

Written by and photos courtesy of Frederico M. Barroso

While most young children soon outgrow their “dinosaur phase”, it is safe to say that I never quite did so. Grown on a healthy diet of abundant outdoor play sprinkled with ad libitum wildlife documentaries (i.e. before the age of tablets), it was no surprise when I decided to pursue a career in the natural sciences. In fact, it was from an early age that I set my mind to becoming the next “Dr Croc” and so it is only natural that I am now doing my PhD on crocodilians.

The idea behind the project is one that I have slowly been developing over a few years now. In fact, it all started with my first expedition with Opwall to Peru (back in my first year of my undergraduate studies). At this point I was merely a Research Assistant accomplishing his dream of visiting the Amazon Rainforest and seeing crocodilians in the wild (little did I know of what was to come!). And just like that, I was hooked. I returned to Peru once more as an RA and then again as a Dissertation Student working on my undergraduate thesis. Since then I’ve graduated, got a job (with Opwall’s Iberian Office) and tagged along as staff herpetologist on a few other Opwall expeditions (Honduras, Croatia) before returning to the project in Peru, now as Science Coordinator, to collect data for my PhD.

For my undergraduate thesis I decided to study the night-time thermalecology of the three syntopic (i.e. living in the same area) species of caiman inhabiting the Pacaya-Samíria National Reserve. I was particularly interested in understanding how the much smaller, semi-terrestrial species, Paleosuchus trigonatus, interacted with the larger syntopic species as the night progressed and temperatures decreased (final report available here).

As I was developing my project, I came across a paper by Zilca Campos and William Magnusson (available here), two authorities in caiman ecology and research, where they tentatively suggest that the other, very similar, species of Paleosuschus (P. palpebrosus) may in fact be a thermoconformer (i.e. a species that makes no attempt to behaviourally, through basking, shuttling etc., maintain a constant body temperature)… an unusual strategy for a crocodilian! That got me thinking as to why would such a strategy would appear in what are essentially top-predators who can easily monopolise their “place in the sun”. Was it maybe a way to reduce competition with the much larger sympatric species of caiman? If so, what would happen when these larger species are not present or not actively thermoregulating (e.g. at night), would Paleosuchus then “sneak in” a bit of thermoregulating?

With that in mind, I decided to investigate the pattern of body temperature change of the three caiman species inhabiting the Pacaya-Samíria National Reserve in the Peruvian Amazon. So I embarked (quite literally, as I was based on a beautifully renovated historical boat from the rubber boom era – Figure 1) on a six-week expedition to the middle of the Peruvian Amazon. I spent my nights on smaller boats, patrolling the Samíria river with a spotlight, looking for those two red beams staring back at you, the unequivocal sign of a caiman (well… not 100% unequivocal, sometimes it was just a nightjar or a potoo which also have a reflective layer inside their eye, the tapetum lucidum, giving them an eye shine when light is pointed directly at them!).

Figure 1: “MF Rio Amazonas” was our home and base camp for the 6 weeks spent collecting data in the middle of the Peruvian jungle. This beautifully renovated historic boat from the rubber boom era housed more than 60 people at any point in time throughout the expedition. Scientists, research assistants (university and high school students) and crew all lived together and shared data, knowledge and experiences in this boat for the duration of Operation Wallacea’s field season.

Having spotted a caiman, it was then essential to catch it. For that I had a noose and Rolin (AKA “Rocky”), Jose and Alfredo – some of the Peruvian guides working with Operation Wallacea on the project. They are from the local indigenous Cocama communities who still inhabit and rely on the forest and hence have an intimate knowledge of the forest and the animals in it. The three of them are also masterful caiman wranglers, eager to pass on their knowledge and skills on how to catch caiman. Upon capture, and after securing the jaws with some duck tape, I needed to measure the animal’s internal body temperature as soon as possible. This was done by turning the caiman swiftly upside down and rubbing its belly (which helps to calm the animal down!) while inserting a fine thermocouple probe into the animal’s cloaca to get the reading. Some morphometric measurements would also be taken to attain a sense of the animal’s size as well as some photos for subsequent individual identification (using software to estimate recapture rate and from it caiman density). The animal would then be released, unharmed, to where it was caught. The temperatures of the animal’s environment (air, soil and water) were also recorded.

Subsequently, from the data obtained, I uncovered a very interesting pattern… As expected, the internal body temperature of the two larger species (the black caiman, Melanosuchus niger, and the spectacled/common caiman, Caiman crocodilus) gradually and consistently decreased as the night progressed and the environmental temperatures dropped (Figure 2). At this point, without the input of the sun, it is safe to assume that these animals are effectively being forced, to some extent, to thermoconform to the temperatures of their immediate environment (i.e. the water).

However, Paleosuchus trigonatus showed a different pattern. Despite an initial gradual decrease in body temperature for the first hours of the night (consistent with the hypothesis of the animal thermoconforming to the also decreasing temperatures of the environment), the average internal body temperature of the Schneider’s dwarf caiman (Paleosuchus trigonatus) would then start to increase again, towards the later hours of the night (Figure 2). This would suggest that, later into the night, the animals may in fact be thermoregulating.

