Chasing Red

A visiting professor named César Nufio came to lead one of the research projects here at La Selva Biological Research Station.  While here, he also gave a few lectures, and in one of them he told us about the research he does at his university back in Colorado, where he is working on a long-term project studying the effects of climate change on crickets.  He talked about thermal performance curves, and the idea that organisms function optimally at a certain temperature, but when their body reaches temperatures too far above or below the optimum temperature their metabolic functions and performance in activities such as jumping starts to diminish, until they reach a thermal maximum or minimum when they can no longer function at all.  This is particularly relevant to ectotherms because they cannot regulate their body temperature metabolically, so their environmental conditions have a large impact on their internal temperature. 

            Andriana, Erin, and I decided to work together on another research project, and we began to wonder if since the lowland wet forest of La Selva is already so hot, some organisms might already be operating at or above their optimum temperature, meaning further warming would decrease their fitness.  We decided to work with the small and charismatic red and blue poison dart frog, Oophaga pumilio, and since it’s an ectotherm its body temperature could potentially rise at the same pace as that of the local climate.  To test this idea, we decided to compare the frog’s jumping ability at temperatures slightly hotter or colder than what they experience in the forest, and to test their thermal preferences since animals often prefer temperatures close to their thermal optimum.  It was easy to spot frogs in the forest; O. pumilio is so abundant at La Selva that we merely had to walk through the undergrowth and one would appear out of the leaf litter every five or ten meters, a small red flash hopping away.  The frogs near the beginning of the trail were apparently fairly accustomed to people passing by on the trail so they weren’t too difficult to catch, and with practice we were able to catch up to 30 in two hours.  However, we needed to catch frogs each morning and release them by the next and couldn’t catch frogs at the same place twice, so each day we went further down the trail.   By the fourth and final day of data collection, we were a few kilometers into the forest where the frogs were much more skittish.  Many probably hid when they heard us coming, and the ones we did see would often disappear into the leaf litter before we could catch them, and that morning we only caught four.

            Back in the lab we tested each frog’s jumping ability at the ambient outside temperature as a control by placing the frog on a plastic lid covered with paint to get some on its feet and let it jump on a white sheet.  They left very cute little frog footprints, and we measured the distance between them to see how far each of the first five jumps were.  We then either warmed the frog up using a heat lamp or cooled it down in an air conditioned room and did the same jumping test again.  To see if the frogs preferred a certain temperature, we put them in a thermal gradient we made out of a wood box and some heat lamps and let them roam around for 15 minutes. 

            We were somewhat surprised to find that the frogs jumped the farthest when they were at the hottest temperatures we tested, around 35°C.  Frogs that were at a higher temperature in the field visited warmer spots in the gradient, and these temperatures were often higher than what they were at in the field.  While this is certainly good news for the frog and suggests that it may be able to tolerate warming of at least a few degrees, weren’t sure how to present these results at first.  Many people predict that amphibians will be particularly vulnerable to global warming since they are ectotherms and many require water to keep their skin moist and to lay their eggs in.  Our results don’t discount the other potential threats of warming temperatures, or suggest that O. pumilio.  It may still affect other species in the same ecosystem, which would in turn affect O. pumilio, and further research would be needed to see if warming temperatures will change precipitation patterns and how that would affect O. pumilio.  However, it is heartening to know that not all species will respond to climate change in the same way, and that some may even be better at tolerating it than we expect.

Sequoia Grettenberg
Wesleyan University