Eavesdropping on nature

One of the two core themes of the Integrative Tropical Biology course is that detailed and intimate understandings of the natural history of a given model system are essential to be able to ask meaningful questions and apply modern technologies to experiments in that system. In biology, there seems to be an increasing trend toward more and more complex and technical conclusions made about systems, based primarily on laboratory or bioinformatics studies alone. As our technological capabilities keep accelerating, I think that there will be an increasing need for scientists who understand the basics of natural history and ecology, in a broad sense. These types of people will be essential to keep our conclusions grounded in biological and ecological relevance.

Understanding natural history begins with keen observation of the natural world. Daily attentiveness and curiosity must be cultivated. An excellent example of a scientist who embodied these talents is the late Thomas Eisner, an entomologist and chemical ecologist extraordinaire. In his astonishingly illustrated reflections on a lifetime of discovery, For Love of Insects, he writes in the afterword that many of his discoveries were made by “eavesdropping on nature.” He would then follow his observations with slight manipulations of the system, to see how a perturbation might lead to ideas for controlled experiments.

Bullhorn acacia (Vachellia cornigera) spines and foliage

On 12 January 2015, on a course field trip to the Smithsonian canopy access crane at Ft. Sherman, our group had ample time and space for making observations as we walked on the gravel road and peered into the ruderal vegetation along the edges. One plant encountered on the walk was the bullhorn acacia, Vachellia cornigera. Known as cachito (“little horn”) in Panamá, this small leguminous tree is an oft-cited textbook example of a myrmecophyte. Wanting to get close enough to see if I could smell the alarm pheromone of the protective ant, Pseudomyrmex ferruginea, and watch the hordes emerge from their hollow stipular spine domatia, I approached with caution. After seeing the ants, one of the first things that I noticed on the acacia was a caterpillar resting on a spine, in a still posture with its body held erect from the spine (see photograph below). The ants would occasionally contact the caterpillar, but did not seem even to pause and sense its presence. Intrigued, I decided to remove the medium brown larva from its perch and place it on foliage to see if that would elicit an aggressive response from the ants. To my surprise, the geometrid (I now knew the family by the caterpillar’s “Earth-measuring” gait), inched along rapidly, immediately back to one of the nearest spines. The ants did seem to respond with increased activity, but when they encountered the caterpillar, they merely antennated the body of the larva and moved on. The caterpillar returned to a resting state. From this, I wondered what the caterpillar was doing in the acacia. Was it an herbivore? Was it providing any benefits to the ants? How did it avoid being attacked by the ants? Did it have chemical signatures that the ants might be recognizing as self semiochemicals?

Geometrid larva on acacia spine

In the few minutes that I spent at the tree, while enduring several ant stings, I happened to notice two other arthropods in the tree – a scarab beetle (I think likely a ruteline) and a salticid (jumping) spider. I was surprised to see the scarab feeding on the acacia leaflets, and apparently tolerating ants that were swarming on it. Could the ants not penetrate its smooth, tough exoskeleton? What about the spider? It was resting on the Beltian bodies at the leaflet tips. I had heard of a fairly recent report of a “vegetarian” jumping spider, but the one that I saw did not appear to be Bagheera kiplingi. Unfortunately, my hiking partner and I were already far behind the group and we could linger no longer. Our observations would remain only observations and extrafloral fodder for thought for the time being.

-Luke

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