Since joining the lab, I’ve heard nothing else more than field seasons in Alabama; from the heat, lizards, those darn fire ants, and the wonderful people they come in contact with. So of course, I was anxious as ever to finally embark on my first field season in the Langkilde lab. After months of preparation, designing projects and all the logistics involved, May 9th had finally arrived and it was time to head south.
The first round of people in the Alabama crew this year had 3 newbies to the lab; Dustin Owen (our personal herpetology specialist), Dr. Kirsty Macleod (our Scottish Post Doc who should’ve been born in the southern US) and Myself aka Frog stallion (long story). The last member of the crew was David Ensminger. This was his third year there, sort of making him our expert of all things and everything that we needed to know (in other words he was our ear to ask a million questions).
The wonderful staff of The Solon Dixon Forestry Education Center welcomed us to the 5 starred field station (in my opinion – I don’t know who could possibly refute that). There were also lots of Auburn University students that were super interested in our work and simply cool people to hang out with. Essentially, everyone we met made sure we left knowing that southern hospitality is 100% real. I now understand why everyone in the lab loves and talks about it so much.
Some Auburn students were successful on their first lizard hunt. It was Herp week in their class, luckily for them, they had some experienced catchers around
Now on to the fun science! In case you’re reading this and don’t know, the Langkilde lab is well invested into the Fence Lizard and Fire Ant system, but I’m just going to focus on my specific part for now. In the broadest of terms, I am interested in the diet of the Fence Lizards, but of course I can’t help but wonder about other aspects of this creature. A former member of our lab (the undergrad king Mark Herr) published a paper that suggests fence lizards seemingly build an addiction to the fire ants. My first thought was the possibility that the fire ants may be more nutritious. In Alabama, I collected loads of ants to quantify carbs, lipids, and proteins in comparison to fire ants, hoping for something to support a risk-reward relationship. My next thought was, if the lizards are presented with a second option, what will they pick? To test this, The Lizard Queen (Dr. Tracy Langkilde) and I ran food preference trials. We used the fire ants and “Dory ants” (still waiting for a true identification, but we call them Dory ants) in tubs with one lizard to see the choices they made. Now back in State College, I’m going through and analyzing all the data. I won’t spoil the surprise, which will hopefully be published, so stay tuned in the near future.
My first Catch of the trip. Gotta love when they just pose and hug your thumb!
Short answer: Nope. Fire ants like their lizard eggs raw. Our recently accepted paper in the Journal of Herpetology “Invasive Fire Ant (Solenopsis invicta) Predation of Eastern Fence Lizard (Sceloporus undulatus) eggs” shows that fire ants do indeed eat fence lizard eggs in a natural setting, but, despite being known as “fire” ants, these hymenopterans haven’t quite mastered the art of cooking their food prior to chowing down. Get your non-final, still with a couple of mistakes, pre-print copy online (paywalled) or download one from this blog (posted for personal use of our readers courtesy of SSAR)!
At this point, you may be asking yourself, “Why should I care if fire ants eat fence lizard eggs?”, so I’ll discuss the impetus behind this project. Previous research in the lab had given us a good idea of how fire ants can impact juvenile and adult fence lizards: they are found frequently on the ground by fire ants, stung if they don’t run away, and can be envenomated when eating fire ants. BUT we knew incredibly little about what impacts fire ants might have on one unexplored life stage of fence lizards: eggs! And unlike juveniles and adults, these eggs can’t flee or twitch when attacked by fire ants; they remain in a nest for 55-70 days (depending on site and temperature), during which time they might be vulnerable to fire ants. Additionally, fence lizards often (though not always) prefer sandy sites with low canopy cover, where sunlight can warm the nest, exactly the type of microhabitat beloved by fire ants. And fence lizards build their nests 4-8 cm underground, right at the same depths where fire ants construct their underground foraging tunnels. We surmised that fire ants might come into contact with fence lizard eggs with some frequency, and, if fire ants ate the eggs, this might have a large impact on fence lizard populations.
