Friday, September 9, 2011

Things That Creep in the Night

I. inornatus

         Earlier this year, I found myself in a tropical downpour, surrounded by a Central American jungle, and with my nose running like a faucet.  I was afflicted with what my advisor had dubbed, “Black Elk Disease”, resulting in my words turning into grumbling sounds that seemed to emanate from the depths of my stomach. I tried to shake my grogginess off; I was supposed to head into the forest that night to look for snakes crawling through the dense shrubs and trees. So far, I would imagine this sounds like Hell on Earth for many people. But, it was actually an incredible opportunity.

            I had returned to La Selva in Costa Rica to conduct a study that aimed to determine how two closely related and superficially similar species of snakes could inhabit the same area. One of basic foundations of ecology (i.e., the study of factors that influence the distribution and abundance of organisms, and my area of interest) is that species compete for resources. If two species eat the same type of food and live in the same type of habitat, then one species will eventually be outcompeted (assuming that there is a limited amount of these resources). What happens next is a tricky question, but the answer probably occurs over a long period of time. In some cases, the lesser competitor could go extinct. Another potential is that the lesser competitor eventually evolves to reduce competition. The classic example of this type of change, often dubbed Character Displacement, is that of the Galapagos finches. Finches evolved different beak shapes that allowed them to feed on unique seeds (and other items). By each species using a different resource, they were all able to thrive; this resulted in a high diversity of species in one area.

I. cenchoa
            The snakes I was interested in are both considered blunt-headed vine (or tree) snakes that are in the same genus (Imantodes). Both species are extremely slender, similar sized, arboreal, and their head shape (which can be an indication of what a species eats) looked identical. The only difference between them seemed to be their coloration: one species tended to be light brown with dark spots and the other was yellowish. At first glance, you might expect that these two snakes competed for everything, but the fact that they were both able to occur in the same jungle suggested that there was more to these snakes than their appearances.

            My hunch was that these two species, although they seemed to be in the same forest at a large scale, actually divided up their habitats at a smaller scale, which allowed them to reduce competition between them. To test this hypothesis though, required finding the snakes and comparing their locations, hence my trip to Costa Rica.

            Fortunately, my sickness was temporary, and I was able to head out most nights to look for snakes. Accompanied by my advisor, the protocol was to slowly walk along the extensive trail system of La Selva while shining our headlamps into the leaves and branches that enveloped us on all sides. It was slow going; the snakes were so slender that they can easily blend into the dense vegetation. We were required to spend a lot of time hiking around before we could find many snakes. In the meantime, I tried to convince myself that the occasional noises we heard emanating from the forest were unlikely to be from a jaguar stalking us (although they are known from the site).

I. cenchoa
Kinkajous, which are nocturnal and terribly curious animals, were frequently noticed as they crashed through the tree canopy above us to get a better look at the herpetologists below. Kinkajous are in the same family of animals as raccoons, but these fruit-eating creatures look more like a cross between a ferret and a lemur. Shining my headlamp into the trees above us would occasionally reveal the eye-shine of a kinkajou looking back down at me.

            Preliminary research had suggested that one of the species, Imantodes inornatus, is found more often in swamps while the other species, Imantodes cenchoa, occurred more often in dryer portions of the forest. This could be the key difference in behavior that resulted in reduced levels of competition. Once we had found a snake, we would record its location. This location would later be placed on a map of the site and by measuring the distance to the nearest swamp, we could determine whether the different species were located closer to swamps than we would expect than if they were found in random locations.

I. inornatus
In addition, the previous study had suggested that the two species of snakes might prefer to eat different things. Since snakes investigate scents by flicking their tongue, it is possible to know how interested a snake is in a particular item by counting how many times they flick their tongue. So, we also captured various lizards and frogs we encountered during our night hikes and later in the study we would rub a Q-tip across one of these animals and put it in front of a particular snake. If one of snakes flicked its tongue at one type of scented Q-tip more often than expected, we could conclude that the snake prefers that prey type.

We were only able to find about ten or so snakes after about two weeks at La Selva, not a particularly good haul and not enough to come to any firm conclusions, but our preliminary data did seem to support the suggestion that although the two species appeared very similar, they are probably using different resources. On my last day in La Selva, as I released all the animals we had captured, I considered how amphibians and reptiles seem to be declining overall at the site and I wondered if our inability to catch more snakes could be attributed to this trend. It is difficult to know.

In any case, I am looking forward to my next study at the site: I will be trapping mud turtles in a few La Selva swamps and comparing my results to some old data to determine how their populations have changed over the last few decades. Of course, I’ll also be keeping an eye out for more blunt-headed snakes.

Not all of the snakes I found were harmless. Rainforest Hognose Pitviper
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Much of what I write is based on my own experience, but I also rely on the research of others.

Whitfield SM, Bell KE, Philippi T, Sasa M, BolaƱos F, Chaves G, Savage JM, & Donnelly MA (2007). Amphibian and reptile declines over 35 years at La Selva, Costa Rica. Proceedings of the National Academy of Sciences of the United States of America, 104 (20), 8352-6 PMID: 17449638

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