Saturday, January 21, 2012

Trapping Turtles in Costa Rica: Week in Review

A respectable haul

With my first week of trapping turtles in Costa Rica coming to a close, I think it’s an appropriate time to take stock of the results.

I set my traps on Monday and left them there all day and night before checking them on Tuesday morning. Although I had high hopes for five traps all full of turtles, the reality was a little less rewarding. Nothing. In the language of turtle trappers everywhere, I got skunked.

For me, this was some cause for concern. The first night of trapping, when the bait is most fresh, is often the most productive. To not catch a single turtle on what was supposed to be the best of nights led me to question whether I had made an major miscalculation. In looking through some of the old data from previous trapping efforts, I saw that most of the trapping had taken place in June and July, months that see considerably more rain. This rain results in full swamps. I was hoping that my project would add some new information by trapping in the dry season, when water levels were low. Specifically, I wanted to know what turtles did as the habitat dried up. But, the first step was to catch some turtles. As I looked at my empty traps, I wondered if turtles were less susceptible to being trapped when the water was low. Maybe they had already left the swamp to find a place in the surrounding forest to spend the dry season.

The snapping turtle is in the background and there
are two mud turtles in the foreground

Much soul-searching and hand-wringing occurred until Wednesday morning, when the turtle gods saw fit to have me check four empty turtle traps before coming up on the fifth and final trap, which was brimming with some familiar rustling. I immediately made out the form of a snapping turtle, but it was only until I had taken the trap out of the water that I noticed there were also three White-lipped Mud Turtles inside, the target species.

The South American Snapping Turtle
The turtle on the right probably looks very familiar but it is actually considered a different species than the animal we know from North America. That species is Chelydra serpentina, known simply as the Snapping Turtle. However, this turtle, which is found from Honduras south to Columbia and Ecuador, is known as the South American Snapping Turtle, Chelydra acutirostris. In looking through the data from past trapping efforts in this swamp, which date back to 1991, only two of these animals have been identified here.

Of the White-lipped Mud Turtles, all appeared to be adults. Two were females and one was a male. The male appeared to have been captured before. By coding each scute (scale) on a turtle shell with a different number, we can make marks in the shell to give each turtle an individual number. By marking turtles individually, we can begin to estimate how many turtles are in the population. In any turtle study, if most of the captured turtles have been marked before, we can have high confidence that we have captured most of the turtles in the population. If only a small percentage of the turtles we catch are recaptures, then we know our trapping effort is not intensive enough to make accurate estimates. This could be because the animals are hard to trap, it could mean that animals simply have large home ranges and we’re only sampling a small fraction of all the animals out there, or it could just mean that animals don’t live long enough to be captured in multiple efforts.

A White-lipped Mud Turtle burrows into the
mud after being released
In any case, I believe the male had been marked with number four, which would mean he had originally been captured in June of 1991, the year the Soviet Union was dissolved and the year Bill Clinton announced he would run for President of the United States. Presumably, this turtle, which was already an adult when it was first captured, has been crawling around the swamp since then.

The next two days were not as productive, as I only caught one turtle each day. Interestingly, another turtle was a recapture, this time a female. She had been originally captured in June of 2003, when 50 Cent’s “In da Club” was the top single in the United States. On Wednesday, I caught sight of a Gray-necked Wood-rail investigating one of the traps. I imagine it was interested in the fish that were swimming around the turtle bait. On Thursday, as I was walking around the swamp looking for some more pools to set a trap (White-lipped Mud Turtles are unlikely to be evenly distributed across the whole swamp and I did not want to overlook a spot with more animals), I found a turtle walking in some dry vegetation. Perhaps she was also looking for some water.

The final tally for the week was one South American Snapping Turtle and and Six White-lipped Mud Turtles. It's something, but not yet enough turtles to say much about the population. Today I was once again skunked. I have been watching with some dismay as the shallow pools in the swamp quickly dried up. Several of my traps are now sitting on top of the mud. If it does not rain soon, my trapping will be over sooner than I had anticipated.

The study site is looking less and less like a swamp each day

Much of what I write is based on my experience in the field. However, I also rely on the research of others. Citations to relevant scientific articles are included below.

Morales-Verdeja, S., & Vogt, R. (1997). Terrestrial Movements in Relation to Aestivation and the Annual Reproductive Cycle of Kinosternon leucostomum Copeia, 1997 (1) DOI: 10.2307/1447847

Monday, January 16, 2012

Trapping Tropical Turtles Today

            Some animals, like turtles, can live for decades. But most turtle studies only last a few years (one big reason is that many studies are conducted by graduate students and, contrary to popular belief, students do in fact want to graduate quickly). So, because turtles live for decades and most studies are completed after just a couple years, that means that much of what we know about these animals is based on snapshots in time.

