Interesting Creatures
Exploring Frogs With Transparent Skin
They’re called glass frogs and you can see their insides right through their skin!


I’m a big fan of frogs and toads. I have toad and frog tchotchkes scattered throughout my home. When I was just a little tyke, we lived in a suburb that had a pond at the end of our street. A little stream flowed out of that pond, and every spring, we kids on the block would see tadpoles wiggling around in the shallow parts of the stream, and we could watch them mature, grow legs, and eventually become frogs.
It was inevitable that at some point, I was probably about 9 or 10 years old, I brought a bunch of tadpoles home and, with the help of my Mom and Dad, set up some kind of system where I could raise them in our backyard to see them change and become adult frogs.
That was super cool. And, of course, all summer long, we’d hear the frogs in the pond croaking and calling to each other. It was also super cool to watch them catch a bug! Ok, I’m about to really date myself here (warning, frog dissection description below).
In my college introductory biology course, to study spinal cords and transmission of nerve signals, we would obtain and paralyze live frogs by pithing them (inserting a dissecting needle into their neck and severing the connection of their spinal cord to their brain. Since they were still alive, we could connect the frogs to a power source and show that by electrical stimulation, we could move the legs.
Apparently, it’s still being done. Here’s a video showing how you have the stomach for it. Nowadays, I could still do it if I had to, but back then, as an avid biology student, I was a champion pitcher and volunteered to do it for anyone else in the class who was unable to for various squeamish reasons.
Ok, enough of the gross stuff! Let’s get to the fun stuff. These days, we often run into frogs and toads on the trail when we’re hiking, and I love to take pictures of them. Here’s a little guy I came across not too long ago.
In my never-ending search for creatures to write about I had come across poison dart frogs and actually included a piece about them in the first ebook I put together on my Biology4Everyone website to get people to sign up for my blog, but I had never heard of glass frogs. And since they’re frogs I knew nothing about, I just had to know more about them!
And my guess is you’ve probably not heard much about them either because you won’t find any in northern climes, where most of the Medium readership lives. So, let’s hop along together and see what we can learn about these exotic croakers.
Basic info
Ok, they’re frogs. We’ve all seen frogs, we know what frogs look like, and some of us know what their legs taste like!
There are three living orders of amphibians: Anura (frogs), Urodela (or Caudata, salamanders), and Gymnophiona (or Apoda, caecilians).
Interesting note: The word amphibian is derived from the Ancient Greek term ἀμφίβιος (amphíbios), which means ‘both kinds of life’, ἀμφί meaning ‘of both kinds’ and βιος meaning ‘life’.
Also, remember that frogs are vertebrates which means they have an internal bony skeletal system around and along a spinal cord. The tiniest frog known is Paedophryne amauensis and is also the world’s smallest known vertebrate. Here’s a picture of a fully grown one sitting on a US dime.

It looks so big because the picture is huge so if you have any spare change around, find a dime and get a real sense of this creature’s size. Frogs are found all over the world, from the tropics to subarctic regions, but the greatest number of species are found in the tropical rainforests. So, let’s keep hopping along and see what these glass frogs are all about.
Their home
Glass frogs are in the Centrolenidae family, which currently comprises 12 genera and 156 recorded species of glass frogs, and they all reside mainly in the Northern Andes and other parts of Central America.
Of these 156 species, almost 40% of them are listed as threatened, 10 as critically endangered, 28 as endangered, and 21 as vulnerable to extinction.
The first recorded species, Centrolene geckoideum, found in Ecuador, was described in the Western science literature in 1872 by Marcos Jiménez de la Espada a Spanish zoologist, herpetologist, explorer and writer.
[Alert! A “decolonizing” note. It would be naive, prejudiced, and demeaning to think that tropical rainforest Indigenous Peoples of the region had no knowledge of C. geckoideum and many other species of glass frogs before 1872. I don’t know how these creatures fit into their worldview and what uses and qualities they may have attributed to them, but to assume Espada was the first human to see a glass frog would be a bit arrogant, IMHO.]