Fire ants are known to eat the eggs of other reptiles, including those of snakes, turtles, and some lizards (such as anoles). To determine if fire ants were physically capable of eating fence lizard eggs, Jill Newman (a former lab undergrad who just started her master’s at Clemson…wooooooo!), Tracy, and I designed a small experiment. We presented fence lizard eggs to captive fire ant colonies and observed them penetrate the eggs in less than 30 minutes…a rather dramatic response! However, we also wanted to see whether fire ants might eat fence lizard eggs under more natural conditions. To address this, Jill dug holes in the ground to the depth of fence lizard nests and placed 12 eggs near fire ant mounds overnight. Upon examination, 11 of the 12 eggs had been punctured and eaten in less than a day!
The next summer, I designed a follow-up experiment to learn more about this type of predation. Specifically, I wanted to know how many eggs fire ants might eat, how quickly they might find them, and whether any environmental variables, like distance of a fence lizard nest from a fire ant mound, might affect predation. To address this, I started by collecting fence lizard eggs. A LOT of fence lizard eggs (over 150!…I resisted the urge to make my own omelet).
I couldn’t, however, just bury the eggs in the ground and check them after 24 hours (as Jill did) to answer my question about how long eggs might survive…I had to be (a little) creative. After reading about a similar problem faced by Kurt Buhlmann and his solution when he wanted to monitor turtle eggs, I developed a method allowing me to monitor the eggs daily without disturbing them (which might attract fire ants and increase predation).
For each nest, I dug a hole into the ground and sunk into it a clear, capped acrylic tube. I carefully replaced the sandy soil around each tube and placed six eggs (a small, but reasonable size for a fence lizard nest) next to the tube. I inserted a small piece of plastic transparency above the eggs and then carefully covered the whole arrangement with the soil. The transparency prevented soil from entering between the eggs and the acrylic tube, and, by lowering a video camera into the tube, I could count the eggs and see if they were being attacked by ants (or other predators). At each “nest” I also measured the amount of canopy cover and the distance to the nearest fire ant mound.
Diagram of nest tube with camera for viewing eggs underground.
…aaaaand what one of them looks like in the ground.
Each day, for up to 20 days, I monitored the nests and recorded if all the eggs were present. If I found ants attacking the eggs, I waited a few hours to let them eat the eggs (or at least let them make a start) and then dug up the nest to catch and identify some of the ants. I can definitively say that fire ants do not like to be disturbed when they are in the middle of a meal! Because of our innovative setup, it only took about 3 minutes a day to monitor each nest (check out a couple of examples below).
In our trials, we found that 24% of nests were attacked by fire ants within 20 days. Extrapolating this to the full incubation period of fence lizards using a mathematical model, we estimated that up to 61% of fence lizard nests are in danger of being preyed on by fire ants. We also did not find any relationship between how far nests were from fire ant mounds and how likely they were to be eaten. Given the high densities of fire ants at many areas in the Southeast, it seems likely that fire ants prey on a substantial portion of fence lizard nests in the wild. Of course, we know that fence lizard populations where we do our research are not in danger of disappearing. Fence lizards are doing fine even in the face of this predation, which is great news, and suggests that survival in other parts of a fence lizard’s life or high reproductive output may allow them to persist in fire ant invaded areas. In the future, I am aiming to build mathematical population models to understand the impacts of egg predation by fire ants, and see how this predation may affect populations over the long term.
One other point of note is that, for many species of southeastern herps that are declining, such as kingsnakes or southern hognose snakes, fire ants are often suggested as a culprit without any definitive proof. My project suggests that fire ants can indeed prey on large portions of the nests of some species, but also shows that one species is doing just fine even when fire ants may be making a buffet of about half of its nests. Moving forward, I would recommend that lab and field trials like those we’ve done be used to pinpoint if fire ants are indeed a threat to the eggs of these other species, and, if so, what proportion of nests are at risk.
