The Lizard Log

The Langkilde Lab in Action

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Does This Predator Make Me Look Fat?: Morphological Responses of Tadpoles to the Presence of a Predator

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.

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.

AwattsFig 3Comb copy

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.


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.

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Not much to look at, but tadpoles have a great personality

Brad Carlson here again. I’m a doctoral candidate in the midst of my fifth and final year with the Langkilde Lab, where I’ve been occupied with a number of projects on amphibian (mostly tadpole) behavior and ecology. There’s one aspect of my research, however, that tends to elicit laughter or raised eyebrows: tadpole personality.

I’m not talking about some tadpoles being stoic, or outgoing, or irritable, or creative – though if I every found evidence of those traits, they very well could be signs of personality. Instead, what animal behaviorists mean when they talk about “animal personality” is a simpler concept that incorporates our ideas of human personality. “Animal personality” is characterized by consistent individual differences in behavioral patterns. If one individual tends to act one way and another individual tends to behave in a different manner, and these individual differences remain for a reasonably long period of time, we have evidence of personality. Put in those terms, I think anyone would agree that many animals they’ve interacted with have personality. My dog, for instance, is shy of strangers, unprotective of her possessions and food, and very quiet except when she’s dreaming. Her best friend – my parents’ dog – is the opposite in nearly every way, and has been for years. (Some of these differences are probably  because we are comparing a pit bull and a 10 lb poodle. By the way, the poodle is the bold one, stealing food and toys from the pit bull.)

Personality has been found in basically every animal that we’ve tested for it. From apes to lizards to hermit crabs to sponges (!), we’ve found that when we measure an animal’s behavior, it often remains similar over time (i.e., consistent) and differs from others of the same species. When I began my research, no one had really looked at whether amphibians – and particularly tadpoles – exhibit signs of personality. That’s unfortunate, because amphibians offer a lot for researchers interested in understanding why personality has evolved (why doesn’t everyone act the same?) and what the larger scale impacts of personality are. So I began investigating tadpole personality.

I hoped I could expect more than the personality of a sponge from my tadpoles.

My goals were 1) to establish whether tadpoles exhibited consistent individual differences in behavior that can be easily measured, and 2) to determine how consistent their behavior remains as they grow from small to large tadpoles and then into frogs. My primary method was filming tadpoles in open-field trials, a standard class of behavioral measures in animal research. In general, an individual animal is placed in an open, empty environment and a variety of behaviors are measured. I focused on the distance moved in the open-field (which for tadpoles was a small plastic tub) and the amount of time they spent close to the walls of the open-field vs. in the central area. Distance moved was my attempt to measure a general personality trait known as exploration-avoidance: whether an animal shrinks away from a new stimulus or environment or whether they “explore” it. A tadpole plucked out of its familiar home environment can either explore this strange new tub (covering a lot of distance) or sit there and avoid all that novelty. The proportion of time spent in the center vs. the edge of the open-field was my measure of boldness-shyness: how risky an individual’s behavior is. A shy tadpole might be expected to remain close to the wall of the tub where it is presumably safe, while a bold tadpole should comfortably cruise around the wide open even though it is vulnerable to predators. Additionally, I also recorded the behavior of the tadpoles in their home enclosures, so that I could tell how much their behavior reflects being in a new environment (the open-field) and how much their behavior in the open-field is simply indicating their overall activity level (yet another personality trait).

A tadpole in an open-field trial.

I first tested this with a set of 8 bullfrog (Rana catesbeiana) tadpoles. While initially intended as a pilot test, the data was nice enough that we were able to publish it in Journal of Herpetology recently. Exploration varied clearly among individual tadpoles, with some evidence indicating that boldness might vary too.

Bullfrog personality findings. From Carlson and Langkilde 2013, J. Herp. 47(2):378-383

Bullfrog personality findings. From Carlson and Langkilde 2013, J. Herp. 47(2):378-383

In the primary experiment following the bullfrog study, I kept 50 wood frog (Rana sylvatica) tadpoles in the lab from hatching until 6 weeks after turning into frogs. During this time, I performed the same trials as above, twice at three different tadpole stages and three frog stages. Unfortunately, I don’t have much to report yet as we are still going through the 100+ hours of footage of tadpoles and carefully documenting every little movement they make. Initial looks at the partial dataset suggests that some personality traits may remain consistent as the tadpoles turn into frogs, which is supported by another recent study that did something similar (but not the same) in a different frog species. This is a bit surprising, as you might think it would make the most sense to have personalities than “start over” when they turn into frogs. A certain personality type (e.g. being bold vs. being shy) could be advantageous in the aquatic environment of the tadpole and costly in the terrestrial realm of the frog, and the brain undergoes some significant rearrangement during metamorphosis. But apparently some personality traits lie deep within the individual, so to speak. 

