Though glass frogs are tiny in size, research done on these amphibians has led to enormous findings: an entirely new form of camouflage.
Previously, scientists had assumed that the glass frog’s see-through skin had something to do with camouflage, but no one had pursued any experimental studies to determine its exact ecological importance.
When James Barnett went to Ecuador for a lab field trip in 2019, he was interested in spotting some of these frogs — he had done previous research on them as part of his PhD at the University of Bristol beginning in 2014. Now based at McMaster University, Barnett realized that no scientist had actually done a study to determine why these amphibians have transparent skin and how it works.
The more he looked into these frogs, the less they seemed transparent. Instead, Barnett says he found they demonstrated a subtly different mechanism — one associated with translucency rather than transparency.
“The gradient of translucency from the frog’s legs towards the more opaque centre of the body means the relative contrast of the outline of the animal to the background is lessened,” Barnett explains. “That smooth gradient means there is not a highly salient contrast boundary which your eyes would be very sensitive to.”
This gradient translucency highlights a completely different form of camouflage called edge diffusion, he says.
Alongside making the edges of the frog less of a stark contrast to the leaf, this camouflage mechanism also involves adapting to different light conditions to match the colour of their surroundings. These green frogs change in brightness and luminance at the same time the leaf does, Barnett explains.
Understanding how and why these glass frogs use camouflage to survive should be studied through the eyes of a predator, like a bird or a snake, says David Green, a professor at McGill University and Barnett’s post-doctorate professor from 2015 to 2017.
“These frogs aren’t trying to hide from us. We’re not predators,” Green says. “We tend to interpret the world as if it was designed for people, but it isn’t. We have to take ourselves out of the equation and think through other eyes in order to figure out what’s going on.”
Seeing the world through a predator’s eyes is what Barnett aimed to do, alongside his supervisor, Professor Nick Scott-Samuel. A series of experiments were conducted, both in the wild and in a lab, to determine exactly how edge diffusion works.
In the first experiment, Barnett says he hoped to characterize how translucent these glass frogs are. Though there are up to 150 species of glass frogs, the studies focused on two they knew demonstrate edge diffusion: Espadarana prosoblepon and Teratohyla midas.
While in Ecuador, Barnett says he photographed the frogs on two different backgrounds: one on a bright white sheet of paper, and another on a dark green background, to analyze how much the frogs changed in colour and brightness between backgrounds.
He then took the photographs back home and made a computer-based detection experiment. In this experiment, he manipulated how conspicuous the frog-shaped targets were and then asked people to try to find them as quickly as possible.
“What we found is that frog’s natural patterns make them harder for people to find and gives a clue that there is something going on which is not quite transparency,” Barnett says. “So we have this idea that they do seem to change colour in a photograph, and it does affect people, but how does it affect real predators in the real world?”
In their final experiment, they headed to Ecuador equipped with gelatin-model frogs. Some frogs were transparent and some were opaque, he describes. The goal was to find out how quickly these frogs were eaten by predators when they are left on leaves.
Over a three-day study, twice as many opaque frogs than translucent ones were eaten.
Glass frogs are the only known animal to demonstrate this camouflage mechanism so far. Edge diffusion could be quite common among aquatic species where there is a range of levels of transparency in the oceans, but both Barnett and Green say more research needs to be done.
“There’s always new surprises. These frogs are endlessly fascinating and there’s no shortage of questions to ask,” Green says. “We don’t know all the answers, which is what makes it so exciting. Anytime you think you know the answers, you should tell yourself, ‘No I don’t.’”
With an understanding of this new kind of camouflage comes an enhanced understanding of predator-prey relationships, Barnett says, which is important if we want to protect and conserve animals.
“If habitats change, perhaps that could be something that affects these complex interactions between species in ways that we cannot predict unless we know more about it,” he says.
In glass frogs, the mechanism works well on natural forest backgrounds. However, Barnett says if these natural backgrounds change, perhaps it would undermine this effect. Conserving and protecting natural environments is essential in ensuring that these species can continue to thrive, he says.