Testing new methods to study reptile thermal ecology – PhD adventures

Testing new methods to study reptile thermal ecology – PhD adventures of another PhD student in Mexico

It’s been a while since you’ve heard all about my PhD dreams of studying the thermal ecology of the dwarf caimans in the Peruvian Amazon (see former blog post here), so an update is well overdue… Despite a promising start back in the Summer of 2019, much has changed, courtesy of your not-so-friendly neighbourhood COVID pandemic… Hence, in the face of this event, it was time for my project to adapt or perish. I chose the former, and while my dream remains to one day study the thermal physiology of the elusive Amazonian dwarf caimans, I decided to shift my focus away from the species and towards the methodological development necessary to approach such questions in a broader sense. Little did I know the laborious task I had set myself to!

The pandemic-related travel restrictions also meant that OpWall’s Peru project had to take a bit of a hiatus, fortunately the Mexico team was more than happy to adopt my orphaned PhD project, with all its new changes. Hence, I was now focused on developing and testing new tools and methods to study thermal physiology of crocodilians and squamates (lizards and snakes), resorting to the diverse herpetofauna of the Calakmul Biosphere Reserve as the study subjects.

Over the course of three field seasons, I tagged along with the herpetology team to catch as many reptiles as possible (for Science! Of course…). From these captures (on transect or opportunistic) I would measure the animal’s cloacal temperature (i.e. a well-established, albeit quite invasive, internal temperature standard) as well as get a thermal radiometric (i.e. thermal) photo of its eye.

The idea is that eye temperature (measured with a thermal camera), may serve as a non-invasive proxy of internal body temperature, which could potentially replace contact thermometry. To validate this, however, I need to collect many temperature readings from as many animals as possible to establish the relationship between these two variables. I have done this before with several Mediterranean lizards where I found promising results for these calibrations (see here and here). However, tropical reptiles often exhibit very different thermal physiology from their Mediterranean relatives and hence the need to investigate further if this eye vs cloacal temperature relationship holds true in with neotropical reptiles.

Opportunistic data collection – Taking a thermal photo (after measuring the cloaca temperature) of a speckled racer (Drymobius margaritiferus) that wandered into camp. Photo by Matthew Wallace

Methods were simple… catch a reptile, measure its cloacal temperature (with a thin probe on a contact thermometer), as soon as possible take a thermal photo of its eye (using a FLIR thermal camera integrated on a CAT S61 phone), take some morphometrics (e.g. snout-to-vent length and mass) and let it go. Easy enough, right? Well… not so fast, a few challenges presented along the way.

First, you must catch the animal while avoiding touching its head or cloacal area, so as to not transfer our own heat and bias our results. This can be “simple” for larger or more elongated (e.g. snakes) animals as you can just grab them somewhere mid-body, but not so straightforward for smaller lizards like the anoles and sceloporus lizards we often encounter in our transect surveys. Instead, for these we resorted to noosing these animals with a fine lasso dangling from the end of a long fishing rod… I know what you are thinking and yes, we really were “fishing” for lizards in the middle of the Mexican forest! And believe it or not, this is a well-established and published method for effectively capturing smaller lizards (see here).

What about faster, ground-dwelling or more secretive animals? For those we employed a more passive method: funnel traps. When well placed (i.e. hidden in the vegetation along areas prone for reptiles to pass through or hide), these can prove an effective and mostly effortless method for capturing species you don’t often encounter on transects. So, we set a few funnel traps in the area around camp and checked on them daily. Any animal caught was then quickly processed and released a few meters away from where the trap was reset. And hence, slowly but surely, we’ve been building our dataset over the course of 3 seasons. Nevertheless, even though some species are quite abundant and often caught (e.g. the ghost anole, ironically!) most species are actually not caught as often, hence building up the necessary sample size to come up with the intended calibration curves will take some more effort over the come field seasons, so stay tuned for future updates on this project.

Measuring cloacal temperature of an anole (Norops sp.) caught on transect. Photo by Matthew Wallace

But wait! What about your beloved crocodilians?! Fear not, they were there… lurking in the muddy aguada waters (see blog post by fellow Portuguese croc scientist, Joto, here to know more about Mexican aguadas and the Morelet’s crocodiles inhabiting them), and don’t doubt for a second that I got some nice data from them too! But that I’ll leave for a later story (although you can get a sneak peek from fellow herper, Matthew Wallace’s blog post available here).

For the time being, though, I am getting to grips with my new Research and Education Officer position at OpWall HQ, focusing on the science being done at OpWall’s neotropical sites. Hence, you can expect many more croc and lizard (and tropical science in general) stories in the upcoming future, so stay tuned!

Title photo – Taking a thermal photo of a Schwartze’s skink (Mesoscincus schwartzei). The animal was caught in one of the funnel traps. Since we did not know if had been trapped for a couple of minutes or a couple of hours, we allowed it to thermoregulate for some time in a large box (partially shaded) before measuring its body temperature with the thermal camera and the contact thermometer. Photo by Matthew Wallace

Related References:

Barroso, F. M.; Carretero, M. A.; Silva, F; Sannolo, M. (2016): Assessing the reliability of thermography to infer internal body temperatures of lizards. Journal of Thermal Biology, 62(A), 90-96. DOI: 10.1016/j.jtherbio.2016.10.004

Barroso, F. M. & Riaño, G. M.; Sannolo, M. & Carretero, M. A. & Rato, C. (2020): Evidence from Tarentola mauritanica (Gekkota: Phyllodactylidae) helps validate thermography as a tool to infer internal body temperatures of lizards. Journal of Thermal Biology, 93: 102700. DOI: 10.1016/j.jtherbio.2020.102700

Sillero, N., García-Muñoz, E. (2010). Two new types of noose for capturing herps. Acta herpetologica, 5(2), 259-263