This is the first discovery of such a phenomenon for bats located in North America. The tricky thing now, the researchers said, is figuring out why. (Credit: Andrea Piazza)
The discovery is both spooky and thought-provoking, but researchers are left with more questions than answers
In A Nutshell
- Century-old bats glow like new: Museum specimens collected in 1922 emit the exact same green glow under UV light as bats caught last decade—proof the trait doesn’t fade with time.
- Six species, one identical shade: Big brown bats, red bats, and four other North American species all glow the same green (520-552 nanometers), suggesting they inherited this trait from a common ancestor.
- Bats might see their own glow: The green wavelength matches what bat eyes are designed to detect (536-560 nanometers), but scientists don’t know yet if it helps with communication, hunting, or avoiding predators.
- The mystery deepens: Dark caves and tree hollows may not have enough UV light to trigger the glow in the wild, leaving researchers puzzled about whether this century-old secret serves any real purpose.
A bat collected in 1922 and one caught last decade have something unexpected in common: both glow green when exposed to ultraviolet light, producing the same range of wavelengths. Scientists at the University of Georgia discovered this peculiar trait while examining museum specimens, some more than a century old. The color of their fluorescent properties hasn’t changed.
Researchers Briana Roberson, Steven Castleberry and their colleagues tested 60 preserved bat specimens ranging from 22 to 103 years since collection. They tested whether specimen age would shift the wavelength of the glow. It didn’t. A bat that’s been sitting in a museum drawer since the Roaring Twenties produces the same green emission, peaking between 520 and 552 nanometers, as bats collected in recent years.
What Makes Museum Bats Keep Glowing
This discovery goes beyond just validating museum collections for fluorescence research. It shows that whatever causes bats to glow under UV light is remarkably stable. The trait survives decades of preservation and storage. Something this consistent points to a stable biological trait with an unknown mechanism. Whether the glow comes from true fluorescence or light scattering remains unclear and requires further testing with different wavelengths of excitation light.
All six North American bat species examined displayed bright green photoluminescence on their wings, uropatagium (the membrane stretching between tail and hind legs), and limbs. The species included big brown bats, eastern red bats, Seminole bats, southeastern myotis, gray bats, and Brazilian free-tailed bats. Every specimen glowed green.
Photoluminescence describes what happens when molecules absorb photons at one wavelength and emit light at a longer wavelength. Think of it like a biological light converter. While this phenomenon has been well-documented in plants, invertebrates, and marine organisms for generations, mammals have only recently attracted attention for displaying the trait.
“It’s cool, but we don’t know why it happens,” admits Castleberry. “What is the evolutionary or adaptive function? Does it actually serve a function for the bats?”
How Scientists Measured the Glow
Researchers used a spectroradiometer to quantify the exact characteristics of emission from each bat. This instrument precisely measures light wavelengths. Before taking readings, the team had to account for all other light sources. They recorded scans with no light present and reference scans of the UV light itself, then subtracted these from specimen scans to isolate only the light produced by the bats.
Photography documented the findings separately. Researchers photographed specimens under UV light alone, then through a yellow UV-filtering lens to reduce visual noise from UV and blue wavelengths. Finally, they used a longpass filter that blocked wavelengths under 470 nanometers. This filtering technique captured the pure emission color without interference from the excitation light.
Museum specimens from the Georgia Museum of Natural History provided the raw material for the research. Bats originally came from across Georgia, South Carolina, Tennessee, Illinois, and California. Using preserved specimens offered advantages beyond their longevity. Researchers could eliminate potential confounding factors like bacteria or fungi that might naturally fluoresce on live animals.
White-nose syndrome lesions are known to be photoluminescent under ultraviolet light, as are certain bacteria common in bat skin microbiomes. Both are unlikely in well-preserved museum specimens.
Why All Six Species Glow the Same Green
The consistency of wavelengths across all six species examined points to a shared physiological mechanism. In scientific terms, the trait appears to be homologous among these species. This means it evolved once in a common ancestor rather than independently in different lineages.
Previous research has identified porphyrins as responsible for red UV-fluorescence in various mammals. Tryptophan metabolites produce other pelage fluorescence colors. The green glow in these bat species differs from both patterns.
Researchers found no differences between male and female bats in their emission characteristics. This rules out sexual selection as a likely explanation for these species. If photoluminescence helped bats attract mates, males and females would likely show different wavelengths or intensities.
The wavelengths also don’t match what would be expected if the glow provided camouflage among foliage. That would require emission peaks around 680 nanometers to align with chlorophyll fluorescence.
Social behavior varies considerably among the species examined. Myotis species, big brown bats, and Brazilian free-tailed bats form social aggregations, sometimes numbering in the thousands. Eastern red bats and Seminole bats are foliage roosters with more solitary habits. Despite these differences in lifestyle and roosting preferences, all species showed the same emission characteristics.
Can Bats Actually See Their Own Fluorescence?
