This may be the last post of the day as I’m a total wreck from insomnia (about two hours of sleep last night). But, as always, I do my best.
Today Lou Jost, an ecologist and evolutionist who works at a conservation-based field station in Ecuador, sent some photos of fluorescent frogs, something recently mentioned by another reader. Lou’s text is indented, and you can enlarge the photos by clicking on them.
Fluorescent frogs
A couple of days ago Ephraim Heller made an interesting Readers’ Wildlife Photos submission about his Brazilian Pantanal experiences, including what scientists have described as the first fluorescent frog in the world. Fluorescence occurs when light hits a molecule and excites it to a higher energy level. This is unstable and a photon (generally of a longer wavelength than the exciting wavelength) is emitted when the molecule drops back to its ground state. The discovery of a fluorescent frog was published in 2017 in the prestigious Proceedings of the National Academy of Sciences (PNAS), and got lots of press and media attention. The article and the press gave the impression that fluorescence is unusual among frogs. The article’s main photo (Fig 1a) shows the fluorescent frog glowing bright green when illuminated by a deep blue or near-ultraviolet lamp, and this is the photo that all the popular media republished:
Fig. 1A:
I didn’t know any of this but I do a lot of fluorescence photography, so when I had the opportunity in 2021 to photograph a rare glass frog (Nymphargus anomalus) from one of our reserves, I wanted to see its fluorescence. Almost everything fluoresces, and I was not surprised to see that this frog had a beautiful aqua fluorescence, especially its bones. The normally green mosses and liverworts beneath it fluoresced red, green, yellow, and blue. Because this was a randomly chosen frog species out of the hundreds that exist here, by the Copernican Principle I assumed that this was not unusual.
“Normal” photos (by Lou)
Same frog fluorescing (this and one at bottom also by Lou):
So when I read Ephraim’s RWP and his helpful response to my comments there, I was surprised to learn of all the hype about the first fluorescent frog, Boana punctata, formerly named Hypsiboas punctatus. It seems the authors did not bother to check for themselves whether that frog was really unusual in its fluorescence. Today, checking the internet, I see that 100% of the 150+ South American frog species tested in 2024 by researcher Courtney Witcher were fluorescent!!! The Copernican Principle works (usually)!
Well, maybe there was still something special about the original frog, whose photos show a bright green fluorescence instead of the aqua fluorescence I had observed. Unfortunately the authors of the original PNAS article took that green photo through a filter that only lets green or longer wavelengths pass through it. They filtered out the blue fluorescence. In reality their frog fluoresces with exactly the same aqua blue color as my frog, as can be seen in their Fig. 1C, center photo. They had mistakenly labeled that photo as having been taken through a filter that only passes wavelengths greater than 516nm (green, yellow, orange, red). If their figure legend were correct, there would be no blue in the photo, but the frog in that photo is completely blue.
Fig 1C (caption from paper):
This conclusion is confirmed by a photo of the same species taken under UV light in 2024.
Anyway there may still be an interesting story to reveal about the biological significance of frog fluorescence. But caveat emptor…
Nice post! The interesting question is, does this fluorescence give the frogs some sort of survival advantage, of is it just a spandrel? These seem to be rain forest frogs. Do frogs in very different environments also show fluorescence?
Yes, that is the big question. Most fluorescence is definitely a spandrel, but there are increasing numbers of reports of actual biological roles for fluorescence itself. Many of these claims are suspicious, but some are well-supported. The jury is still out for frog fluorescence.
Fluorescent frogs are less likely to have their natural camouflage compromised by creatures that lack a fluorescence lamp, which is 99.99999…% of their total interactions in evolutionary history – hence a clear evolutionary advantage.
Oooo beautiful .. and… exciting (😁)
I’m always intrigued how the wavelength profile of the vision of the all the animals involved factors in that of the fluorescence. Clearly the backgrounds shown are not fluorescent – but could be! (E.g. I think some fungi fluoresce).
Actually the backgrounds are fluorescent. Chlorophyll in particular has a very bright red fluorescence, which you can even see very well in daylight if you have a strong UV flashlight.
But frogs do seem to have night vision tuned to blue and green.
