A Walk in a Panamanian Rainforest

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The research crew analyzing data (From left to right; Brendan Dula, Dr. Alex Trillo, Meghan Brady, and Sarah Smith)

THE PLACE 

We are conducting research at the Smithsonian Tropical Research Institute (STRI) in Gamboa, Panama.  Gamboa is a small town that was originally built to house the US workers of the Panama Canal. Today, it houses many STRI researchers who conduct studies nearby in Soberanía National Park or Barro Colorado Island. Our research is focused on the relationships between two species of frog (The túngara frog and the hourglass treefrog), a species of frog eating bat (the fringe-lipped bat), and a genus of frog-biting midges (Corethrella). Before we explain our projects, here is a little more information on the cool organisms that form part of this story.

THE ORGANISMS IN THIS STORY

The túngara frog, Engystomops pustulosus, likes to breed in large puddles or ponds, where they call to attract females. These frogs are an important part of the food chain, as a lot of predators, such as snakes, herons, bats, and other frogs, eat them regularly. Hence why Dr. Trillo, our advisor, calls them “the popcorn of the jungle.” Túngara males can produce simple or complex calls. Simple calls consists of a single “whine” that decreases in frequency. In complex calls, the whine is followed by another note call the “chuck.” Females choose a male based on its call, preferring males that produce complex calls. These calls, however, are also highly attractive to predators and parasites. The fringe-lipped bat, Trachops cirrhosis, is a leaf-nosed bat found in tropical dry and wet forests that uses echolocation to hunt insects and frogs in the early hours of the night. Fringe-lipped bats are highly attracted to the complex calls produced by túngara males. Frog-biting midges in the Corethrella genus are small flies that parasitize frogs. Adult female flies of this genus are attracted to the mating calls of male frogs and they also parasitize males using complex calls to a greater degree than those making simple calls. Finally, the hourglass treefrog, Dendropsophus ebraccatus, has large breeding habitats and shares breeding ponds and swamps with túngara frogs.

THE QUESTIONS

Many frog species call from the same pond during a given night, forming mixed species choruses. In such aggregations, individuals of one species could be affected by the presence of frogs of other species calling around them. The question that we are trying to answer is whether hourglass treefrogs are affected by calling near individuals of another frog species that is highly attractive to predators and parasites, such as the túngara frog. Our advisor, Dr. Trillo, came up with two alternative hypotheses: (1) Hourglass treefrogs suffer more predation and parasitism when calling near a calling túngara frog than when calling near members of the same species (Collateral Damage Hypothesis) because both bats and flies will sometimes make foraging errors, attacking the neighbors of frogs they originally targeted; (2) Hourglass treefrogs suffer less predation and parasitism when calling near a túngara frog than when calling near members of their own species (Shadow of Safety Hypothesis) because both bats and flies will preferentially choose the more attractive individual. Previous research by Dr. Trillo showed that although there is no difference in bat predation on hourglass treefrogs when calling near túngara frogs versus those calling near members of their own species, frog-biting midges were much more attracted to hourglass treefrogs whenever they called next to túngara frogs.

The questions that came up after these findings were: Does this increase in parasitism vary depending on the type of túngara call (simple versus complex)? In other words, do whine-chuck calls cause higher levels of collateral damage on nearby calling hourglass treefrogs than the simple whine calls? and do you see the same effect on the hourglass tree frogs when túngara frogs are highly abundant versus when there is only one túngara calling? So, these are the questions we are trying to answer this summer!

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Brendan and Meghan after setting up the equipment at one of the field sites

THE METHODS:

So, how do we answer these questions? Everyday around 5:30 pm we go to Dr. Trillo’s house and gather everything that we’ll need for the night. We have seven sites, and for each site, we alternate between four different treatments: (A) A speaker with a calling hourglass treefrog next to another speaker with a calling hourglass treefrog, (B) A speaker with a calling hourglass treefrog next to a speaker with a túngara simple call, (C) A speaker with a calling hourglass treefrog next to a speaker with a túngara complex call, (D) A speaker with a calling hourglass treefrog next to five speakers with túngara complex calls.

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A still image of a bat predatory visit to one of our speakers. It was attracted by the frog call and is now attempting to echolocate the frog.

Once we arrive at the first site of the night we set up either 2 or 6 speakers depending on which of the treatments we’ll be doing. We set up video cameras and IR lights to film the bats that visit our speakers. We also apply sticky goo to plastic sheeting placed over two of the speakers in order to capture frog biting midges. Then we get to hang out in the forest for 80 minutes and sometimes we get to see really cool forest critters like glass frogs or caimans!!! (see picture below). We repeat this same process one more time, later in the night, to get as many data points as we can. We are only here for 2 months! The next morning we count the frog biting midges that we caught the night before and watch bat videos and score the number of predator visits, normally bats. Every now and again we also get to head to Panama City for project supplies, groceries, or lectures given by other STRI researchers, which are super interesting!

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A baby caiman we came across one night

CALL SITE SELECTION PROJECT

We’re also collaborating with Dr. Ximena Bernal on another project, which we’ve unofficially dubbed the Poolito Project. That’s because we’ve set up a modified circular kiddie pool, with 24 upside down terra cotta pots in it for hourglass treefrogs to call from. We assembled this incredibly scientific set up to test if hourglass treefrogs choose different calling locations in response to the collateral damage they experience near túngara frogs. We go out and catch male hourglass treefrogs, transport them in super high tech Tupperware containers or Ziploc Baggies, and place them in the center of our kiddie pool/arena. After 3 minutes of settling time below a funnel we let them loose and play them either hourglass treefrog calls, tungara calls, or both together, and record whether they choose to call away from or close to the speaker for each treatment.

LIFE AS A TROPICAL RESEARCHER

Life in Gamboa is definitely not like life at Gettysburg. It’s not unusual to run into fellow researchers in the forest carrying machetes or weighed down with gigantic bags of scientific equipment. Every day there seem to be new creatures to see – from the family of capybara crossing the road in the early morning to the lizards that have taken up residence in our bathroom. The animal diversity here is incredible! We’ve learned a lot about field work trouble shooting, too. There was one night where we had to make our own camera tripods out of palm fronds and another where we walked all over town searching for a building with working wifi (ours goes out when it rains) so that we could post a certain blog before its deadline…. The most important thing we’ve learned? If you want to be a field researcher, be prepared for a lifetime of adventure.

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Sarah Smith investigating a tree near a field site

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A toucan that likes to visit Dr. Trillo’s yard

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We also have had opportunities to work with other researchers by helping them to capture bats using mist nets!

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