The green wavelengths detected in the study fall within ranges that bat eyes can detect. Bats possess medium to long-wave sensitive vision. Other research has found that bat eye proteins (called opsins) respond most strongly to wavelengths between 536 and 560 nanometers, which overlaps with the green glow observed in this study. However, this study did not test whether the bats examined can actually perceive or respond to their own photoluminescence.
Just because bats might see these wavelengths doesn’t necessarily mean the glow serves a purpose. Several theories exist for why mammals might photoluminesce: predator evasion, communication between individuals, or improved vision in low-light conditions. Proving ecological relevance requires more than demonstrating that an animal glows in a laboratory, though.
The amount of UV light present in natural settings may not be sufficient to produce noticeable photoluminescence, especially in dark caves or hollow trees where many of these species roost. The glow was clearly visible on the undersides of wings and limbs. These are areas more visible during flight than while roosting.
What Other Bats Glow
Published in Ecology and Evolution, the study adds to growing evidence that photoluminescence in mammals is widespread. Mexican free-tailed bats sport photoluminescent bristles on their feet. Eastern tube-nosed fruit bats in Australia show glowing wings similar to what the research team found. Greater Antillean long-tongued bats display piebald spots that become more visible under UV light.
Understanding whether the trait serves a purpose requires specific types of studies. Scientists need to compare photoluminescence in live individuals with museum specimens. They need experiments testing bat responses to photoluminescent signals. And they need measurements of UV light availability in natural bat habitats during active hours.
“Bats have very unique social ecology and sensory systems, and the characteristics we found in these species differs from many other observations in nocturnal mammals,” says Roberson, lead author of the study. “It’s possible for glowing functions to be more diverse than we previously thought.”
What This Means for Future Research
The durability of photoluminescent properties in museum specimens opens new avenues for research. Scientists can now confidently examine historical collections to document the presence and characteristics of photoluminescence across species and time periods. No concerns that preservation has altered the trait. Looking back through collections could speed up the mapping of photoluminescence across mammalian lineages and help identify patterns related to evolution or ecology.
Researchers cannot yet answer the biggest questions. Do bats respond behaviorally to photoluminescent signals? How much UV light exists in natural habitats? Is the emission true fluorescence or light scattering? For now, the discovery remains a biological curiosity, but one with staying power.
“The data suggests that all these species of bats got it from a common ancestor. They didn’t come about this independently,” Castleberry says. “It may be an artifact now, since maybe glowing served a function somewhere in the evolutionary past, and it doesn’t anymore.”
The trait has persisted unchanged in museum drawers for more than a hundred years, waiting for someone to shine the right kind of light and notice.
Paper Summary
Methodology
Researchers examined 60 adult museum specimens representing six North American bat species: big brown bats, eastern red bats, Seminole bats, southeastern myotis, gray bats, and Brazilian free-tailed bats. All specimens came from the Georgia Museum of Natural History mammal collections. Specimens were originally collected between 1922 and 2003 from various locations including Georgia, South Carolina, Tennessee, Illinois, and California.
The team first observed specimens visually under 410-nanometer ultraviolet light while using a yellow UV-filtering lens to reduce interference from UV and blue wavelengths. They photographed specimens using a Nikon D5600 camera under different lighting conditions to document the photoluminescence.
To quantify emission characteristics, researchers used an ILT950 spectroradiometer with wavelength accuracy of ±0.5 nanometers and an integration time of 4,000 milliseconds. The probe was positioned 10 millimeters over the ventral surface of the uropatagium. The team recorded scans with no light present and reference scans of the UV light alone, then subtracted these values from specimen scans to isolate only the light produced by the specimens themselves. They identified the wavelength value at the emission peak for each specimen and used statistical tests to examine differences between species and sexes, as well as relationships with specimen age.
Results
Bright green photoluminescence was observed on the uropatagium, wings, and hind limbs of all 60 specimens examined. Spectral analysis revealed emission peaks ranging from 520 to 552 nanometers, consistent with the observed green color. No differences in peak wavelength were detected among the six species examined. Similarly, no differences were found between male and female specimens. Specimen age since collection, which ranged from 22 to 103 years, showed no relationship with wavelength values, supporting the reliability of using museum specimens for photoluminescence detection.
Limitations
The study examined only museum specimens rather than live bats, which may not perfectly represent photoluminescence characteristics in living animals. Researchers could not evaluate whether sufficient UV light exists in natural bat habitats to produce observable photoluminescence, particularly in dark roosting environments like caves or tree hollows. The study measured emission wavelengths but did not test whether bats actually respond behaviorally to photoluminescent signals. All specimens examined were adults at the time of collection, so the research provides no information about photoluminescence in juvenile bats. The team could not definitively determine whether the emission results from true fluorescence or light scattering, which would require additional testing using excitation at multiple wavelengths.
Funding and Disclosures
The authors received no specific funding for this work. The authors declared no conflicts of interest.
Publication Details
Roberson, B. J., S. Perea, D. DeRose-Broeckert, and S. B. Castleberry. 2025. “Glowing Green: A Quantitative Analysis of Photoluminescence in Six North American Bat Species.” Ecology and Evolution 15:e71885.