Physical characteristics
This group of frogs is generally fairly small and ranges in size from 3–7.5 centimeters ( 1.2–3 inches). Most glass frogs are a kind of “lime-green” colour combined with patterning on their upper body surfaces. Here’s a figure from the Sulbarán paper cited below, showing the non-translucent dorsal patterns of 6 different glass frogs.

And here’s another showing glass frogs sleeping on the underside of a leaf. You can really see the lime green color in this one.

They have expanded tips on their “toes,” which help them climb into the trees and shrubs along the waterways they frequently inhabit.
If you look at other frogs’ pictures, you’ll see that their eyes are facing out towards the side, whereas all glass frog eyes face forward. Ok, how about their transparency; the reason they’re called glass frogs!
When a creature has a unique feature, the first question any scientist or curious person asks is, Why? What purpose does it serve?
The first thought that comes to mind for this transparency is camouflaging to avoid predation and increase survival.
To quote a glass frog researcher looking at how this feature might play out for glass frogs, Dr James Barnett writes,
Camouflage patterns prevent detection and/or recognition by matching the background, disrupting edges, or mimicking particular background features. In variable habitats, however, a single pattern cannot match all available sites all of the time, and efficacy may therefore be reduced. Active color change provides an alternative where coloration can be altered to match local conditions, but again efficacy may be limited by the speed of change and range of patterns available. Transparency, on the other hand, creates high-fidelity camouflage that changes instantaneously to match any substrate but is potentially compromised in terrestrial environments where image distortion may be more obvious than in water. Glass frogs are one example of terrestrial transparency and are well known for their transparent ventral skin through which their bones, intestines, and beating hearts can be seen. However, sparse dorsal pigmentation means that these frogs are better described as translucent. [bolding is mine]
Ok, if their bones, intestines, and beating hearts can be seen through this transparent skin, does it still contribute to helping them hide from predators?
Barnett and colleagues set out to test this and published their results in this article. I’m not going to dive into the details here as that’s a lot of science you don’t want or need to read.
So here’s the bottom line results from their experiments;
- they were always green but seemed to brighten and darken depending on the substrate against which they were viewed
- they were definitely more difficult for humans to detect
- and they were less likely to be attacked by wild predators than opaque frogs.
The translucent appearance of many glass frogs acts to provide camouflage in a manner conceptually distinct from both true transparency and active color change. Rather than allowing the background to be directly seen, diffuse light transmission through the frog adjusts a generalist camouflage pattern to more closely match the immediate background. This change in perceived luminance then transforms the frogs’ salient high-intensity outline into a less conspicuous graduated boundary. Thus, the imperfect glass of the glass frog provides effective camouflage, disguising the frogs’ outline and blending the frog and the leaf more smoothly together.
I do want to describe one of their “fun” experiments, though.
In one of their tests, they designed and constructed 180 translucent and 180 opaque “frogs” out of gelatin, put them in vegetation in leaves in Ecuador, and assessed them over three days to see which ones were eaten more often.
The opaque gelatin frogs were the clear losers, 53 of them being eaten versus 24 of the translucent ones!
The imperfect glass of the glass frog provides effective camouflage, disguising the frogs’ outline and blending the frog and the leaf more smoothly together.
While we’re on this topic of transparency/translucency, another question arises. How do they achieve this characteristic? Again, this is not so simple.
Remember, the glass frogs are vertebrates and have a circulatory blood system. That means they, just like us, have red blood cells that use hemoglobin to transport oxygen and nutrients to their various organs and tissues.
And that’s a problem because the cells containing hemoglobin strongly absorb blue and green light. Since this would make them opaque rather than translucent, how do they get around that?
Well… maybe it is simple.
When Taboada and colleagues investigated this question, they came up with an answer.
They hide these cells. Essentially, what they do is sequester the red blood cells away in their liver while they sleep. When they do that, their total transparency increases by 2–3 times what is when they are awake.
They found this out using photoacoustic microscopy, a technique that combines sound and microscopy to get morphological, functional, and molecular imaging of living subjects.
In this instance, they were specifically tracking where the red blood cells were.
This figure from their paper highlights these differences!

You can see exactly where the red blood cells are during these treatments. There are striking differences in their location when the frogs are sleeping versus when they’ve been anesthetized to mimic sleep or are actually awake and exercising.