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.
Fisher checking out den site. These are some pretty carnivorous animals!
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.
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.
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.
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. 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! Not one, but TWO BEARS!!! …double jerks.
Perhaps the bears just like to mess with novel items placed in their habitat.
Bear Hug….
Bear chewing on camera…
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?
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
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.
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.
Imagine you’ve passed your deadline for filing a report at work. The report isn’t finished and now you need to decide where you’re going to finish it. This is critical, because your boss hasn’t confronted you about it yet, and you just might get off scot-free if you manage to get it in soon enough. Here is the dilemma: If you work from home, you won’t see your boss and so won’t get a thrashing. Without running into you, your boss might not even recall the fact that you were supposed to submit the report, and you’ll get off free and clear. Your job fully permits you to work from home, but unfortunately you don’t have access to all of the company resources you need to get it done as efficiently as possible. It might take twice as long to finish from home, and if your boss already knows it’s missing you are going to be in even more trouble if it’s extra late. If you go into the office you’ll be able to finish the report in half the time and might get it submitted before anyone realizes it’s missing, but you’ll also risk the thrashing from your boss if he is aware. This is the type of situation that people encounter all the time: problems with multiple solutions, each with positives and negatives and no clearly superior alternative.
It just so happens that scenarios like these are common in nature as well, and how organisms respond to problems like these has important impacts on ecology and evolution. When we discuss the ways that organisms respond to problems like these, we refer to them as trade-offs: situations where an individual, population, or species gives something up in return for something else. It seems simple enough, but it turns out that this concept is tremendously important. For example, if a predator inhabits a landscape with diminishing prey resources, it may face a trade-off in how to respond. Some individuals may become more active in order to find more prey to sustain themselves, while others may take the opposite route and stop moving in order to conserve what energy they do obtain. In this way, a trade-off like this could result in one species diverging into two – an active forager and an ambush hunter.
This brings me to the research project that I’m going to be working on this upcoming spring and summer. I’ll be working with lab post-doc Chris Howey, who’s currently studying the way that proscribed fire impacts Timber Rattlesnakes here in PA. Chris is particularly interested in the impacts that these fires have on gravid (pregnant) female rattlesnakes.
So while I’ll be working with Chris helping him out on his larger project, we’ll also conduct a side project on the trade-offs that gravid female rattlesnakes make during the active season. Shortly after emerging from their winter dens, gravid female rattlesnakes will congregate in open rocky areas to incubate their developing embryos. Typically they stay at these spots, termed gestation sites, for nearly the whole active season – until they give birth to live young sometime in late summer. They are spending their time thermoregulating in order to develop their embryos as efficiently as possible, so it’s obviously important for them to choose sites with good thermal qualities.
This is where they might encounter a trade-off, though. The largest, most open rocky areas will have the most sun exposure, and so one would think they would be the best places for the snakes to choose as gestation sites. However, we think that these sites might leave the rattlesnakes even more exposed to predators than they would be otherwise – this is especially important because the snakes are already more vulnerable while out basking than they would be if they were foraging in the forest where their camouflage is most effective.
This past summer we found that some pregnant females chose smaller, more closed canopy spots with less sun exposure as their summer gestation sites. Why would these rattlesnakes be choosing sites like this when big open sunny spots are available? We think that this might be a classic ecological trade-off: with snakes weighing the thermal quality of the spots with the risk of being attacked by predators.
This rattlesnake is doing a terrible job of trying to avoid predators.
This might be what’s going on, or it might not. Perhaps the snakes are just choosing the spots that are closest to where they denned up. Maybe the closed spots and the open spots don’t even have different predation risks attached to them! We’re interested in exploring this issue to see if this is really what’s going on, and understanding this dynamic might help conservation authorities understand the ways that a species under threat (like timber rattlesnakes here in the Northeast) use the different parts of their habitat.