            Since many of us want to know best how to conserve turtle populations, it is important that we have a better understanding about how they change over time. Knowing how populations naturally fluctuate allows us to better understand how they might respond to conservation threats. Establishing this baseline data is particularly important for undisturbed populations.

People have been trapping turtles in this swamp for 20 years
            When I heard that researchers and students had been trapping, on and off since 1991, a population of White-lipped Mud Turtles, Kinosternon leucostomum, in a Costa Rica swamp surrounded by old-growth rainforest, I realized that this population represented a unique opportunity. By continuing the trapping myself, and then compiling and examining the data collected by many different people over the last two decades, it would be possible to see how a turtle population has changed over time. And I wouldn't need to conduct a 20-year field study to do it.

            So, thanks to the Organization for Tropical Studies, I find myself back at La Selva. I arrived yesterday and will be here, walking amongst the peccaries and listening to the toucans, on and off until April.

            Today, the first task was to find the swamp (those familiar with La Selva may know this small forested pond as the Research Swamp). Unlike my previous research here, this work is to take place during the day; I was pleasantly surprised to find that the path to the swamp was well-marked with flagging and easy to follow (when it is light out).

            Deep in thought, I stumbled into a herd of collared peccaries, they let me know I was too close by snapping their teeth together; doing so produces a sound meant to warn off potential predators. Collared peccaries are generally thought of as relatively harmless creatures, but I give them my respect. You don’t last long in tropical forests filled with pumas, jaguars, and venomous snakes by being a pushover.

            Speaking of venomous snakes, there is no shortage of them here. Perhaps the most infamous is the Fer-de-lance, also known as the Terciopelo, Bothrops asper. These large vipers are most often observed at night, perhaps as they travel across a path or when they are coiled up in an ambush position along a tree trunk, waiting for a rodent or frog to scurry by. Swampy areas are thought to be the preferred habitat of the Terciopelo, so I reminded myself to watch my step when I was mucking through the mud to set my turtle traps. It was good advice.

Notice the trap in the waterway
            The water level was relatively low, so it was somewhat of a challenge to find areas deep enough to cover the bait containers within the traps. It is important to have the bait underwater so that the scent (in this case, of sardines in tomato sauce) can easily reach any hungry turtles.

            Another important thing to consider when trapping turtles, or any wildlife for that matter, is to make it easy for the animal to encounter the trap. Although the bait might entice a turtle to crawl over somewhere where it might not go otherwise, you will probably have better luck putting the traps where the turtles are going anyway. So, with this in mind, I tried to put the traps in areas where small streams ran through the vegetation, I figured these pathways would be how the turtles got around.

My initial view of the Terciopelo
            As I stumbled through the mud and stepped over large tree roots and downed logs, I caught the familiar shape of a coiled snake. Basking in the sunlight sat the infamous Terciopelo, a relatively small one. If it noticed me as well, there was no indication. I couldn’t help but think about what kind of animal was, for lack of a better word, confident enough to sit on a log where other predatory animals might easily see it. I did not need anyone to tell me not to get too close.

            If you read this blog, then you know that I am fond of snakes and I do not fear them. I know that snakes do not represent much of a danger to me because I take a few small precautions (like wearing boots instead of sandals). However, even though I knew there were Terciopelos around me all along, I couldn’t help but start overreacting a bit whenever a branch scraped across my back. I also noticed my steps became a little slower and more deliberate. In any case, if I walked by any more snakes, I did not see them.

A closer look
            But, we are here to talk about trapping turtles. I am most interested in White-lipped Mud Turtles; I hope some wander into the traps tonight. Of course, there are many animals that like to eat sardines, so we will have to see what shows up tomorrow morning.

A White-lipped Mud Turtle I noticed at a
nearby swamp  a couple years ago


Much of what I write is based on my experience in the field, however, I also rely on the research of others. Citations to relevant articles are provided below.

Wasko, D., & Sasa, M. (2010). Habitat Selection of the Terciopelo (Serpentes: Viperidae: Bothrops asper) in a Lowland Rainforest in Costa Rica Herpetologica, 66 (2), 148-158 DOI: 10.1655/08-064R2.1

CONGDON, J., DUNHAM, A., & VAN LOBEN SELS, R. (1993). Delayed Sexual Maturity and Demographics of Blanding's Turtles (Emydoidea blandingii): Implications for Conservation and Management of Long-Lived Organisms Conservation Biology, 7 (4), 826-833 DOI: 10.1046/j.1523-1739.1993.740826.x

Friday, January 13, 2012

Readers Write In: Grey Ghost Rattlesnakes in Wyoming?

