The discovery is both spooky and thought-provoking, but researchers are left with more questions than answers
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.