You know, as I wrote it, and the edit window was closing, I’m looking at the leaves and the gears in my head are going “squeak squeak squeak” 😆
Fascinating stuff! When I was a grad student learning, and then teaching, the biology of arthropods, I learned that insects could see in the ultraviolet and that some plant flowers reflect ultraviolet in such a way as to appear as a bulls eye to insect pollinators. The plants had figured out a way to attract pollinators and the pollinators had figured out a way to oblige the plants, all to their mutual benefit. (I don’t know if that has held up to further scrutiny over the years.)
Of course, I am led to wonder whether phosphorescence in these beautiful frogs might also have some function. If conspecifics can see the phosphorescence, maybe frogs choose mates based on the robustness of the (visible) skeleton. (I’m mostly kidding, but natural selection has a way of making the strange plausible. Whatever works.)
The theory among these frog authors is that frog night vision is tuned to blue and green light, and the green fluorescence therefore makes them more conspicuous to other frogs at night. Maybe, but the evidence mentioned in the PNAS paper seems very suspicious to me.
The phosphorescence could just be an artifact of the physiology of their skin, but once that trait is present it can be used for something—eventually.
Very very cool!! So fluorescence in frogs is not a great novelty, as you so beautifully show, so why did the press get so hyped about this one example? That seems to be about the pursuit of clicks and viewer engagement rather than about the pursuit of information.
You can actually see the fluorescent blue bones in the glass frog!
Yes, the investigators, their university publicity staff, and the media all are in it for clicks and funds. I do think that the authors, though sloppy, did not deliberately mislead. But I fault them for not bothering to check the extent of fluorescence in frogs before making such a big deal about this one. It would not have taken much work to learn that fluorescence is widespread.
Having a fluorescent skeleton would be a useful feature at Halloween for these frogs.
Sorry about the continuing insomnia. Hopefully you were awake and energetic for your time with Prof Pinker last night.
Lou, thanks for following up on this fascinating subject! It’s good to have readers like you educate me.
Thanks a lot for your stimulating RWP submission!
Wow! Fluorescent frogs! Had no idea these existed.
Salamanders and newts. I just checked online, and of course they to can do it.
The challenge with photographs is that through adjustment of exposure any fluorescence, no matter how weak, can be made to appear bright. Similarly, when we illuminate something with a bright excitation light in the dark our eye/brain system, which is very sensitive, can also make things appear bright. So I particularly home in on the images of the frog in settings with other fluorescing features, especially the chlorophyll fluorescence from the plants. That chlorophyll fluorescence is fairly weak, typically on the order of just a few percent of the light that is absorbed. The blue-fluorescing frog does not appear very bright next to the plants, making me think that the fluorescence is fairly weak (in absolute terms).
If the argument is that it makes the frogs more conspicuous to conspecifics at night, I would ask what the excitation source is? The frogs are fluorescent, not bioluminescent, so they are not producing their own light. Moonlight has been suggested as the excitation, but moonlight is reflected sunlight and is fairly broadband. Also not strong. And if moonlight is present to excite fluorescence it is also present to be reflected, and in order for fluorescence to have a visual function there must be enough of it to make a significant difference relative to the light that is reflected from the same surface.
Yes, those are all excellent observations, and I agree. The PNAS authors claim that the fluorescence light makes up 11-29% of the total light leaving the frogh. This seems very suspicious to me. And you should be able to observe that light even with an ordinary photo taken in sunlight. The authors could easily have tried to make direct observations, but instead they calculated the percentages from other parameters. I wish the authors were here to discuss these issues.
I am familiar with that group to a certain degree, and of the various groups that have been publishing on fluorescence and visual function I am inclined to respect their work. I have not analyzed it in excruciating detail but their approach is solid. You need to do their kind of lab measurement approach to find the excitation and emission spectra and make a good estimate of the fluorescence yield. This enables you to model reflectance/fluorescence under arbitrary illumination conditions. But you are right – the final proof-is-in-the-pudding measurement would be an ACTUAL spectrum measured from a frog, in the field, under the stated illumination conditions.
The last field biology trek that I made was collecting jellyfish, Periphylla periphylla from a fjord N of Bergen. The reason for the quest was for their light-emitting proteins. They come to the surface near/at the full moon, and the thought was that this was to facilitate mating – they come from three-dimensional space to two-dimensional, and then flash when they collide with something, like another jellyfish. There must be some similar thought that the frog’s flourescence facilitates finding a mate.
enjoyable, better than HiSchool biology! Our poor frogs