And that clearly (pun intended) translates into the extent of transparency
They hide the red blood cells away in their liver while they sleep
Is that too cool or what?!
For your additional viewing pleasure, here’s a poster that was presented at a scientific conference that has all kinds of interesting information and pictures about Ecuadoran glass frogs and how to tell them apart by their physical differences.
Alright. That’s enough for physical traits. Let’s move on to mating.
Reproduction
Here’s a typical frog life cycle diagram: Eggs → tadpole → adult.
This all looks pretty straightforward, but there are several interesting aspects of each of the phases of this cycle that are unique to glass frogs.
Like all other frogs, glass frogs are sexually dimorphic, which means there are two sexes, male and female.
Their sexual mating “play” is initiated by the male. Perched either on the top or underside of a leaf that is hanging over the edge of a lake or stream, the male begins his advertisement or mating call.
In a study of glass frog vocalizations, Sulbarán looked at “…the association between body size, calling site, parental care and call properties (call duration, number of notes, peak frequency, frequency bandwidth, and call structure) of the advertisement calls of glass frogs.”
That’s how serious scientists investigate and unpack a frog call!
They learned that,
Peak frequency of calls is significantly associated with body size, whereas call structure is significantly associated with calling site and paternal care. Thus, the evolution of body size, calling site and paternal care could constrain call evolution.
If you want the nitty-gritty details, feel free to read the article. It’s unrestricted, open access. And in a much earlier study, Jacobsen compared two species of glass frogs and found that how long males called on a given evening did not affect their mating success. What did matter was whether they stayed at or returned and repeated their calls over a few days.
So remember, if at first you don’t succeed, try, try again! Ok, that’s probably more than you wanted to know about glass frog calls but it is a good reminder that to really do science, you have to carefully pick apart what you want to study if you want to actually learn or prove something.
If he is successful and attracts a female, then fertilization can occur. And that is not as simple as you might think! Frogs mate using a position herpetologists call amplexus. Here’s a graphic posted a few years ago on what is now known as X, showing 7 of the 10 different possible positions of amplexus.
The most commonly used amplexus positions are the Inguinal and Axillary. Basically, what happens is the male gets on top of the female and squeezes her with his legs. That stimulates her to release her eggs, and as they are shed from her body, the male fertilizes them with his sperm.
And not unlike other creatures, there is often strong competition among males; it is not uncommon for a “free” male to attack one already amplexed with a female. These attacks actually involve combat!
If the attacker is successful, he then assumes amplexus and gets to fertilize the next batch of eggs released.
Alright, our frogs have called to each other, “hugged”, had frog sex and there are now a bunch of fertilized eggs, usually a bit more than 30. Now what?
Well, almost all the links I came across say that the male glass frogs guard the eggs until they hatch, and the tadpoles begin their lives in the water below. But, is it always the male that does this brooding task?
As part of their Ph.D. research, Jesse Delia, with research partner Laura Bravo Valencia, a graduate student at the Universidad de los Andes in Bogotá, Colombia, had to jog up and down streams every night for 18 months to observe glass frog behaviour during embryonic development.
This was not a simple task! Here’s Delia describing it.
“In Colombia, we would take buses…into the mountains and try to find somebody who would put us up, somewhere close to a forest, a couple hours’ hike into the stream. Streams in the Andes are really steep, with impassable waterfalls every so often, and in many sites they are cascades of freezing cold water. So we were soaking wet all night.”
What they found completely overturned the idea that it was always the males that guarded the eggs. In fact, in most of the species they looked at, it was the mothers who sat on the eggs. The time from egg deposition until tadpoles were mature enough to be on their own could take almost 20 days.
But dedicated Moms, they weren’t! In most cases, they only stayed one night! Since they were looking at embryonic development and what affected survivability, they were curious to know if Mom watching over the eggs had a significant effect. So, in one set of experiments, they removed the mothers immediately after the eggs were fertilized.
Not surprisingly, what they found was that the eggs guarded by Moms survived significantly better than those that weren’t.
And that’s because “Glassfrog eggs, laid on leaves hanging over streams in tropical rainforests, are tasty snacks for snakes, insects, and other predators until they hatch and drop into the streams to begin life as tadpoles.”