Wood frog tadpoles whose personalities were tracked throughout development.

So that’s the story so far. Tadpoles have personality (more than some people, maybe), and there remains a lot to be understood. I’m still chipping away at this and trying to better understand what has shaped all this variation (one of my “dead ends” is in press …) Biologists still have a long way to go towards making sense of animal personality, but I hope at this point you are maybe thinking about tadpoles – and indeed any animal – in a slightly different way.

Or maybe you won’t – that’s just part of your personality.

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When Do Siblings Become Food?

Continuing with our undergraduate blogging we are featuring Aaron Jacobs, a junior majoring in the Vertebrate and Physiology option of Biology and minoring in Psychology.  During his time here at Penn State he has been on the executive board for Camp Kesem, an organization that provides camps for children who have been affected by a parent’s cancer.  He is a resident assistant, a teaching assistant for Biology 230W and president of a campus organization called Cancer Outreach.  He is planning on furthering his education after Penn State by going to medical school with the hopes of becoming a surgeon. Aaron’s blog post is below:

I have been working on various projects in the Langkilde Lab for almost three semesters, and one of those projects has been published recently.  This publication started with an observation I made one day while caring for the lab’s lizard colony. I was feeding and maintaining the  housing bins of our fence lizards (Sceloporus undulatus), when I noticed that a lizard was missing. Sceloporus is a highly studied genus of lizards, commonly known as spiny lizards, and it includes some of the most common lizards in the United States. Sceloporus undulatus, the Eastern Fence Lizard, is found throughout the eastern U.S., and is the subject of a lot of research in our lab. After a brief check to make sure that this disappearance wasn’t a bookkeeping error, I realized that the lizard that was missing was still in the housing bin, but was just in the stomach of one of the other lizards.

Cannibalism is a rare occurrence in reptiles, but this situation represented an even rarer event. This wasn’t just an act of cannibalism; it was an act of siblicide.  It turned out that the two lizards, the eater and the eaten, weren’t just the same species, but were family; they were hatched from the same clutch of eggs.  This act of siblicide is a rare occurrence and the first ever documented in the genus Sceloporus. Siblicide in general is probably rare because it reduces the fitness of an organism’s genes.  By eating a sibling, which shares genes with an individual, the chance of that individual’s genes to get passed on is decreased. For more information about some hypotheses regarding this rare occurrence see our recently published paper.

The cannibalized Sceloporus undulatus lizard (right fecal pellet) and one of his siblings (left).

The cannibalized Sceloporus undulatus lizard (right fecal pellet) and one of his siblings (left).

Cannibalism is not unknown in Sceloporus, and may occur for many reasons. One reason for cannibalism is the fact that Sceloporus species are opportunistic feeders and, due to size disparities, will eat fellow conspecifics because the smaller lizards make easy prey.  The supply of food also plays a role in determining if a species will undergo cannibalistic behaviors.  If food resources are low, lizards will consume fellow lizards due to it being more beneficial to consume a conspecific than to perish and have no chance of passing on your genes. The density of adults to juveniles plays a role in cannibalistic events as well: the more adults that are near juveniles the higher the chance that the larger lizards will eat the smaller juveniles.  The last proposed hypothesis for why cannibalistic events occur is the distance to the nesting sites where oviposition occurs.  S. undulatus seem to pick nesting sites based on the temperature and moisture levels. The farther the distance from adult territories to a nesting site, the more time the juveniles have before coming in contact with adult lizards, which provides them with more time to grow larger and avoid cannibalism.

Cannibalistic events show a positive relationship with size disparity and adult/juvenile density (a) . Cannibalistic events show a negative relationship with food availability and distance to oviposition (b) .

Cannibalistic events show a positive relationship with size disparity and adult/juvenile density (a). Cannibalistic events show a negative relationship with food availability and distance to oviposition (b).

Publishing a note on this observation was a way of documenting an extremely rare occurrence in captivity and in the wild.  This project has provided data for further experiments to build from and, due to the rarity of such an event, it has given us knowledge as to how this species and possibly others interact. Siblicide does occur, as this shows, and the reasons why an organism might engage in an act with negative consequences for fitness should be further investigated.  Potentially, some species may not distinguish between siblings and unrelated lizards.

This ended up being a fascinating project, and I was glad to be a part of it. I was able to contribute to this note by collecting data, providing input on possible reasons for such an occurrence and researching other papers on cannibalism in lizards, and specifically looking for examples of siblibide that have already been published. By being a part of this note, I gained inside knowledge of how the process of publishing works, from the sending of the paper to the publisher, receiving a revised copy, resending and so forth.  It is a long process and I now feel that I have gained a better understanding of research and have knowledge that I can apply to future publications.