout the gestation period, and then target those trees that overlap with the path of the sun.  This way I can open up each of the sites just enough, but keep those trees that do not overlap with the path of the sun and could perhaps maintain some decreased risk of predation.

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Hemispherical photograph at one of our gestation sites. The path of the sun throughout the entire gestation period is shown by the yellow arc. Trees that overlap with this arc can be targeted to improve the thermal quality of the site.

Of course I want to address this management idea with as much scientific rigor as possible for an ecological study.  This past summer, I visited six historic gestation sites within Pennsylvania.  Four out of the six sites are pretty shaded over, and I consider thermally poor.  Two of the gestation sites are fairly open and should provide pregnant females with a plethora of sunlight.  This summer, I surgically implanted nine pregnant timber rattlesnakes with temperature-sensitive radio transmitters, which allowed me to track these individuals throughout the summer. I followed each snake and noted their behaviors, body temperatures, and the date that they gave birth to their young.  Additionally, I measured available body temperatures and risk of predation just like we did in the previous experiment that Mark, Michaleia, and I completed last summer.  The catch is, this winter I will go into three of the six sites (along with US Forestry personnel), and we will remove specific trees blocking out the path of the sun.  Then next summer I will repeat everything and determine if Daylighting improved these thermally poor sites.  Will I see warmer available temperatures within the Daylighted sites?  Will snakes within these sites maintain warmer body temperatures and for a longer duration of the day?  Will these snakes give birth at an earlier date, allowing their young to shed and get a first meal before hibernation?  Will I still see an increased risk of predation?

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Biophysical models laid out at a gestation site and measuring the potential body temperatures that a snake could achieve at that site.

Many of these questions I won’t be able to answer until next year.  But, I am collecting some interesting data thus far.  As expected, the two sites that were more open did have warmer available temperatures.  Snakes occupying these sites maintained warmer body temperatures, moved less often, and were the first to give birth to adorable baby rattlesnakes!

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Neonate rattlesnake coiled up next its mother.

But, these sites were also visited by more potential predators.  In fact, in one instance we observed a red-tailed hawk swoop down and nab a garter snake that was basking alongside our pregnant rattlesnakes!

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Red-tailed hawk grabbing a quick dinner at a gestation site. I’m fairly certain this was a garter snake that got nailed.

The more enclosed sites were indeed cooler and snakes at these sites maintained cooler body temperatures and moved more often.  It appeared that some of these pregnant females shuttled between nearby sites in order to track the path of the sun.  In the morning the snake may be at one site, and in the afternoon that same snake would move to a nearby site.

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Female at a thermally poor area was typically found in the morning at a cut tree stump. In the afternoon she typically moved to another location.

Other snakes continued to move throughout the entire summer from one site to the next; constantly searching for a thermally suitable site where she could continue to develop her babies.  Unfortunately, all of these movements brought one of our mommas too close to a nearby road where someone swerved into the shoulder in order to run her over.  Although the loss of this mom was a little tough on me, it did show me just how important it is to improve these historic gestation sites.

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Our big momma rattlesnake could never find a suitable gestation site. She moved from potential site to potential site for about a month before coming too close to a nearby road.

Currently, as I am writing this blog, we are still waiting on some of our snakes from the thermally poor sites to give birth.  As we find neonates (babies) at each of our sites, I will also collect data on each of them so that we can compare body condition (health) among sites.

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Collecting data on newborn rattlesnakes is a fantastic way to start anyone’s day. I’m collecting data on body length and mass so that I can determine the body condition (or health) of each individual.

 

Next year, I will continue to track snakes throughout these same gestation sites.  However, following our daylighting management, we hope that all of our pregnant snakes will give birth at early dates, move less, and stay clear of predators.  To be continued…