Figure 2: Pattern of body temperature change over night for the three species of caiman studied. Note the fact that data for Paleosuchus trigonatus is only available from over an hour later than that of the other two species. Is this because they only move into the rivers later into the night when the other species start to become colder and less active or was this simply a remnant of the relatively small sample size for this species (smaller than for the other two)? Also interesting to note is the relatively cooler starting point for P. trigonatus. Perhaps due to the fact that it may have been in the forest, instead of the water, where temperatures may drop faster due to the lower thermal inertia of air compared to that of water.

Yet, how and why can this be? Where are they getting the heat from (thigmothermy?) and, if they are in fact thermoregulators after all, why do they not thermoregulate all-day and all-night long? (Or do they?).

Perhaps, as the body temperature and therefore the level of activity of M. niger and C. crocodilus drops well bellow suboptimal temperatures, that allows P. trigonatus to move from the forest (where, during the day it is safe from the other larger caiman species) to the water, a more stable and warmer thermal environment, thus being able to increase its body temperature and possibly maximise its performance. Perhaps P. trigonatus is in fact a thermoregulator yet with lower set points and/or a broader thermal range than the other caiman species (but see Figure 3). Finally, perhaps P trigonatus is segregating its thermal niche both in terms of adapting to different (lower) thermal optima and segregating its use of the thermal habitat not spatially but temporally (i.e. it thermoregulates at night while the other species thermoregulate during the day).

Figure 3: Frequency density histogram of the internal body temperatures of the three species of caiman studied. Note the broad shape of the distribution observed for C. crocodilus and M. niger. This shape is indicative of generalist strategy. On the other hand, note the much narrower yet bimodal distribution for P. trigonatus. Is this narrow shape an indication of a thermal specialist (completely contradicting the thermoconformer hypothesis) and would the two peaks indicate two different selected temperatures (perhaps depending on the level of activity of the competitor species) from two different times of the night? Or could this simply be an artefact of the small sample size? It is also relevant to notice that the highest peak for P. trigonatus falls at a lower temperature than those of the other species, possibly indicating a lower preferred temperature for this species. More in depth studies are needed to draw any definite conclusions, though.

For the upcoming 4 years of my PhD project (at CIBIO Lab – University of Porto, Portugal), I will be collecting a range of ecophysiological data to describe the ecology of these animals. Now with an extended sampling site which includes the Pacaya-Samíria, Yarapa river and the Tamshiyaco-Tahuayo Community Reserve, I am now able to obtain data from both species of Paleosuchus (this year we also found P. palpebrosus in Tahuayo, the new site, it was never seen before at the previous site – I was very excited about this discovery!).

A big part of my project is centred around developing new tools and techniques to obtain large amounts of data with minimum disturbance/ invasiveness to the animal. For that I will be testing tools such as InfraRed thermal imaging to infer internal body temperature of caiman (Figure 4) as well as developing and deploying a custom tag (in development by ElectricBlue – www.electricblue.eu/) which will allow me to log internal and external temperature of the animals as well as GPS location remotely and for a large period of time. Hence, in the near future I will be attaching these tags to some animals which will allow me to track them in space (i.e. study their movement patterns, habitat preferences, etc) as well as track their internal body temperature and compare it to the temperature of their immediate environment, thus allowing me to describe their realised thermal strategy.

Figure 4: InfraRed Thermal Image photo of a P. trigonatus. The colours represent different temperatures (red=warm, blue=cold). Actual temperature readings accurate to the nearest 0.1ºC can be obtained from every pixel in the image, using specific software. I am currently trying to determine if the thermograhically obtained temperature of the eye shows a strong correlation to the internal body temperature, as observed for other reptiles by Barroso et al. (2016). If that is the case then in the future IR cameras could be used to easily infer internal body temperature of caiman without the need to catch the animal.

Additionally, I intend to study these animals’ dietary preferences through a combination of stomach sample analysis and stable isotope analysis. These should allow me to determine to what extent these four species of caiman compete for food. With the addition of the fishing data collected by Opwall and FundAmazonia, I also expect to be able to infer to what extent the diet of the caiman overlaps with the fish being caught by the local fishermen and hence determine the extent to which there is human-caiman competition for fish.

Hopefully these tools will then enable me to answer some of the questions mentioned previously. While ultimately, a better understanding of these animals’ ecology will also help us better predict how these species may respond to changes in their habitat and environment (i.e. due to climate change, habitat degradation, etc.) and hence contribute towards developing informed management and conservation strategies for these animals

In the meantime, however, it is time to get my rubber boots deep into the muddy Amazonian waters (sometimes quite literally!) and collect the necessary data to uncover the truths about the illusive dwarf caimans of the amazon jungle.

Stay tuned for updates!

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