A reader recently wrote to me in the hopes of shedding some light on a rattlesnake mystery from northern Wyoming. The original e-mail is below (I have edited for length and clarity).

I was involved in a rescue event in late August in N. Wyoming.  For reasons too complex to go into here I found myself in knee-to-waist high mesquite brush in pitch-black darkness, wearing nothing but shorts, personal flotation, and kayaking slippers.  I was using an LED light, which produced a bluish beam.  I came upon three rattlesnakes within a radius of less than 100 feet.  Two were actually within about ten steps of each other.  They were the palest shade of grey I had ever seen in a snake -- almost white, void of markings.  They were the absolute exact same color as the ground.  My bluish light was probably producing some color distortion, but if so, it was distorting the color of the snakes and the ground in the same way.  

A small rattlesnake (Pigmy Rattlesnake) from Florida
At the time I saw the first snake I had been climbing around in the bush for an hour, and I freaked out.  I was being fairly vigilant but not really expecting to see rattlers in those conditions (or at that elevation). These snakes were not large.  Although it hard to guesstimate the length of a coiled rattler, I would say less than 18".  Their camouflage was so perfect that they were nearly impossible to pick out.   Although I could see their rattles, I couldn't hear them because of the high wind. 

Any guesses as to the species?  Any thoughts as to why I would have stepped by three of these things in such a small area?

Denis O.

There are a number of serpentine mysteries to unravel here. First off, what species of rattlesnake could this be? Although Wyoming is not the first place many people think of when asked to imagine a good place to find rattlesnakes, there are two species native to the state: the Prairie Rattlesnake, Crotalus viridis, and the Midget Faded Rattlesnake, Crotalus oreganus. In Wyoming, the Midget Faded Rattlesnake is only found in desert habitats in the southwestern corner of the state. We can safely conclude that the species the reader observed was the Prairie Rattlesnake.

Prairie Rattlesnakes, along with several other species of rattlesnakes that live in high latitudes or elevations, may be found in large numbers near den sites. I suspect that the reader was trudging through an area close to a den site, which explains why he would find three in a relatively small area (usually rattlesnakes are found few and far between, especially in the southeastern United States where they do not typically den together).

It is difficult to estimate the size of coiled rattlesnakes
(here, another Pigmy Rattlesnake)
Let’s move on to the lengths of the snakes. Eighteen inches (about 46 cm) is not a very large rattlesnake. Considering it is human nature to overestimate the size of rattlesnakes (as well as the reader's admission that it was difficult to determine), let’s assume that the snakes were slightly smaller. This is about the right size for neonate (that is to say, newborn) rattlesnakes, which tend to be born in late summer or early fall. Since the snakes were observed in late-August, this explanation makes sense.

The apparent color of the snakes is interesting. The reader reported them as grey, almost white. Although there are albino rattlesnakes, it would be very unusual to find three in a lifetime, let alone in one night. I think the appearance of the small snakes can be explained with a combination of rattlesnake biology and the choice of flashlight.

Newborn rattlesnakes don’t tend to move far or do much until they shed their skin for the first time, which often happens within their first few weeks. Before a snake sheds their skin, the old skin may appear gray. This appearance was probably heightened by the use of an LED light. I have overheard many debates among herpetologists regarding the use of LED vs. more traditional halogen bulbs. Although LED lights are brighter, they produce a light that is less “natural”. When looking for reptiles at night, I prefer LED lights because they illuminate a larger area. However, they may wash out colors. It is easy to imagine how a snake that is about to shed their old and gray skin might appear even more gray when lit up by an LED.

Finally, because rattlesnakes develop their rattles by adding new segments after each shed skin, a newborn rattlesnake would not have a functioning rattle. This explains why the reader did not hear their telltale warning signs. Although newborn rattlesnakes might not be more dangerous than adults, their venom does pack a punch. This is a good reminder not to handle a snake just because it doesn't sound (or look) like there is a rattle.

So, in conclusion, the most likely explanation for this observation is that the reader stumbled onto a Prairie Rattlesnake den and several newborn babies. That the babies had not yet shed their skin, as well as the use of an LED light, explains why they appeared gray and did not produce any rattling noises when disturbed.

Thanks for writing in with your questions. Keep them coming.


Most of what I write is based on my experience in the field, however, I also rely on the research of others. Here's a relevant article:

HOLYCROSS, A., & FAWCETT, J. (2002). Observations on Neonatal Aggregations and Associated Behaviors in the Prairie Rattlesnake, Crotalus viridis viridis The American Midland Naturalist, 148 (1), 181-184 DOI: 10.1674/0003-0031(2002)148[0181:OONAAA]2.0.CO;2