The Adorable Polka-Dot Frog Has a Party Side
My first assignment as a new Field Museum employee was quite a whirlwind: I started my job as a conservation scientist on a Monday, and two days later, I was on a plane headed to Colombia for a month! The expedition to Bajo Caguán-Caquetá was my first rapid inventory experience and the 30th inventory for the Field.
Zooming through the air at 500 miles per hour from Chicago to Colombia, I felt like my thoughts raced just as fast as the plane was traveling. As a herpetologist—studying frogs, snakes, and lizards—I wondered what amphibians and reptiles I’d see. Did I pack everything I’d need? Or did I pack too many things and would soon regret lugging them through the forest? What do these campsites look like, and where do I do routine things like bathe?
It turns out this last, seemingly mundane, point ended up generating some of the most memorable experiences for me on the trip. So where did we bathe? In the rivers, of course! If you’re envisioning a nice, relaxing swim after a long day of work—not quite. Electric fish in the rivers can electrocute you, so it’s not the best idea to spend a lot of time swimming around. The solution? A bathing raft. These work quite nicely: you stand on a wooden raft and use buckets of water from the river to bathe yourself. This way, you avoid mucking around in the muddy banks of the rivers when your goal is to get clean.
Okay, so maybe now you’re envisioning a long day of work in the rainforest followed by a peaceful respite on the raft—perhaps you can lie back and catch some warm rays and almost feel like you’re vacationing on some remote tropical island. Nope, again. The habits of amphibians and reptiles vary. Some are active during the day, and many come out primarily at night. Our days were occupied by surveys and processing the specimens we caught, and our nights were spent conducting nocturnal surveys. Most nights, we returned to camp around 1:30am.
And that’s really the only time we had to jump into our bathing suits, head to the raft to bathe and do laundry—in the dark with our headlamps—enjoying the reflection of the eyes of our caiman friends in the water nearby. Our nights ended in many near-falls and at least two actual falls off the raft into the water (me, both times). Most of all, the nights ended in a lot of side-splitting laughter while we reflected on our unique bathing situation.
We recorded 55 amphibian species and 42 reptile species during the inventory, including three new frog species records for Colombia (a glass frog, Hyalinobatrachium cappellei, and two tree frogs, Boana alfaroi and Dendropsophus shiwiarum). On the very last night of the inventory, at the fourth campsite, we got a special surprise. We heard a cacophony of frog calls in the same stream where we had been bathing.
Diego instantly knew it was the polka-dot tree frog (Boana punctata, formerly named Hypsiboas punctatus). This frog was recently discovered as the first known amphibian to fluoresce by researchers at the University of Buenos Aires and colleagues. What is so cool about their discovery is that they didn’t stop at “wow, this frog glows!” they actually went back and studied the science behind how it fluoresces.
Within amphibians, so far only B. punctata and closely related B. atlantica are known to fluoresce, but we think there are more fluorescent species out there. Throughout our time in Colombia, we diligently checked amphibian species for fluorescence with a black light—and finally, those frog calls on the last night told us we were close.
We searched and searched the vegetation in the stream where we heard the calls, but as it turns out, it’s really hard to find small green frogs in a mass of green vegetation. So, we decided to turn off our regular headlamps and let the black light do the work for us and, voilà, there they were—little glowing spots dispersed within the vegetation. They were spectacular!
What makes the frogs “glow” under a black light? The compound responsible for fluorescence in these frogs is found in lymph and skin glands. Fluorophores (a type of pigment, in this case, hyloin) absorb light and re-emit it at a lower energy than at which it was absorbed. Because the re-emitted light has lower energy and therefore, longer wavelengths, it takes on a different color. Ultraviolet light is not visible to humans, but the lower energy fluorescence is.
We don’t exactly know why a frog would fluoresce. Many species are known to fluoresce including corals, fish, birds, scorpions, and even flowers, but it is much less common in land-dwelling vertebrates than marine vertebrates. The hypotheses as to why each of these species fluoresces range from communication, to a form of sunscreen, to a byproduct of some other process.
So, which is it for the frogs? The polka-dot tree frog is active at dusk and night and inactive during the day, when they commonly hide in vegetation in or near water. If you think about it, it’s perplexing why a nocturnal species would fluoresce under UV light—there’s not much UV light at night. However, research shows that even in dusk and nighttime conditions, fluorescence accounts for an important fraction of emerging light in the polka-dot tree frog—between 18 and 29 percent. (“Emerging light” includes both light that is fluoresced and also reflected light.) Basically, the fluorescence makes these frogs brighter even at night and is visible to other frogs’ eyes.
Additionally, these frogs leave fluorescent marks on surfaces they contact through skin secretions (including your hands, after holding them). Therefore, it's possible that fluorescence has an important function in communication among individuals. It is also possible that these UV-reflecting compounds act as a good sunblock during the day. While these frogs generally hide in vegetation during the day, they still may be partially exposed to strong sunlight, since bodies of water generally mean gaps in the forest with less canopy cover and more sun exposure.
We’ll need more research to reveal if and how fluorescence has an important function in amphibians, why it appears to have developed in few amphibian species, and exactly how many species have this charismatic glow. Next time you see an amphibian, get out your black light!