How 40 condoms proved essential to an experiment deep in the Amazon rainforest

Art by Lívia Serri Francoio

Marina Méga

The first time I flew over the Amazon rainforest, it felt as though it was saying, “You are crossing into another world.”

As a marine biologist, I spent ten days fully immersed in that biome on a field course with fifteen other young researchers as part of the Serrapilheira Institute’s Training Program in Quantitative Ecology. During that time, the forest’s message took shape in my chosen subject of study: the enigmatic clay towers scattered across the forest floor. I had never seen anything like them before. 

That’s why they took me by surprise the first time I came across them. They were neatly sculpted, cylindrical structures of various sizes. The instructors explained that cicada nymphs build these chimneys in the year before their metamorphosis, molding them from clay and excrement. When the time is right, the nymphs breach the tower, climb to its summit, and undergo metamorphosis there. 

There were four of us—all women—trying to move the project forward, but the limited literature on the topic offered few clues: Why do cicadas build these towers? What are they for? Why do they come in so many sizes? Our instincts told us this wasn’t random. As we stepped back from the towers, we couldn’t help but notice something: ants.

They were everywhere, sometimes alone and sometimes in massive colonies. Any one of them could potentially prey on the nymphs. This led us to our first hypothesis: Could the towers serve as protection during metamorphosis? We returned to inspect them more closely.

What if they also helped regulate conditions inside the tower? We noticed that the tops of many of the towers would open after heavy rains. Maybe the water saturated the clay, clogging its pores and making it harder for the cicadas to breathe. Perhaps the openings facilitated gas exchange. Based on this observation, we developed a second hypothesis: the towers perform a physiological regulatory function.

It was time to put our hypotheses to the test. Collecting data was no easy task. The trails were long, the forest was dense, and the heat was suffocating. We began by measuring the height of each tower. To evaluate the anti-predation hypothesis, we experimented with different types of ant bait until we found the perfect option: small “pizzas” made of flour, water, and sardines. We placed the bait on top of the towers and on the forest floor. The results supported our hypothesis: there were eight times fewer ants on the towers than on the ground.

To test whether the towers played a regulatory role, we racked our brains over how to seal them. Then, one of my colleagues joked, “Look at their shape—what if we used condoms?” The idea made sense, so we tried this unconventional approach. We realized that if a condom inflated slightly around a tower, we would have a safe confirmation.

Now, 40 condoms topped off our field kit, so we got to work slipping them over the towers and observed that indeed they help regulate gases. We also learned that—jokes aside— size matters: towers of different sizes reacted differently. The larger towers inflate the condoms more, suggesting that they are better suited to handling stress.

This experiment led to a major insight and highlights how creativity, freedom, and careful observation go hand in hand in science. As a bonus, we also found the largest cicada tower ever documented, an impressive 47 centimeters from base to tip. 

Stepping out of my marine habitat and delving into the forest helped me grow as a scientist. I learned not only about cicadas but also about the importance of listening, collaboration, and thinking outside the box. I realized that science thrives on the unexpected and that significant discoveries can emerge from challenging paths or seemingly crazy ideas.

*

This text was written by Marina Méga, a doctoral student in ecology at UFRJ, and reviewed by Pedro Pequeno (UFRR) and Rodrigo Fadini (UFOPA).