In order to test to see whether a trade-off really is occurring, we’ll be assessing the different gestation spots chosen by rattlesnakes for their thermal qualities and predation risk. To test the thermal quality we’ll analyze the sun exposure of these sites and we’ll place thermal models designed to mimic rattlesnakes and analyze them against the preferred body temperatures of live rattlesnakes that we measure in the lab.
How will we measure predation risk? This is the part that I’m working on right now in preparation for this summer. We are making realistic foam rattlesnake models that we are going to set out at the different sites. The foam that we are casting the snakes in should hold the imprint of attack from the predators and should give us some idea of what type of predator went after our snakes! This is a technique that’s been used before (on rattlesnakes no less!) by Vincent Farallo to quantify the risk of predation, and we are excited to use it to test our hypothesis!
Making these foam models is taking up most of my time on this project right now, and we are going to need a couple hundred by the time spring starts, so I’m busy! I’ll post more updates on the project here on the Lizard Log in the future as we get rolling – but I’ll leave you off with some photos of the model making process that’s taking up my time now while there’s still snow on the ground!
The first step in making our models is to get an actual snake to cast them from! We took this preserved timber rattlesnake specimen from the teaching collection here at Penn State to make our mold for the future specimens.
Then we posed her in a typical basking rattlesnake position – getting the snake like that isn’t as easy as it seems! When specimens are fixed in formalin during the preservation process it tends to make them rigid and tough to work with, but we managed!
After this we poured a rubber mold compound in the tray and let it set, then we removed the preserved snake and voila – snake mold!
Here you can see we’ve just poured the mixture into the mold – it will then quickly expand to fill the whole mold and once it hardens…
We’ve got our rattlesnake model! The only step after this is painting it to make it look like an actual basking rattlesnake!
Here’s a painted model; painting them is by far the most tedious part of the process, as the rattlesnakes have pretty intricate patterns. By the start of the project I’m going to need a couple hundred of these things painted and ready to go. It’s gonna be a busy semester!
As Chris mentioned in a recently post, we both had the wonderful opportunity to do some research on Guana Island. And it was BEAUTIFUL!
The amazing view from my porch.
Such clear water!
But it was also exhausting! Getting used to temperatures in the 80s and 90s and the incredible humidity was a challenge we were to happy to meet. And it’s hard to complain about doing fieldwork on a beach…
North beach!
We had a number of projects to keep us busy. As Tracy described in a previous post, our lab and our collaborators are interested in how invasive fire ants (Solenopsis invicta) affect one of the world’s most endangered iguanas: the Stout Iguana (Cyclura pinguis). (remember this guy?) This year, Chris and I wanted to address whether fire ants are capable of preying on iguana eggs while in the nest. Thanks to some of Chris’s previous research, we know that fire ants are potential threats to fence lizard eggs: they are capable of foraging at depths of fence lizard nests, can find artificial nests, and can get through the shell of the egg to obtain a meal. We wanted to know if fire ants could get to depths approaching those of an Iguana nest, which are deeper than those of fence lizards. To do this, we installed fake “nests” next to clear plastic tubes in the beach and forested area nearby. This involved digging a hole roughly 16 inches into the soil (or sand!) and inserting a tube. We then placed slices of hot dogs (faux “eggs”) along the outside of the tube at a standard depth and filled in the hole with sand. Every afternoon, we checked our mock nests by sliding a small camera down the tube and taking video of the hot dogs through the tube wall. We immediately checked these videos to determine if any fire ants were present (or beetle larvae, as we observed in one case!).
Chris digging a hole on the beach for our nesting experiment.
Tube in place before we filled in the hole with sand.
During our stay, we also continued to survey the island for fire ant mounds. Our lab has collected this since we started working on Guana in 2010, and the resulting maps help us monitor the spread of fire ants on the island.
Looking for fire ant mounds.
Crabs like peanut butter too!
We also set up baits around the island to see which species of ants are actively foraging in the area. The fire ants love our peanut butter balls, but occasionally a crab would stake claim:
Chris and I had 6 days of hard work and amazing views, but we eventually had to return to the Pennsylvania fall. Next step: data analysis!
