The surprising hidden glow of one of Earth’s largest birds

Ginger, a six-foot-tall southern cassowary, has a calm temperament that makes her receptive to experiments. On a warm March night in Florida in 2021, scientists wanted to find out if keratin—a protein found in human skin and hair—in the horn-like structure on her head would fluoresce in ultraviolet light. This had never been documented in these large, flightless birds.

Todd Green, an anatomist and paleontologist, brought along an ultraviolet flashlight on his visit to the Cassowary Conservancy in Florida, where Ginger lives. Ultraviolet light is invisible to humans, but visible to many birds.

“I went out with one of the owners that night, and he was actually kind of making fun of me because he didn't think anything was going to happen,” says Green, a cassowary expert at the New York Institute of Technology College of Osteopathic Medicine at Arkansas State College. But after shining the ultraviolet flashlight into the dark fencing where Ginger was kept, the story changed. “It was one of the brightest fluorescences I've ever seen,” he says. “I audibly gasped.”

Cassowaries are some of the largest birds on Earth. Native to rainforests and mountain ranges in Australia and Papua New Guinea, they brandish long dagger-like claws and can weigh over 140 pounds. New research led by Green, published earlier this year in Scientific Reports, focuses on cassowary casques, which are bony keratin-covered ornaments that grow atop the birds’ skulls. The study is the first to demonstrate the ultraviolet response and glow of the casques.

“This is one of those huge, high profile, career-defining discoveries,” says Darren Naish, a paleozoologist at the University of Southampton who was not involved with the research. “It's a very, very good set of results.”

Each of the three known cassowary species has these casques, but they vary in size, shape, and color. They typically grow as the animal ages, peaking once the birds reach sexual maturity.

“It’s a bizarre structure,” says Green. “When you see it, it kind of looks more like a horn, but really is nothing like any horns that animals have.” 

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Prehistoric headgear

Researchers have long held an interest in cassowaries and their casques. “Cassowaries are among the best living analogs of many extinct kinds of dinosaurs,” says Naish. Paleontologists often look to these casques to better understand weird headgear on extinct dinosaurs like hadrosaurs or ceratopsians.

Yet cassowaries are understudied and their casques remain enigmatic. Hypotheses about the horn-like structure’s function have varied from roles in defense, heat regulation, visual displays, or to help them bellow out unique low-frequency vocalizations. Casque means “helmet” in French, but the structure is actually fragile, says Green, suggesting they probably did not develop them for defensive purposes. And unlike deer antlers that evolved to help males compete over females during the rutting season, both female and male cassowaries have casques. The leading hypothesis is that they play some role in visual display.  

To investigate further, Green and his colleagues wanted to see whether cassowary casques glow under ultraviolet light. Studies have found that most birds can see ultraviolet wavelengths—so if the casque glowed under ultraviolet light, that could suggest the structure plays a role in visual signaling. 

The work began in 2020, when many parts of the world had implemented COVID-19-related lockdowns and travel restrictions. Researchers were looking for ways to use available resources without having to go far.

“We couldn't travel to Papua New Guinea where these birds were, but we had access to museum specimens and specimens in the lab,” says Paul Gignac, an anatomist at University of Arizona and co-author of the study.

Gignac ordered a few different ultraviolet lights to scan preserved cassowary casques and quickly found the museum specimens—some of which are over 100 years old—glowed vibrantly beneath the UV wavelengths. Green received a text while on a flight from Colorado to New York. “He sent me the picture and said ‘well, we found something,’” Green recalls.

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A surprising glow

The next step was to see whether living animals exhibit the same glow. That’s what brought Green to visit his old friend Ginger, the first live cassowary found to glow in the study.

Experiments with other cassowaries soon revealed that the different species glowed in similar colors, but in distinct patterns across their casques. The dwarf cassowary, which has a jet black casque, did not fluoresce at all, whereas the southern and northern cassowary, which have casques with dull greens, yellows, and browns, glowed in different places.  

“If you'd asked me to predict how the casques would look, I certainly wouldn't have come up with anything that's like what's actually been discovered,” says Naish. “I would have thought that probably all of the keratin would fluoresce the same.”

But to establish the fluorescence was more than just an anatomical oddity, Green and his colleagues had to confirm whether the ultraviolet wavelengths reflecting from the casques were visible to cassowaries. Their results suggest the reflected wavelengths are between 365 and 385 nanometers, which means the cassowaries might see them. Further experiments are needed to determine how visible these brightly glowing patterns are under the natural living conditions within the dark rainforest canopy of their natural habitat.

Ultimately, the new discovery seems to align with the thinking that the casques serve some sort of visual purpose, perhaps by helping cassowaries distinguish between other cassowary species.

In future research, Green hopes to find out whether these ultraviolet signals are visible in the cassowary’s natural environment and uncover further details about their potential purpose. He also hopes the research might be a catalyst for conservation efforts and museum cataloging, since the uniquely fluorescing casques can act as a quick and easy way to track and distinguish between species.

“I'm really eager to see how he'll take this [discovery] and run with it,” says Gignac.