So what did Mom do to protect the eggs so profoundly?
Again, it was pretty simple; they soaked up water from the damp spots on the leaves and added it to the eggs. The eggs absorbed this water, swelled up, and became almost four times thicker. Essentially, now they had a protective mass of gooey jelly.
One of the primary glass frog egg predators is a cricket or katydid. What they observed is that when a katydid bit into the swelled eggs, all it got was a glob of jelly, so it would give up and look for other prey.
So even though it was only one night, it had a high amount of survival payback. Alright, back to the Dads. What role DO they play in all this rearing?
What Delia and Valencia observed was that in almost a third of species, glass frog fathers stay on guard for much longer periods.
Dr. Karen Warkentin, under whose mentorship Delia and Valencia did the research summarized their observations.
This was a “tour de force of extreme fieldwork.
These are relatively well-studied, charismatic frogs, yet we were fundamentally wrong about the reproductive behavior of most glassfrog species. There is still a lot to be learned from basic fieldwork. And that primary information has the potential to change how we think about larger processes, like sex-role evolution.”
“In glassfrogs, maternal care helps embryos survive, but they seem to do the bare minimum. It seems that fathers not only took over the job, when mothers were already doing it, but they also greatly elaborated the amount of care. Even after eggs have started hatching, fathers keep caring.”
From this point on, the eggs mature, and tadpoles emerge. Then they drop off into the water and, like all immature life forms, do their best to avoid being eaten so they can continue developing and becoming adults. And start the whole shebang all over again.
So what about overall risks? How likely are we to see these wonderful creatures survive into the future?
Conservations
As is the case with so many other creatures on this planet, the primary concern in this age of the Anthropocene is habitat destruction by mining, forestry, and development.
Also included are water contamination, pesticides, climate change, introduced species, and emerging diseases. All these factors and more, need to be considered when developing conservation strategies.
In a paper published several years ago, Guayasamin and colleagues predict that
“The most endangered species are: Centrolene buckleyi, C. charapita, C. geckoidea, C. medemi, C. pipilata, Cochranella mache, Nymphargus balionotus, N. manduriacu, N. megacheirus, and N. sucre and likely to go extinct because of any of the aforementioned variables.”
As I mentioned above, the Northern Andes have the most glassfrog species, so efforts directed towards the conservation of Andean forests will greatly benefit them.
For the glass frogs in Ecuador, Guayasamin proposed the following set of recommendations;
(i) The implementation of a biological corridor between the National Parks of Cayambe-Coca and Sumaco,
(ii) creation of a new protected area in the lower montane evergreen forest of the western Andes in Pichincha Province, and
(iii) protection of the endemic Chocoan forest (threatened mainly by wood extraction and palm plantations).
They also note that another one of the dangers that these frogs face is
“the disease caused by the fungus Batrachochytrium dendrobatidis, also known as chytridiomycosis. Chytridiomycosis has been implicated in the extinction of numerous Andean species, mainly harlequin toads.
Ecological modeling predicts that in Ecuador, the highest suitability for chytridiomycosis is in the Andes, at elevations above 2000 m. The impact of the disease on Ecuadorian glassfrogs has not been assessed, but preliminary studies show that Batrachochytrium dendrobatidis has a relatively high prevalence in several species of Andean glassfrogs.”
Yet another threat to glass frogs is the introduction of rainbow trout into waterways. They find tadpoles to be a tasty treat. Since it is not an indigenous species, the recommendation is that like that of many other introduced non-native species around the globe, if possible, efforts should be made to eradicate it.
And last but not least is the impact of climate change. As environmental conditions alter, the frogs will surely be affected. By how much is, at this point, unknown. So we’ll need to keep a close watch on these effects.
Well, I think that’s enough for now!
I’m not writing a book here, although I think if I wanted to, I could easily do so. I’m hoping you enjoyed this foray into yet another one of the many incredible and fascinating creatures we have on our little blue dot.
I sure did and learned all kinds of frog stuff I knew nothing about!
So dear reader, until next time,
Rich
P.S. If you liked this article, you might also enjoy these
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And here’s a fun video about photoacoustic bioimaging for your scientific enjoyment.