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 2, Part 3, Part 4, Part 5). Check it out!
Male Southern Two-lined Salamander (L) and Male Brownback Salamander (R) – Photo Credit: Sean Graham
Andrew Watts is a junior who is majoring in Biology with a Vertebrate Physiology Option and also minoring in Kinesiology. Outside of his studies, he is involved with THON, Camp Kesem, a summer camp for children affected by cancer, and works for Penn State EMS as an EMT. After graduation, Andrew is looking to attend medical school and pursue a career as a surgeon. Andrew’s post about his ongoing research in the Langkilde Lab is below.
I have been working in the Langkilde Lab with Brad Carlson, a graduate student, since Fall 2012 investigating the effects of predator cues on wood frog tadpoles. I am investigating morphological effects, which pertain to how the tadpoles change their size and shape in response to predators. We are attempting to look into whether or not the presence of a predator cue has a significant effect on a tadpole’s morphology, and how these changes make it adapted to living in an environment with or without predators. Also, another factor that we looked into was whether or not the previous adaptations of the first generation tadpoles collected had an effect on how the second-generation tadpoles changed in the presence of a predator cue. These same tadpoles were also part of other work looking into tadpole personality and traits, but I was only involved with the morphology aspect of the project.
The experiment actually started in the spring of 2012 when the original tadpoles were collected from many different ponds. They were all set up into large bins where some were exposed to a predator cue, and others were controls (no predator cue). The tadpoles exposed to a predator cue had a dragonfly larvae (a natural predator of tadpoles) in a small cage placed in the middle of the tube, while the control groups had nothing placed in their tubes. The dragonfly larvae released chemical cues into the tubs which tadpoles can detect as an indicator that a predator is present. If the tadpoles detected a predator, their morphology might change in response. During the time in which the tadpoles were alive they were videotaped for the purposes of the personality and traits experiment. When the trials were finished, the tadpoles were humanely euthanized, preserved in ethanol, and placed into the jars according to which experimental group they were in. Each group had approximately 60 tadpoles and 10 of these tadpoles were randomly picked to be weighed and have pictures taken in two different positions. This same routine was repeated for all of the groups (control and experimental). There were 38 total groups, so, in all, 360 tadpoles were weighed and photographed for a total of 720 pictures. This was an extremely long and time-consuming process!
Figure 1: These are examples of the two types of pictures that were taken. Each is of the same tadpole just taken at different angles so that the multiple morphological measurements could be taken.
Once all of the pictures were finally taken, I was able to analyze them using computer software called ImageJ. For each tadpole several different measurements were taken in order to get a complete representation of its overall morphology.
Figures 3 and 4: These are the reference guides that I used from previous tadpole morphology research done to measure the tadpoles.(Figure 3, Left: Rick A. Relyea (2001), Figure 4, Right: Ronald Altig (2007))
With all of the data collected I then analyzed it with Brad’s help. First, we found that tadpoles that were exposed to the predator cue were actually shorter and wider than the tadpoles in the control groups, which were longer and slimmer. The shorter, wider body types may allow these tadpoles to grow faster or survive better in the presence of predators. These findings were expected since previous research by Dr. Rick A. Relyea found that when tadpoles are exposed to predator cues they adapt by changing their morphology to a shorter and wider shape. However, we also tested if there was any significant link between what kind of pond the tadpole came from and the effect of the predator cue. As previously discussed the tadpoles were taken from many different locations, and some ponds were known to have predators present. This could have possibly primed or prepared the tadpoles to be able to better adapt to the predator cue. However, the results from our data did not show a significant relationship (p>0.05) between what kind of pond the tadpoles originally came from and how their morphology changed when placed in the presence of a predator cue. Even though our data supported the null hypothesis for this experiment, further research is still being conducted. There are many other groups of tadpoles from various experiments, and each has the possibility of revealing something else about tadpole morphology and evolution. This research can allow us to better understand how exposure of previous generations and their adaptations and personality traits can affect how future generations will adapt to different stimuli and changing environments.
