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Feb 24, 2025

Weird? These bat toes can glow greenish-blue

Scientists were surprised by this fluorescence in Mexican free-tailed bats Under ultraviolet light, bristles on the toes of this Mexican free-tailed bat glow a brilliant blue-green. F. Gual-Suárez By

Scientists were surprised by this fluorescence in Mexican free-tailed bats

Under ultraviolet light, bristles on the toes of this Mexican free-tailed bat glow a brilliant blue-green.

F. Gual-Suárez

By Jason Bittel

October 15, 2024 at 6:30 am

You’ve heard of jazz hands? These bats have glow toes.

When ultraviolet (UV) light shines onto the Mexican free-tailed bat’s hairy toes, they light up. That glow is called photoluminescence. It happens when bristles on the toes absorb UV light, which our eyes can’t see. The bristles then re-emit that energy as light in a blue-green wavelength, which we can see.

Scientists have been finding more and more animals that can glow like this. Wasp nests, platypuses, flying squirrels and tardigrades all do it. But most of these animals’ whole bodies glow under UV light. In Mexican free-tailed bats (Tadarida brasiliensis), it’s just the toes.

Researchers found out about the toe glow by accident. They were trying to use a photoluminescent powder to track bats in Mexico.

“We failed at that,” says Fernando Gual-Suárez. He’s a biologist at the National Autonomous University of Mexico in Mexico City. But while they were using UV flashlights to search for signs of the powder, the team saw those glow-in-the-dark toes.

“Is that normal? Do the feet usually look like that?” Gual-Suárez remembers another scientist asking.

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Scientists already knew that the toes on this species are strange.

These and related bats “have what are termed ‘spoon-shaped bristles’ along the outer edges” of some of their toes, notes Nancy Simmons. She’s a bat expert at the American Museum of Natural History in New York City. She did not take part in the new work. “Nobody has ever known why [the bats] have these bristles in the first place,” Simmons says.

But it’s these bristles that glow under UV light.

Gual-Suárez’ team shared its new findings August 8 in Mammalian Biology. It’s the first time photoluminescent structures have been reported in live bats, they say.

The researchers found similarly bright fluorescent bristles on 25 live Mexican free-tailed bats. They came from two sites. The same glowing bristles showed up on a bat captured more than 700 kilometers (435 miles) north of the others.

The team also tested dead bats kept in museums for research. Their toe bristles didn’t fluoresce. But their bodies glowed a dull green all over when bathed in UV light. That’s probably because of chemicals used for preservation, the researchers say. It may also be due to other physical changes that occur in the specimens as they age.

That finding shows scientists would have missed the toe glow “if they’d have used museum specimens instead” of live bats, says Linda Reinhold. She’s a zoologist at James Cook University in Cairns, Australia. She calls the work by Gual-Suárez and his colleagues “good science.”

It’s not clear what purpose the toe glow might have. Scientists don’t even know if the bats can see the glow. And since the bats are nocturnal, they would likely encounter UV light mostly at dusk, dawn and in moonlight.

This species lives in colonies that can run to millions of bats. Such hordes need to coordinate migrations in the dark over of thousands of kilometers. The researchers say it would make sense if the light-up bristles played some role in communication at night.

“We need to test it in the lab,” says ecologist Rodrigo Medellín. He’s a coauthor, also at the National Autonomous University of Mexico. His team is thinking of keeping the bats captive and shaving the bristles off their feet. Afterward, they could test whether this affects the animals.

“This is just scratching the surface of a very deep and very long iceberg that nobody knows anything about,” he says of the mysterious glowing toes.

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bat: A type of winged mammal comprising more than 1,400 separate species — or one in every four known species of mammal. (in sports) The usually wooden piece of athletic equipment that a player uses to forcefully swat at a ball.

biology: The study of living things. The scientists who study them are known as biologists.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

coauthor: One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

colleague: Someone who works with another; a co-worker or team member.

ecologist: A scientist who works in a branch of biology that deals with the relations of organisms to one another and to their physical surroundings.

field: An area of study, as in: Her field of research is biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

fluoresce: (adj. fluorescent) The process of absorbing light of one wavelength (color) and reemitting as a different wavelength. That reemitted light is known as fluorescence.

migration: (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war).

nocturnal: An adjective for something that is done, occurring or active at night.

physical: (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).

platypus: Sometimes known as the duckbill, this shy Australian egg-laying mammal (Ornithorhynchus anatinus) has a streamlined body and flat bill. Its waterproof fur allows it to comfortably navigate in rivers and other waterways, where it feeds on invertebrate animals that live in the sediment. It uses electrical signals given off by the muscles of its prey to find its food. Males have a spur on the inner side of each ankle releases venom for use in their defense.

species: A group of similar organisms capable of producing offspring that can survive and reproduce.

tardigrade: An eight-legged creature not much larger than the period at the end of a sentence. Tardigrades live in many places, including ponds, the sea floor and parts of Antarctica where rock sticks above the ice.

ultraviolet: A portion of the light spectrum that is close to violet but invisible to the human eye.

wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Journal: F. Gual-Suárez et al. Ultraviolet-induced photoluminescent bristles on the feet of the Mexican free-tailed bat (Tadarida brasiliensis). Mammalian Biology. Published August 8, 2024. doi: 10.1007/s42991-024-00441-3.

Journal: W. Daney de Marcillac et al. Bright green fluorescence of Asian paper wasp nests. Journal of the Royal Society Interface. Vol. 18, August 2021. doi: 10.1098/rsif.2021.0418.

Journal: P.S. Anich et al. Biofluorescence in the platypus (Ornithorhynchus anatinus). Mammalia. Published online October 15, 2020. doi: 10.1515/mammalia-2020-0027.

Journal: H.R. Suma, S. Prakash and S.M. Eswarappa. Naturally occurring fluorescence protects the eutardigrade Paramacrobiotus sp. from ultraviolet radiation. Biology Letters. Published online October 14, 2020. doi: 10.1098/rsbl.2020.0391.

Journal: A.M. Kohler et al. Ultraviolet fluorescence discovered in New World flying squirrels (Glaucomys). Journal of Mammalogy. Vol. 100, February 28, 2019, p. 21. doi: 10.1093/jmammal/gyy177.

Jason Bittel is a freelance science writer who specializes in animals.

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Biological Evolution: Unity and Diversity, Energy, HS-LS4-4, HS-PS3-1, MS-LS4-6