Jewels of the Osa
Words and images by Katherine González
If I thought for too long about the countless days that I waited for the rains to come when I worked in the Osa Peninsula, I would laugh at myself.
For many Costa Rican residents, six months of rain every year can be annoying; but for others, rain is crucial for survival and reproduction. When I accepted the position as a researcher at the Osa Peninsula’s remote field station, rain heralded the most amazing experience that a field biologist could witness: the explosive breeding event of the Gliding Tree Frogs.
From my first day at the research station, I was mesmerized by the intensity of the forest and its inhabitants. Spider monkeys were my first encounter; these large and agile primates are abundant in the Osa Peninsula, where they look for palm fruits and leaf sprouts around the buildings. After a few days, the monkeys began to warm up to me, and I was able to get close enough to take pictures of them. From my initial curiosity, I developed a total admiration for spider monkeys; this species is a true indicator of the state of conservation in our forests — when their populations thrive, we know we have a healthy ecosystem. Despite my favorite distraction, I knew that I would soon have to turn my attention to the focus of our research: the Gliding Tree Frogs.
The Gliding Tree Frogs stand out among Costa Rica’s 30+ tree frog species for one important reason: their explosive breeding events. At each occurrence, thousands of individuals congregate around ponds to mate throughout the night, but biologists are unsure about what triggers these events. Experts have assumed that large storms mark the beginning of these reproduction frenzies, but no research has provided evidence to support the claim.
After many days of waiting at the research station, we finally got a heavy rain. It was time. One of the field assistants gave us the hint that they had heard some frogs at the pond, so we pulled on our rubber boots and drove as fast as we could to the site. Fifteen minutes into our hike, we realized that we were on the wrong trail; so much excitement had gotten us confused, and we had to loop back and make our way into the forest again. But we were not delayed for long; after another five minutes, we began to hear a low guttural sound coming from the trees overhanging the pond. It felt like the frogs were having a private party, and their calls were the rhythmic music of the fiesta. Then, we saw it: hundreds of Gliding Tree Frogs in their breeding ritual, one of the rarest and most exciting mating events in the world!
Hundreds of frogs were hopping around looking for a partner — sometimes two frogs, sometimes three or four. Males would jump on a female’s back, and then competing males would join. When the females laid their eggs, multiple males would try to sneak behind their backs and fertilize them. It was a crazy frenzy of frogs that would seem strange at first sight, but for scientists like us, the explosive breeding mayhem was an opportunity to ask questions and test our hypothesis.
One of the most important questions about Gliding Tree Frogs is why they reproduce in the ways that they do. What is the evolutionary advantage of such a messy strategy when we know so much about the benefits of strategically selecting a mate and avoiding competition with rivals? Why do these frogs choose to congregate at the pond without any apparent strategy for selecting mates? The research team on the Osa Peninsula was not interested in these questions alone, but also in understanding the potential consequences of this reproductive behavior on the development of offspring.
Through multimodal mechanosensory mechanisms in the vestibular system, tree frog embryos are able to perceive abnormalities in their surroundings, such as the vibration of a snake as it moves across the forest floor. But snakes are not the only predators — wasps, turtles, birds, and even spider monkeys threaten the unhatched eggs, and invisible enemies like dehydration can also imperil the future generations.
Because their dangers are so numerous, tree frog embryos possess the incredible capacity to respond to potentially hazardous changes in their surroundings — in some cases, even hatching early to avoid predators. According to researchers at Boston University, tree frogs have evolved special enzymes in their noses and backs that dissolve the egg’s membrane, giving the young froglets the chance to escape with an early emergence when necessary. When the juveniles detect these potential threats, they are gambling on two risky choices: to leave the egg prematurely in their larval development, or to stay in the egg and face the threat.
In the case of the Gliding Tree Frogs, it was clear that the frogs had a vital relationship with rain. After the heavy storms, all we had to do was return to the pond to find hundreds of frogs laying their eggs in strings of pearls on the green leaves. However, after the parents left the pond, the tree frog embryos were left to face the many threats to their survival all alone.
After a few days of checking on the egg clutches, we discovered a pattern: with no rain, the eggs changed their coloration to a dark green and became solid to the touch; the embryos were dying. With these observations documented in our field notebooks, we headed back to the lab and asked ourselves if humidity levels were so low that dehydration was killing the embryos of the Gliding Tree Frogs. Ultimately, our experiments confirmed our predictions: a reduction to 92% humidity killed all of our embryos, and only when we increased the relative humidity to 96% were the frogs able to hatch early and escape their eggs. But even this was not good news; when the frogs do manage to escape early in their development, their smaller size makes them more vulnerable to aquatic predators. Even more concerning: global warming is likely to change the precipitation patterns in places like the Osa Peninsula, brining longer periods of drought after heavy rains. For the Gliding Tree Frogs and others that depend on reliable moisture, this is a death sentence.
As I watched the frogs in their congregations on my last day in Osa, the small pond felt full of life but very fragile; I wondered what it would look like whenever I was able to return again; next year, or the year after. Salty air blew in from the ocean beneath a sky that might bring rain and might not.