My work and contributions in the Langkilde lab have given me a whole new perspective and appreciation for scientific research. I never fully understood the amount of time and effort that goes into the research. I also never fully understood how much evidence you need to have in order to produce significant findings and support a claim. I have also learned a great deal about ecology and especially predator prey interactions, which has even helped me in biology courses involving these concepts.
References:
Relyea, R.A. 2001. Morphological and behavioral plasticity of larval anurans in response to different predators. Ecology 82:523-540.
Altig, R. 2007. A primer for the morphology of anuran tadpoles. Herp Con Bio 2:71-74.
This week in our undergraduate blogging, we’re featuring a post from Danielle Rosenberg, a senior in the Schreyer Honors College majoring in Veterinary and Biomedical Science and minoring in Equine Science. While at Penn State she has been the Community Service Chair in the Block and Bridle Club and in charge of planning the largest blood drive on campus. She’s also just applied to veterinary school and hopes to become an equine surgeon in the future
Last fall, I applied for and received an Eberly College of Science Undergraduate Research Grant to conduct an independent research project on wood frog tadpoles. Last spring, I planned my project with Brad, a graduate student in the Langkilde Lab, and waited for the ice to melt, the frogs to come out, lay their eggs, and for the eggs to hatch. I waited and waited and waited a little bit longer. The past spring happened to be a very cold one, and unfortunately the tadpoles hatched much later than expected which delayed my research plans, but did not deter me from addressing the following questions: “What risks do specific predators impose on tadpoles of various sizes, and how do tadpoles respond to these threats?
Tadpoles are very important components of aquatic ecosystems. They help to cycle nutrients through the ecosystem by feeding on detritus and on phytoplankton and periphyton, which are both primary producers in aquatic ecosystems and provide energy for living organisms. Tadpoles’ role in an ecosystem can be drastically affected by the presence of specific predators. Predators can have a huge effect on an ecosystem by not only eating the prey and decreasing their population density, but by also causing prey to react to the predator’s presence and change their behavior in some way. For example, tadpoles are known to reduce their activity in the presence of predator to avoid detection. Behavioral and morphological changes brought about by predator presence can cause the allocation of resources to change from reproduction to camouflage or defense mechanisms, greatly affecting a population and thus an entire ecosystem.
Tadpoles are prey for a number of insect and vertebrate predators, such as fish, newts, dragonfly larvae, salamanders, and water bugs. Each predator presents a unique level of risk to tadpoles based on their size, speed, effectiveness at capturing their specific prey, etc. and thus can cause varying responses on prey of different sizes. For example, smaller predators may be limited in the size of the prey they can eat (their mouthparts may only be so big!), while larger predators may not have that limitation. In addition, predators feed at different rates and eat varying numbers of tadpoles at a time. Similarly, the risks associated with each predator may change as tadpoles grow. As tadpoles age, they become larger and faster and may be able to escape or avoid some predators.
Wood frog tadpoles (Rana sylvatica) are a model system for this type of study because they are known to exhibit a strong response to predators and play an important role in aquatic ecosystems. For my study, I was looking to determine the susceptibility of tadpoles to various predators based on their size and to see if this influenced their behavioral response. To examine this I observed tadpole behaviors in response to a predator cue and followed-up with predation trials which allowed predators to feed on the tadpoles. I predicted that a more dangerous predator would cause a greater effect on the behavior of the tadpoles at their most susceptible size class.
Three of the different predators used; from R-L: a backswimmer, a dragonfly nymph, and an Eastern Newt
The study seemed like it would be pretty straightforward and simple, but science never goes as expected! I started my research at the beginning of summer and had literally thousands of tadpoles at my disposal along with a group of predators that were known to consume tadpoles. I was using backswimmers, newts, dragonfly nymphs, and diving beetles as my predators and wood frog tadpoles as my prey. I would weigh tadpoles and separate them into two different size classes based on the limits I had set. One size class was 100 mg +/- 20%, and the other was 400 mg +/- 20%. I began weighing tadpoles and noticed some mortality of my populations in the lab, so Brad and I decided to move my research project out to the field where the environment might be less stressful on the tadpoles.
I began my research over again, weighing and sorting. Once I had sorted enough tadpoles into the two prescribed size classes, I began to run behavioral trials. For these I needed a predator cue to make the tadpoles think that a potentially dangerous predator was present during trials. I created this cue by allowing each predator type to eat a single tadpole in a small container and then collecting the water the predation event took place in. For the trials, I put 8 tadpoles from one of the size classes into 1800 mL of water and recorded their natural behaviors. I added 10 mL of regular water to the tubs as a control and observed the tadpoles’ movement every three minutes for a half hour and recorded the number of tadpoles swimming at each time interval. I then added 10 mL of the prescribed predator cue to each tub and again recorded the movements every three minutes for a half hour. The pre-predator cue data provided me with a way to detect tadpole behavioral changes due to the presence of the predator cue. I am still in the early stages of analyzing this data, but the preliminary analysis showed a decrease in tadpole activity across all predators and size classes. It did not seem to matter which predator or what size the tadpoles were; it seems as though the tadpoles respond by decreasing their behavior in the presence of any predator regardless of size. This makes sense as reducing activity might be a good way to avoid the attention (and subsequent attack) of potential predators.
An example of the two different size classes of wood frog tadpoles
Next, I ran predator trials to determine the susceptibility of each size class to the individual predators. I ran these trials by placing 8 tadpoles of a specific size class into a tub with a prescribed predator. I allowed each predator 2 hours to eat as many of the tadpoles as they desired. At the end of the 2 hour time period, the number of tadpoles remaining was recorded, and I am currently analyzing this data to determine predation rates on the two size classes.
These experiments will help to provide information that can be useful in understanding the workings of aquatic ecosystems and the various effects that predators can have on tadpoles during their various pre-metamorphic stages. Conducting this research was a great experience, and it gave me a new respect for the life of a researcher. While I always assumed that once a method was put in place, the experiment was just carried out, it turns out it does not always go exactly as planned. There are a lot of bumps and holes that need to be worked around and fixed to create a successful project. Some methods that look very simple on the outside probably went through a lot of revisions to get results and conclusions. Researchers need to have a lot of patience and be willing to constantly update and rework their methods. I think it is impressive how much time researchers can put into a project yet can seem so easy and laidback.
No, it’s not the name of a cool new band you haven’t discovered yet; the Hanging-thieves are a genus (Diogmites) of robber flies that is known for hanging by one or two legs from a perch site while consuming their prey. Robber flies (family Asilidae) are also known as assassin flies, because they catch other bugs while on the wing and eat them with their piercing mouthparts. In general, robber flies are considered to be beneficial, as they eat many pest insects.
A hanging-thief chows down on a freshly caught wasp .
I was lucky enough to stumble upon a Hanging-thief (it looks like Diogmites salutans) as it was engrossed in eating a wasp. The robber fly was perched on one of my AED’s (Armadillo Excluder Devices), small wire cages that I set up to protect my fake lizard nests from predation by wily armadillos (that story another time…). The fly had already paralyzed its prey with venom, which also contains enzymes which liquefy the prey’s innards (as in spiders). As I watched, the robber fly carefully manipulated the wasp, turning its body over and around, and inserting its proboscis into likely sites, as if its prey were some sort of delicious juice box. In the video below, you can see the mouthparts of the fly working to slurp up its presumably delicious meal.
My favorite part of seeing this robberfly was observing its moustache, or mystax, which is so tough that it may help protect the fly from injury when it is subduing struggling prey.
The mystax is among the most fearsome of moustaches in the animal kingdom.
The Lizard Log 